Drainage network dynamics and knickpoint evolution in the Ebro and Duero basins: From endorheism to exorheism

Accepted Manuscript Drainage network dynamics and knickpoint evolution in the Ebro and Duero basins: From endorheism to exorheism

Lucía Struth, Daniel Garcia-Castellanos, Marc Viaplana-Muzas, Jaume Vergés PII: DOI: Reference:

S0169-555X(18)30482-3 https://doi.org/10.1016/j.geomorph.2018.11.033 GEOMOR 6599

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

5 July 2018 21 November 2018 30 November 2018

Please cite this article as: Lucía Struth, Daniel Garcia-Castellanos, Marc Viaplana-Muzas, Jaume Vergés , Drainage network dynamics and knickpoint evolution in the Ebro and Duero basins: From endorheism to exorheism. Geomor (2018), https://doi.org/10.1016/ j.geomorph.2018.11.033

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Drainage network dynamics and knickpoint evolution in the Ebro and Duero basins: From endorheism to exorheism

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Lucía Struth, Daniel Garcia-Castellanos, Marc Viaplana-Muzas, and Jaume Vergés

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Institute of Earth Sciences Jaume Almera, ICTJA, CSIC, Lluis Sole i Sabaris s/n, 08028 Barcelona, Spain

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([email protected], [email protected])

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Abstract

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The study of fluvial network rearrangement provides a key to understand past and future landscape evolution. Large perturbations of hydrographic basins such as the change from endorheism to exorheism have repercussions in the steady or disequilibrium state of the

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basins and their drainage divides. Such transitions from internal to external drainage imply a major lowering of the geomorphological base level causing a major retreating knickpoint wave that separates the upstream low-relief area (inherited from the endorheic period) from the

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downstream incised area. Subsequently, the water divide migrates to reach an equilibrium in which erosion rates at both sides of the divide are similar. Previous fluvial analyses suggest

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that both the Duero and Ebro drainage networks, the two largest catchments in Iberia, experienced a change from endorheism to exorheism sometime between the Late Miocene and the Pliocene. Fluvial capture evidence argues for a Pliocene westward migration of the Ebro-Duero divide implying an area decrease for the Duero fluvial network (victim) in favor of the Ebro (aggressor). We used river profiles, knickpoint distribution and Chi-map calculation to understand the different degree of erosion of the Duero and the Ebro catchments and the dynamics of their drainage network. The results show an equilibrated Ebro drainage network

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ACCEPTED MANUSCRIPT in contrast with a disequilibrium in the erosional state of the Duero drainage, which remains reorganizing and adapting to the newly-imposed Atlantic base level. We identified at least two knickpoint wave trains in the Duero drainage resulting from the onset of exorheism: a fastpropagating wave through the cover and a low-propagating wave affecting the bedrock. Field evidence and topographic analysis suggest a westward migration of the Ebro-Duero divide,

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resulting in an ongoing headward erosion of the Ebro against the Duero catchment. Chi analysis provides the degree of disequilibrium of the drainage network indicating a large-scale

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aggressor role for the Duero and a victim role for the Ebro. We interpreted this seeming

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contradiction as the result of a different time scale perspective: local divide observations

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indicate a victim Duero in the short-term, whereas basin-scale dynamics support a victim role for the Ebro fluvial network in the long-term (multi-million-year time-scales).

Introduction

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Key words: Ebro Basin, Duero Basin, water-divide migration, knickpoint.

The study of drainage basin rearrangements provides a key to understand past and future

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landscape evolution. Drainage networks evolve as a dynamical system adjusting to perturbations in the landscape (such as those associated to tectonic deformation) in order to

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reach an equilibrium between fluvial erosion and base level variations, then maintaining a steady fluvial network and stationary drainage divides (Howard, 1965). Rivers with important convex-up sectors, knickpoints, in their longitudinal profile represent a transient response to external perturbations, such as a tectonic or climatic signal (e.g., Whipple and Tucker, 1999; Montgomery and Brandon, 2002; Wobus et al., 2006; Kirby and Whipple, 2012). However, river profiles can record drainage area changes, complicating the interpretation of the profiles and their inflections (e.g., Willett et al., 2014; Yang et al., 2015). A common dramatic scenario of large drainage area change is that caused by the change from endorheism to exorheism

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ACCEPTED MANUSCRIPT conditions, when a captor river connects with the terminal lacustrine system of the closeddrainage basin. This process, driven by both sediment overfilling of the basin, headward stream erosion, and climate oscillations (Garcia-Castellanos et al., 2003), implies a sudden lowering of the base level that is communicated upstream by the migration of an incision wave as was reported by Hasbargen and Paola (2000) with experimental modelling. The adjustment

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of the drainage network to the new base level results in a disequilibrium state of the basins (Hack, 1973; Howard and Kerby, 1983; Schumm, 1993) and their drainage divides (e.g.,

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Whipple, 2001; Bonnet, 2009; Castelltort et al., 2012; Willett et al., 2014). Drainage divides of

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catchments in a transient state are dynamic features that migrate horizontally in order to

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reach steady state (e.g., Gilbert, 1877; Davis, 1903; Glock, 1931; Bishop, 1995; Willett et al., 2014; Struth et al., 2015, 2017; Whipple et al., 2016).

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The northern part of the Iberian Peninsula encompasses two major hydrographic catchments, the Ebro and Duero (Fig. 1), which were opened from endorheic to exorheic

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conditions since the Miocene (e.g., Garcia-Castellanos et al., 2003; Arche et al., 2010; Antón et

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al., 2012). Recent works (e.g., Mikes, 2010; Antón et al., 2018; Vacherat et al., 2018) use field evidence, geomorphic analysis and Chi-maps to show the migration to the west of the Ebro-

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Duero drainage divide through a series of captures. These works are focused only in the EbroDuero shared divide and not the entire watershed. We provide a basin-scale relief and a

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detailed Chi (χ) analysis including Chi-maps and Chi-plots (e.g., Perron and Royden, 2013; Willett et al., 2014) for the entirety of both drainage networks. The extraction of this information agrees with a knickpoint analysis, and the comparison with the surrounding basins are the key to understanding the maturity and dynamic/static pattern of the Ebro and Duero networks obtaining the drainage dynamics for the northern Iberia.

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Fig. 1: (A) Topographic map of Iberia with the main hydrographic divides (thick black lines) and drainage network (thin blue lines). Red thick lines show the boundary of the Duero and Ebro catchments. Mean elevations have been calculated with a moving window diameter of 30 km. (B) Iberian tectonic map (from Vergés and Fernàndez, 2006 after Rodríguez-Fernández, 2004 and Vergés et al., 1995). CM is Cantabrian Mountains CCR is Catalan Coastal Ranges, CS is the Central System, GB is Guadalquivir Basin, CZ is Cantabrian range and ALZ is Asturian-Leonese Zone.

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2.

Geologic and geomorphologic setting The Ebro and Duero catchments are the major catchments in the northern portion of the

Iberian Peninsula that include homogeneous Cenozoic sedimentary basins (Fig. 2). During the

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Late Cretaceous-early Miocene, the collision between the Eurasian and Iberian plates occurred, resulting in the Alpine buildup of mountain chains like the Pyrenees, the Catalan

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Coastal Ranges, the Iberian Range, and the Central System (see Fig. 1B; Roure et al., 1989;

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Muñoz, 1992; Teixell, 1998; Vergés et al., 2002; Mouthereau et al., 2014). The Ebro basin was formed by the lithospheric flexure under the main thrust system of the Pyrenees and the

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Duero Basin was formed in front of the Cantabrian Mountains, the western continuation of the Pyrenean orogen (Alonso et al., 1996; Pedreira et al., 2003). These two basins were filled by

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siliciclastic and carbonate alluvial sediments related to the growing Pyrenees and Cantabrian Mountains, the Central System, and the Catalan Coastal Ranges (Pulgar et al., 1999; Guimerà

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(Alonso-Zarza et al., 2002).

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et al., 2004; López-Blanco et al., 2000) and were opened to the Atlantic by the Bay of Biscay

The closure of the Ebro and Duero basins occurred during the late Eocene at 36 Ma (Costa

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et al., 2010), with the uplift in the westernmost part of the Pyrenees, resulting in an endorheic basin (Puigdefàbregas et al., 1992; Costa et al., 2010). From the Oligocene to Miocene, the

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basins remained endorheic filled with alluvial and lacustrine deposition (e.g., Riba et al., 1983; Sáez, 1987; Alonso-Zarza et al., 2002; Garcia-Castellanos et al., 2003, Garcia-Castellanos, 2006). The Ebro and Duero basins were connected as a unique endorheic basin across the La Bureba corridor (Fig. 2; e.g. Mediavilla et al., 1996; Pineda, 1996). In the late Oligocene-Early Miocene, the Catalan Coastal Ranges were cut by a system of normal faults related to the formation of the Valencia Trough (e.g., Bartrina et al., 1992; Roca et al., 1999). In the late Miocene, the Ebro opened towards the Mediterranean Sea by the capture from a Mediterranean stream, by overfilling of the sedimentary basin, by an overflow of the endorheic lake system during a

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ACCEPTED MANUSCRIPT transition to a wetter climate stage, or probably by a combination of these three mechanisms (Garcia-Castellanos et al., 2003). According to Fillon and Van der Beek (2012), the underfilling and excavation of the basin started at this time. The more recent Neogene sedimentary deposits in the Ebro Basin are located in the Tarazona and San Caprasio areas (Fig. 2) and are dated as late Miocene units (~12 Ma and 13.6

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Ma, respectively; Pérez-Rivarés et al., 2002; Garcia-Castellanos and Larrasoaña, 2015). Numerical modeling provides an age range for the opening between 12 and 7.5 Ma (García-

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Castellanos et al., 2003), in accordance with the beginning of the siliciclastic sedimentation in

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the Valencia Trough by the Castellón Group with an age of about middle Serravallian (Martínez

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del Olmo, 1996). The total volume of sediment eroded from the Ebro Basin since the change from endorheism to exorheism ranges between 25,000 and 45,000 km3 (Garcia-Castellanos et

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al., 2003; Babault et al., 2006; Antón et al., 2017). According to the eroded sediment in the Ebro basin center (~1 km; Garcia-Castellanos and Larrasoaña, 2015), a first estimation of the

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denudation rate for the Ebro ranges between 47 and 80 mm/ka for 12 and 7.5 Ma, respectively

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(Antón et al., 2018).

The final infilling of sediment in the Duero Basin is related to the top of the Páramo Formation (located in Fig. 2) with an age of 9.74 - 9.58 Ma obtained by magnetostratigraphy

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(Krijgsman et al., 1996). The Páramo Formation was tilted post-deposition to the SSW, with a

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total maximum uplift value of 175 m, in agreement with the fluvial incision in this area (see Fig. 3C, where the maximum elevation values in the Duero basin are related to the Páramo Formation; Vergés et al., 2004). This situation argues for an incision response to the uplift and, according to Vergés et al. (2004), with no relation to the local base level variation to the Duero area. The Ebro Basin underfilling results in an isostatic rebound that, according to GarciaCastellanos and Larrasoaña (2015), affects to the western part of the Duero Basin, partially explaining the tilting of the Páramo units towards the center of the Duero Basin. There are extended thin mantled pediments (“rañas”) of Pliocene-Pleistocene age (Pérez-González and

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ACCEPTED MANUSCRIPT Gallardo, 1987; Martín-Serrano, 1991, 1994; Gutierrez Elorza et al., 2002) that overlain the Cenozoic basin that is now dissected by the drainage network, arguing for an incision onset of Late Pliocene to Pliocene-Pleistocene (Pérez-González and Gallardo, 1987; Pereira et al., 2000; Cunha, 2008). Silva et al. (2016) dated the onset of fluvial downcutting by the effective Atlantic capture in the Arlanzón River area on 1.1 - 1.9 Ma, based on geochronological data from the

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fluvial terrace systems for this area. Therefore, the opening of the Duero River Basin to the Atlantic was in a range between 9.74 - 9.58 and 1.9 - 1.1 Ma. Estimates of the total sediment

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volume eroded since the start of the exorheism for the Duero Basin yield a minimum value of

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2800 km3, lower than the 25,000-45,000 km3 for the Ebro (Antón et al., 2018).

Fig. 2: Geology of the Duero and Ebro catchments (adapted from IGME 1:1,000,000). The analyzed fluvial network is shown with blue lines. Last sediment record of the endorheic infill is represented by the Páramo Formation for the Duero Basin and by the Tarazona (TA) and San Caprasio (SC) location for the Ebro Basin. The La Bureba corridor location is indicated with the BC label.

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ACCEPTED MANUSCRIPT The present topography of the Central System and Tajo Basin can be explained by the nearly flat Moho and base of the lithosphere at around 32 and 135 km of depth, respectively (Fernández-Viejo et al., 2000; Fernàndez et al., 2004). Westward of the Iberian Range, there is an extended area in the Iberian Peninsula with a high topography between 600-1000 m that corresponds with a negative Bouguer anomaly, including the Duero and Tajo basins,

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interpreted as a thickening of the crust under the sedimentary basins as a result of the

Fernàndez, 2006; Carballo et al., 2015; Torne et al., 2015).

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compressive tectonic evolution in the Cenozoic (Casas-Sainz and Faccenna, 2001; Vergés and

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The premise of this study is that the current drainage network dynamics of the Duero and

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Ebro watersheds are the result of the fluvial reorganization after their respective exorheism onset. Opening times can be modulated by different outlet characteristics such as, for

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example, the outlet lithology: the Duero River network flows across a wide region of Variscan crystalline basement (location in Fig. 2; Díez Balda et al., 1990, 1992; Ábalos et al., 2002) in

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contrast with the Ebro network that drains across a narrow barrier of folded cover rocks of the

Methodology

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3.

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Catalan Coastal Ranges (Gaspar-Escribano et al., 2004).

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To test our hypothesis and to highlight potential disequilibrium of erosion we used quantitative morphological analysis of the two catchments. We used the 90 m resolution digital elevation model (DEM) SRTMv4 (Jarvis et al., 2008). Morphometric analysis and calculation of the geomorphologic parameters described in the following sections was carried out by using the D8 flow routine (eight-flow direction matrix; O’Callaghan and Mark, 1984; Tarboton, 1997; Mudd et al., 2014).

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ACCEPTED MANUSCRIPT 3.1.

Topographic analysis

The distribution of the topographic slope provides information about the different degree of incision of the drainage networks. We extracted the elevation distribution of an area from a rectangular swath through a set of parallel profiles and plotted it into a single profile: the swath topographic profile (e.g., Isacks, 1992; Masek et al., 1994; Telbisz et al., 2013). We

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calculated and projected the maximum, minimum and mean elevations with the Swath Profiler add-in automatic algorithm from Pérez-Peña et al. (2017). The mean elevation provides the

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general topographic signal of the landscape within every swath profile band, whereas the

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maximum and minimum elevations provide landscape variations in a direction perpendicular

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to the swath profile. The maximum elevation gives us the location of the ridgelines and, combined with low slope values, allows identification of elevated low-relief surfaces. The

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minimum elevation line corresponds to valley floors and the incision (or relief) is given by the difference between the maximum and minimum elevations. The swath width was fixed to 10

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km with 111 parallel profiles separated by 90 m (1 pixel). This width allows us to represent the

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information about major valleys and ridgelines. Generally, low values of local relief and swath profiles where all lines merge together are related to low-to-moderate incision, as in stable basins or plateaus. High relief and wider swath profiles are characteristic of dissected

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landscapes related to high uplift or fluvial incision in mountain ranges. Deviations of minimum

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and maximum values from the mean are present in the profiles, for example, if the mean approaches the maximum, it indicates a transient state of adjustment to high uplift rates (Keller and Pinter, 2002; Wobus et al., 2006). 3.1.1.

Local relief and relief anomaly

Local relief (difference between maximum and minimum elevations) describes the complexity of the landscape, reflecting the degree of incision by exogenic agents (wind or rivers). In addition, the local relief describes the regional relief characteristics (Bishop et al., 2003) and classifies different landforms (e.g., Brown et al., 1998). A relief anomaly (Scotti et

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ACCEPTED MANUSCRIPT al., 2014) highlights elevated and low-relief landscapes and represents the elevations normalized by the local relief (hmax-hmin). Both analyses are calculated with a mobile window of 1 km width. 3.2.

Fluvial analysis

In order to analyze the fluvial drainage network, we extracted all the parameters from 90 m

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resolution SRTM data with the sea as the base level for all the rivers, using the D8 flow routine

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(eight-flow direction matrix; O’Callaghan and Mark, 1984; Tarboton, 1997). We excluded all

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the pixels with a drainage area less than 1 km2, because they are considered to be governed by hillslope processes, landslides or debris flows (e.g., Gani, 2015).

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Analysis of slope-area relationships is often used to reveal spatial trends of erosion and/or

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rock uplift in channel networks by the mean of the channel steepness index or Ksn (e.g., Kirby and Whipple, 2001; Kirby, 2003; Snyder et al., 2003; Wobus et al., 2006; DiBiase et al., 2010).

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However, in areas with DEMs with low resolution or with narrow valleys, the provided slope

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values can be incorrect. In order to resolve this problem, we calculated the channel slopes (Ksn) using the Chi () gradient (Royden and Perron, 2013; Mudd et al., 2014), where we applied an elevation versus Chi instead of a slope versus area calculation. Assuming that rock

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uplift is balanced by erosion (steady-state condition) and that uplift (U) and erodibility (K) are

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constant in time and space, stream power theory predicts that a river will have a linear profile in a Chi- plot and Ksn will be proportional to erosion rates (Royden and Perron, 2013; Mudd et al., 2014). To obtain the Chi value, we introduced a reference drainage area (A0) with the aim of providing units of length (meters) on both axes and integrating upstream from base level (xb) to location (x) using the following equation (Perron and Royden, 2013):

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A0

m n

χ = ∫x (A(x)) dx

(1)

b

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ACCEPTED MANUSCRIPT where A0 is the reference drainage area and m/n is the concavity (Willet et al., 2014). Following the methodology of Perron and Royden (2013), we applied a scaling area (A0) of 1 m2 and calculated the best concavity for all the basins, obtaining an average value of 0.45 (see supplementary material). The application of Eq. (1) provides a test of the linearity of steadystate Chi-plots and allows the identification of transient signals in river profiles (Perron and

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Royden, 2013). If the resulting transformed profile shows a linear trend, that will indicate that the river is close to steady-state conditions with a slope proportional to the ratio U/K (Whipple

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and Tucker, 1999; Perron and Royden, 2013; Mudd et al., 2014; Willet et al., 2014). Therefore, we analyzed the dimensionless slope of the transformed Chi profiles for the

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main rivers and tributaries showed in Fig. 2, and extracted the longitudinal profiles analyzing the knickpoints. We localized the different knickpoints from the longitudinal profiles and made

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a first filter of these features if their nature is related to the location of anthropogenic dams, tectonic structures or a lithologic changes that we labelled as minor knickpoints (based on the

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DEM, topographic and geologic maps). We made a second classification with knickpoints that

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are associated with a low-relief surface upstream with a prominent concave-up profile shape and base level fall, and we labelled those as “major knickpoints”. According to Loget and Van

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Den Driessche (2009), a base level fall can produce a wave train knickpoint migration. This base level fall can be related to the opening of an endorheic basin resulting in an opening wave,

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which can be divided by more than one knickpoint wave according to the erodibility of the different lithological layers. This migration will function on the upstream drainage area, the lithology, and the amplitude of the base level lowering (Loget and Van Den Driessche, 2009). According to these authors, we suggest at least two different knickpoint waves related to the erosive dynamics and erodibility: a first advance of the cover waves and then wave trains related to the bedrock that correspond, according to Loget and Van Den Driessche (2009), to the real time response of rivers. With this analysis we want to highlight the link between the major knickpoints and their relation to the location of bedrock knickpoint waves.

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ACCEPTED MANUSCRIPT Dynamics between tectonics and surface processes can be extracted from the analysis of transformed river profiles (Chi-plots) (Perron and Royden, 2013). To compare the basins, we analyzed the Chi-plots and selected a representative stream for each basin, containing the main shape and knickpoint distribution shown on the same plot. The Chi-plots allow the identification of perturbations in the profile from the initial linear trend indicating a transient

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response of the river profile. According to Perron and Royden (2013) and Willett et al. (2014), if a steady-state river gains area by capture, the Chi-plot will shift toward lower Chi values and

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above the steady-state trend. However, if a river loses area by capture, the plot will be shifted

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either higher uplift rates or less erodible lithologies.

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to the right, below the steady trend. The same effect as a capture gain area is produced by

To extract information about the horizontal dynamics of the river network, we elaborated a

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Chi-map following the method of Willett et al. (2014). Observing the Chi-maps, differences in Chi across the drainage divides will suggest disequilibrium and dynamic drainage network

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rearrangement. Therefore, the analysis of Chi values across water divides reveals ongoing

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drainage network reorganization. Similar Chi values at the channel heads on opposite sides of a drainage divide suggest that the river network is near geometric equilibrium, while large

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differences in Chi values across the drainage divide imply that network geometry is far from equilibrium. Identifying contrasts in Chi indicates possible divide migration and river capture

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(Willett et al., 2014). If uplift and bedrock erodibility are spatially uniform, drainage divides generally migrate from low to high Chi values to achieve equilibrium and hence, channels displaying high Chi values are prone to capture and may eventually disappear (Willett et al., 2014). We calculated the Chi-map of the entire Iberian Peninsula and in detail for the Ebro and Duero basins to investigate the dynamics of the drainage divides. To test the hypothesis that a recent stream capture is recorded in the Chi-plots, we recalculated these after removing the Cenozoic Duero Basin extension of the DEM, following the methodology described in Giachetta and Willett (2018). We considered all the drainage

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ACCEPTED MANUSCRIPT area interpreted as endorheic including the Cenozoic sedimentary basin (Alonso-Zarza et al., 2002; Alonso-Gavilán et al., 2004) and the boundary hillslopes that drain to the Duero Basin. If substantial area gain occurred recently, the Chi-plots of capturing streams should collapse on the trend displayed by the streams that did not capture. To visualize the gradient of Chi and relief for the entire divide of the Duero and Ebro watersheds, we calculated the difference of

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Chi-map values and relief results under a smoothing mobile window 20 cells wide and with a divide radius of 5 km. We used the higher values of Chi and relief and move on to the divide

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location in order to proceed to calculate the difference in these parameters.

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We generated topographic profiles of the drainage divides, calculating their mean elevation

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using the method of adjacent-average smoothing. We compared the location of divide depressions (low-elevated segments below the mean elevation) with the geometry and

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characteristics of the current drainage network. A fluvial capture leaves gaps in the drainage divide where water does not flow anymore (wind gaps) and where fluvial sediment relicts may

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be preserved.

Results

4.1.

Topographic analysis

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An initial topographic analysis was developed in the Iberian Peninsula using a swath profile,

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relief anomaly and local relief calculation in order to extract information about the incision signal in the topography in Iberia (Fig. 3). The local relief map in Fig. 3A shows highest values (in red) in the north related with topographic highs as the Pyrenees and the Cantabrian Mountains. Lowest values of local relief (in blue) related to the Duero and Ebro basins and the La Mancha region. The topographic analysis (Fig. 3A) of the Ebro catchment shows that the Ebro sedimentary basin is heavily incised by the fluvial network and is bounded by the high values of local relief that correspond to the Catalan Coastal Ranges (east), the Pyrenees and Cantabrian Mountains (north), the Iberian Range (south) and the Sierra de la Demanda (west)

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ACCEPTED MANUSCRIPT (see location in Fig. 2). The Duero catchment highlights a flat surface in the inner part (Duero sedimentary basin, meseta), which is now partially incised by the fluvial network. The meseta is a high plateau bounded by high relief corresponding to the Sierra de la Demanda and the Iberian Range (east), the Central system (south-southeast), the Cantabrian Mountains (north) and the Montes de León (Iberian Massif, west) (see location in Fig. 2).

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High values of relief anomaly (Fig. 3B, red color) indicate flat and elevated surfaces. In the

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Iberian Peninsula, these values are principally located in the Duero Basin and in the Mancha

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region, which corresponds to an upper Cenozoic continental basin, an extremely flat area.

Fig. 3: (A) Local relief for the Iberian Peninsula. Highest values in red and lowest values in blue. (B) Relief anomaly in the Iberian Peninsula, where highest values are represented in purple and lowest in green. (C) Swath profiles following the river paths of the Duero and Ebro (see Fig. 1A for location). Zmax indicates the maximum infill elevation for the Cenozoic sediments.

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ACCEPTED MANUSCRIPT The calculated swath profile for the Duero and Ebro watersheds (Fig. 3C) highlights the topographic signal of the low or high relief distribution within the band. The Duero drainage area shows high relief (reaching 1000 m of river incision) in the low-medium section of the profile, followed by an elevated low-relief area with very low incision (all lines merge together showing a river incision of about 175 m in Fig. 3C). The Ebro also shows high relief in the lower

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(reaching 1000 m of river incision) and higher parts where lithological changes and tectonic structures are located, whereas in the middle the signal of the mean, maximum and minimum

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4.2.1. Channel slope gradient (Ksn)

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4.2. Fluvial analysis of the Duero and Ebro basins

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remain closer arguing for a more mature landscape.

Significant variations in channel slope are observed between the Ebro and Duero drainage

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networks (Fig. 4). The map distribution of the Ksn shows generally low values for the upper part of the Duero fluvial network and for the central part of the Ebro, coinciding with the lower

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local relief (Fig. 3A) and is consistent with the results of relief anomaly (Fig. 3B) and field

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observations. The upper part of the Duero drainage network shows a meandering river pattern with very low slope, meanwhile in the lowest part of the network, the local relief and channel

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steepness became higher. These two incision areas are limited by a major knickpoint (655 m asl; Fig. 5B). The Ebro network is characterized along the entire basin by incised and concave-

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up river profiles (Fig.5B), with higher slopes in the upper parts and lower slopes in the direction of the outlet.

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Fig. 4: Channel steepness gradient (Ksn) for the Ebro and Duero catchments, where red colors

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indicate high values and blue colors indicate lower values. The black dashed line indicates the

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4.2.2. Knickpoint distribution

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limit between lower Ksn values in the Duero meseta from the higher downstream values.

The analysis of the longitudinal profiles and Ksn in the Ebro and Duero drainage networks

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(Fig. 5) reveals different types of knickpoints related with lithologic changes, tectonic structures, anthropogenic dams or little upper captures (minor knickpoints). Knickpoints that

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are located in with sharp and prominent concave-up inflection with an extended low-relief surface upstream, are typed as major knickpoints (yellow circles in Fig. 5). This type can be related to the propagation of an incision wave produced by capture. Rivers in the Ebro network show a convex-up profile with minor knickpoints related to dams, tectonic or lithologic sources, and the signal of an elevated low-relief surface is very low and only observed in the trunk channel near the headwaters with a small extension (200 km2). From the analysis of the longitudinal profiles and Ksn in the Duero catchment, the knickpoint results reveal a different distribution of these features across the basin. Knickpoints are concentrated

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ACCEPTED MANUSCRIPT in the medium-lower western part are classified as minor knickpoints, limited to the east with the location of major knickpoints (yellow points, Fig. 5). This last type (located at 655 m asl in the Duero trunk river) is related to an upstream elevated low-relief surface associated with the preservation of the Cenozoic sedimentation infill. The rivers in the Duero catchment present longitudinal profiles that are concave-up below the 655 m knickpoint and convex-up above it,

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suggesting that they are non-equilibrated rivers (Fig. 5B). However, the preserved Cenozoic low-relief infill is dissected by the fluvial network (see red points in Fig. 5) including the Páramo

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Formation (9.74-9.58 Ma; Krijgsman et al., 1996). The mantled pediments (rañas) are also

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incised, as in the Arlanzón River (see location in Fig. 2) where the pediments have an age

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between 1.1-1.9 Ma (Silva et al., 2016), arguing for a very young fluvial incision.

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ACCEPTED MANUSCRIPT Fig. 5: (A) knickpoint distribution for the Ebro and Duero catchments. Knickpoints associated with elevated low-relief areas are yellow, those related to lithologic or tectonic sources are black, and Duero sedimentary cover are red. The black dashed line connects the yellow points defining a major knickpoint wave. (B) Duero and Ebro longitudinal profiles without dams,

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where the yellow point is the position of the major knickpoint in the Arribes zone.

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4.2.3. Chi analysis

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The inferred dynamics of the divides of the Ebro catchment is summarized in Fig. 6B, with a comparison between a representative river for each domain: Ebro, Duero, Adour,

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Mediterranean and Cantabrian rivers. In general, the Ebro River plots show a near equilibrium state, with no meseta segments in the plot that relates to a recent captured area. The

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Cantabrian rivers (cant1) have a clean mark of an aggressor pattern situated above the plots from the Ebro rivers (eb2), indicating that the divide will move to the South in order to reach

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an equilibrium state. Comparing the plot of the Adour River (ado1) with a tributary of the Ebro

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(eb3), indicates a southern divide migration. Mediterranean rivers (med1) lie above the Ebro (eb5), indicating a western migration of the drainage divide towards the inner part of the Ebro

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basin. The Tajo River (tajo3) lies below the Ebro (eb4), reflecting an aggressor pattern of the Ebro against the Tajo and then a migration of the southwestern portion of the divide. The

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Duero plot (due1) shows a marked inflection at 655 m asl and is situated above the plot of the Ebro tributary (eb1), meaning that the Duero one has more erosive potential (aggressor) than the Ebro river (victim).

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Fig. 6: (A) Local relief map with the location of the selected rivers plotted in B and C. (B) Chiplots with a representative plot for the Ebro (eb) and the shared divide rivers: Adour (ado), Mediterranean (med), Duero (due), Tajo (tajo) and Cantabrian (cant) basins. (C) Chi-plots with

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ACCEPTED MANUSCRIPT a representative plot for the Duero (due, black) and shared divide rivers: Atlantic (atl), Tajo (tajo) and Cantabrian (cant) basins.

The Chi-plot of the Duero River (Fig. 6C) has a marked inflection in the plot at an elevation

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of 655 m asl. The Cantabrian (cant2) plot reveals a more aggressive pattern than the Duero (due3), as in the case of atl2 against the due6, promoting an area decreasing of the Duero by a

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divide migration to the South. The Tajo (tajo2) stream plot lies below the Duero plots (due2

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and due5) indicating an aggressor pattern for the Duero against the Tajo streams and then producing a Duero expansion. The Tajo1 plot lies below the due5. We plotted the Sil (atl1)-

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Luna (due4) rivers (capture reported in Santos-González and Fernández-Martínez, 2011 and Peláez González, 2013) where we can observe the low-relief morphology in the upper Sil, due

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to the recent area capture.

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Mapping the Chi contrast in the divides provides information about the degree of

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disequilibrium of the drainage divides as well as the horizontal motion needed to reach equilibrium (sensu Willett et al., 2014). A marked Chi contrast across the divide was found in the Cantabrian Mountains with Duero and Ebro catchments (Fig. 7). The higher Chi value on

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the southern side suggests that the drainage divide will migrate to the south. A Chi contrast is

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also found in the area of the Almazán Basin, where the Duero, Ebro and Tajo share the drainage divide. From the Chi-map results, the divide of the Ebro and Duero will move to the south, to the inner part of the Tajo. A Chi contrast is found also along the divide between the Ebro and the Mediterranean basins, with higher values in the former suggesting a divide migration towards the inner part of the Ebro Basin. Results are more intriguing along the divide between the Ebro and Duero catchments as was reported in Vacherat et al. (2018). The Chi values are similar across the divide with a locally higher Chi for the northern Duero-Ebro

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ACCEPTED MANUSCRIPT divide along the La Bureba region (Fig. 2) and lower to the south, which is similar to the

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Pyrenean region.

Fig. 7: Chi-map for the northern part of the Iberian Peninsula. Higher values are

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represented by red colors defining a victim pattern and lower values in blue for the aggressive

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pattern.

To test if the current inflection in the Duero Chi-plot is the result of a recent drainage area gain, we removed the hypothesized captured drainage area above major knickpoint wave

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amounting 70,000 km2 (red thick line in Fig. 8) and recalculated the plot (proto-Duero in Fig. 8)

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using a concavity of 0.45. The results show that the proto-Duero Chi-plot (grey line, Fig. 8) collapses to a similar trend displayed by the neighbor streams (atl3, atl4, atl5; see location in Fig. 6A), evidence that the perturbation in the plot is in agreement with the captured area. In this test, we assumed that the proto-Duero reached an equilibrated profile just before reaching the Cenozoic Duero Basin following the general trend displayed by atl3, atl4 and atl5.

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Fig. 8: Original Chi-plot for the Duero River (due1), neighbor Atlantic river plots (atl3, atl4, atl5)

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and the result from removing the meseta area (pale pink polygon) for the Duero River (our

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reconstruction of the proto-lower Duero River).

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Results of the Chi and relief in the divides are summarized in the Fig. 9 where we show the

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difference between the values of each basin in a radius of 5 km for each side. We displayed the results under a color ramp for each basin where we highlighted the pattern of aggressor (if the Chi inside the basin is lower or if the relief is higher than in the opposite side of the divide).

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This helps to visualize the divide migration expected from Chi contrasts and relief.

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Impressive divide migration exists in the Cantabrian Mountains, with a direction to the inner part of the Ebro and Duero basins (southwards), as well as for the short Mediterranean rivers (westwards). Therefore, the short Cantabrian and Mediterranean rivers have an aggressor pattern while the Ebro and Duero tributaries have a victim pattern. The shared divide between Ebro and Duero catchments points to a westward migration in the southern part (Almazán Basin region, between 42-41oN), suggesting an expansion of the Ebro drainage network at the expense of the Duero one. The Chi-map contrast (Fig. 9A) in the north (between 43-42oN) shows the opposite sign. The other conflicted area is the shared divide of

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ACCEPTED MANUSCRIPT the Duero with the short Atlantic rivers where the Chi range argues for an aggressor pattern in the south and a victim pattern in the north. Meanwhile, the relief range (Fig. 9B) argues for an aggressor pattern for the incised Duero watershed (west to the dashed line). Topographical and geological observations are highlighted by the relief results, where in the lower part of the Duero catchment the rivers are incised and the knickpoints are located in the headwaters,

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eroding and increasing their drainage area, and powered by the Cenozoic meseta capture that provide erosion capacity to these rivers. The shared divide between the Sil (atl1) and Luna

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(due4) rivers, shows a green ramp in the relief map (Fig. 9B), arguing for an aggressor Duero

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pattern related to a higher relief in the Duero catchment than in the Sil, that actually records a

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non-incised part that resulted from a recent capture event, concurring with previous works that described this capture area (Santos-González and Fernández-Martínez, 2011; Peláez

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González, 2013). In the Duero-Tajo divide, the analysis of longitudinal profiles and Chi-plots results in more equilibrated rivers for the Tajo than for the Duero rivers, where the first shows

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a linear trend in the plots (Fig. 6). The Duero River presents a knickpoint that separates the

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incision zone (downstream) from the low-relief zone (upstream). Therefore, the divide area has a non-erosive drainage of the Duero and an erosive drainage in the Tajo, arguing for a divide migration towards the Duero Basin. The north of the Sierra of Guadarrama (Fig. 1),

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affected by glaciation processes (e.g., Bullón, 2016), argue for a relic of higher slopes in the

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north that are preserved by the glaciers, and a completely incised and mature fluvial network in the south, with remnants of small captures. Currently, the relief pattern argues for a southward divide migration based on higher slopes in the Duero than in the Tajo catchments.

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Fig. 9: Chi range (A) and Relief range (B) along the boundary of the Duero and Ebro catchments. The color for the Ebro-Duero shared divide is represented with the Ebro colors.

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Black arrows indicate the divide migration for each parameter.

4.2.4. Drainage divide analysis Two topographic profiles of the drainage divides of the Ebro and Duero catchments (Fig. 10A) show a set of depressions (low-elevation segments below the mean elevation of the drainage divide) of the Ebro-Duero divide (Fig. 10C) that usually coincide with reentrants of the divide trace (Fig. 10B). These reentrants are segments of the divide that diverges from the main orientation and can be the topographic expression of divide migration (e.g., Willett et al., 2001) or the result of lithological changes, tectonic structures, different climate, or zones of

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ACCEPTED MANUSCRIPT plunging folds. These divide depressions are usually located in areas with higher Ksn contrasts, as in the La Bureba corridor (number 6 in Fig. 10; see discussion) and with elbow geometries along the river paths, as can be seen in Hontomín (number 5 in Fig. 10; and Fig. 11A and 11C). In the southern part of the depression number 5 we can observe the result of a capture in Peñahorada (Fig. 11D). In this area, a very impressive beheaded valley is observed: an ancient

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drainage area of the Hoz River that drained at the east was captured and incorporated to the

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Ebro network by the Oca River, leaving a gorge and a beheaded valley along the current divide.

Fig. 10: (A) Drainage divide profiles for the Ebro and Duero with the smooth profile of 1000 pts (red line). (B) Geologic map with detail of the Ebro-Duero divide with the reentrants in red. Black box indicates the location for the Hontomín capture (see discussion). (C) Location of the

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ACCEPTED MANUSCRIPT reentrants related to topographic depressions below the mean elevation. Numbers and arrows indicate the depressions under the mean elevation (smoothed red line).

Additional divide depressions with reentrant geometry are located along the divide near La

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Piedra (depression number 5, Fig. 10) that shows a reentrant towards the Duero catchment associated with a low elevated depression below the mean elevation drainage divide (star 10,

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Fig. 6A). This capture highlights the high potential of erosion of the Ebro tributaries. In the

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same area, we interpreted a capture segment associated with a large drainage area (40 km2)

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near Fuencalenteja, just to the north of La Piedra (star 10, Fig. 6A; Fig. 11B).

Fig.11: (A) Interpretation of the fluvial capture of the Homino-Ubierna rivers with the related knickpoint (black star) and the position of the Ubierna fluvial deposits. (B) Interpretation of the divide migration (from the dashed black line to the red line) and location of a suggested future

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ACCEPTED MANUSCRIPT capture near Fuencalenteja. (C) Field view of the Hontomín capture with the knickpoint location indicated with a black star (field of view 1.5 km; see location in Fig. 10, depression 5 north). (D) Field view of the beheaded Hoz valley from the Duero-Ebro divide (field of view 2.5

5.

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km; see location in Fig. 10, depression 5).

Discussion

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Geomorphic analysis shows systematic differences in relief and channel morphology

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between the Ebro and Duero watersheds, highlighting the transition from a low relief and partially dissected topography in the Duero catchment to relatively high relief topography in

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the Ebro. We have explored how these features and the opening of the two basins towards the

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Atlantic and Mediterranean, respectively, affected the erosive response of the fluvial networks and divide migration.

5.1.1.

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River network morphology and erosive response Ebro drainage network

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5.1.

In the Ebro watershed the relief signal, knickpoint distribution and river longitudinal profiles

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argue for a basin whose rivers are near the steady state and where the incision wave produced by the drainage opening has reached the headwaters. The Ebro Basin presents a homogenous

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Oligo-Miocene sedimentary infill and a very narrow barrier of folded cover rocks of the Catalan Coastal Ranges (Gaspar-Escribano et al., 2004). This sedimentary configuration allowed the rapid incision wave propagation in the basin. In this way, all the tributaries experienced the erosion wave, and then were supplied with a high potential to erode and incise. Vacherat et al. (2018) analyzed the knickpoints in the upper parts of the Ebro tributaries in the Ebro-Duero divide, showing that the knickpoints are located in the headwaters. Our results agree with this and with the interpretation of a mature Ebro drainage network.

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ACCEPTED MANUSCRIPT The divide between Ebro and Cantabrian domain suggests an ongoing divide migration to the south. The headwaters of the Ebro trunk river are threatened by high and steep Cantabrian rivers near Reinosa (star 1, Fig. 6A). Atristain (1999) documented a capture area between the Nervión River (Cantabrian domain) and the Bayas stream (Ebro tributary) near Orduña (star 2, Fig. 6A), and between Deba (Cantabrian domain) and Zadorra streams (Ebro domain) (star 3,

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Fig. 6A). The authors interpret these captures as the result of a contrast in the fall of the

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elevation along a shorter distance in the Cantabrian rivers than in the Ebro tributaries.

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The easternmost Ebro tributaries, with headwaters in the Catalan Coastal Ranges, are threatened by the short Mediterranean rivers draining directly to the Mediterranean Sea (Figs.

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6 and 9). The steeper Chi-plots of the Mediterranean rivers (e.g., med1 in Fig. 6B), argue for a westward divide migration and then a reduction of the area of the Ebro basin. Lewis et al.

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(2000) and Castelltort et al. (2016) documented fluvial captures of small portions of the Ebro basin from the erosive Mediterranean rivers, such as the Francolí and Llobregat (stars 4 and 5,

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Fig. 6A), due to the Neogene extension and from Mediterranean base level change.

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Ebro River tributaries draining the divide with the Tajo and Turia watersheds show an aggressive pattern against both basins. From the topographic and fluvial analysis carried out in

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5.1.2.

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this study we envisage a southern migration of the divide.

A major knickpoint set (yellow points in Fig. 5) separates the upstream low-relief area of the Cenozoic Duero Basin from the incised area farther downstream. Among this set of knickpoints, the one at the Duero River itself (655 m asl) displays an incision of 470 m in the Arribes Gorge (Antón et al., 2012; Fig. 5). The area below the major knickpoint set is represented by steep and incised rivers with an approximately linear Chi-plot (except for the little tectonic/lithological/dams features) and with a concave longitudinal profile, arguing for a high erosive pattern. The tributaries that flow through the flat meseta area are characterized

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ACCEPTED MANUSCRIPT by low slope gradient (Fig. 4), high Chi values (Fig. 7) and a convex-up profile (Fig. 5B) and Chiplot (e.g., due rivers in Fig. 6C), arguing for a low erosive pattern. These rivers have very low erosive potential and are therefore prone to be captured by more aggressive rivers with steeper profiles, such as the Atlantic or Cantabrian. In general, below the major knickpoint set (yellow points in Fig. 5), several knickpoints are located at lithological changes or tectonic

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structures that are beyond the scope of this study.

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The area located upstream from the major knickpoint set is characterized by very low relief,

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but incision can be observed in the Páramo Formation (Fig. 5A), as was reported by Silva et al. (2016). We interpreted this as a wave-train knickpoint migration, as was described in Loget

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and Van den Driessche (2009) in different rivers draining towards the Mediterranean Sea, due the different erodibility between the bedrock and the sedimentary cover. Accordingly, we

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postulate that the Duero drainage network undergoes at least two waves of fluvial incision: a bedrock wave and a cover propagation wave. According to this theory, the current position of

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the bedrock knickpoint wave is limited by the low erodibility of the Paleozoic rocks (yellow

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points in Fig. 5), whereas the cover train propagates faster to the headwaters reaching the upper parts of the streams (red points in Fig. 5).

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Knickpoint migration velocity can be estimated from the current location of the bedrock and cover knickpoints and assuming a common origin and start time. We propose a starting

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time of ~10 Myr according to the age of the Páramo Formation (Krijgsman et al., 1996) in order to obtain a minimum migration rate. Further, we propose the starting point of knickpoint migration near the locality of Peso da Régua (see Fig. 2), where the topography is higher and coincides with the neighbor divide trend location. The location of the knickpoint origin is used as a fixed value to calculate the migration rates. The Arribes bedrock knickpoint set is located at 215 km from the origin (Peso da Régua, Fig. 2), resulting in a migration rate of 0.02 m/yr. Cover knickpoints are situated in the headwaters of the Duero fluvial network (we used the

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ACCEPTED MANUSCRIPT knickpoints located near the Ebro- Duero divide) at a mean distance of 500 km from the origin, providing a migration rate of 0.05 m/yr. Therefore, we obtained a cover knickpoint wave propagation rate that is two and a half times faster than for the bedrock wave. This result agrees with Loget and Van den Driessche (2009), who concluded that for a same drainage area, the rate of rivers that flow in alluvial cover is higher than in bedrock. According to these

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observations, we propose a schematic evolution model of knickpoints for the Duero fluvial

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network (Fig. 12).

Fig. 12: Schematic model of knickpoint evolution related to the capture of a yellowish lowrelief area and the development and migration of knickpoint wave trains: knickpoints related to the bedrock wave in yellow and related to the cover wave in red. Times (t1-t5) only indicate the chronology but not steps.

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We propose that the exorheism onset in the Duero results in an incision wave that when it arrived at Cenozoic basin, was divided into at least two different waves: one that continues incising the bedrock (yellow points in Fig. 12) and one with a faster propagation through the sedimentary cover (red points in Fig. 12). We can observe in Fig. 5 that many of the cover

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knickpoints are in the headwaters of the small tributaries and that this is the reason that we

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cannot observe them in the Chi-plots or longitudinal profiles: the location is just in the head of

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the stream and the knickpoint cannot advance because of the insufficient drainage area upstream. Therefore, we agree with Loget and Van Den Driessche (2009) that the migration

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rate of the knickpoints can differ in relation to the erodibility and the drainage area upstream. The major bedrock knickpoint wave is located in the basement (granite and metamorphic

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rocks; Fig. 5). These rocks configure the high topography of the knickzone that in Cenozoic times allowed the isolation of the basin from the Atlantic margin (Santisteban et al., 1996;

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Antón et al., 2014) where the NNE-SSW faults controlled the evolution and architecture of the

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western part of the Duero catchment. These are currently active strike-slip faults with moderate tectonic activity, without evidence of current uplift in the area (Antón et al., 2010;

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Martín-González et al., 2012; Antón et al., 2014). We argue that the limited upstream migration of the major knickpoint is the result of the low erodibility of the granite that forms

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the outlet of the Duero Cenozoic basin. The change from endorheic to exorheic conditions by the capture of an ancient Atlantic stream is the most likely mechanism for the drainage evolution in the Duero Basin, as was reported in other areas like in the Sorbas Basin (SE Spain) (Mather, 2000) or in New Zealand (Crosby and Whipple, 2006). According to Antón et al. (2014), the fluvial profiles and slope gradients in the Duero streams are not explained by a lithology effect, sea level fluctuations or active tectonics. Following this interpretation, the Atlantic stream that eventually captured the

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ACCEPTED MANUSCRIPT Cenozoic Duero Basin (proto-Duero) had a fluvial profile and a Chi-plot similar to the present short Atlantic rivers, as those labeled as atl3, atl4 and atl5 in Fig. 7. If our interpretation is correct, the proto-Duero stream significantly enlarged its drainage area due to the capture, resulting in a Chi-plot steeper than that of the neighbor Atlantic streams (atl3, atl4 and atl5; Fig. 8). The plots of the short Atlantic rivers show a more linear trend than the Duero trunk

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river (Fig. 8), indicating that the Duero has not had enough time to re-adjust the profile. Following the model of Perron and Royden (2013) and Willett et al. (2014), when a river

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captures a neighboring basin, it increases its slope in the Chi-plot and develops an inflection in

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the upper part. When removing the captured area (red thick line in Fig. 8) from the Chi

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calculation then the lower part of the due1 stream returns to the trend of rivers not affected by captures (atl3, atl4 and atl5 in Fig. 8). This supports the hypothesis of a recent capture of

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the Duero basin by a previous stream with a Chi profile similar to the grey line in Fig. 8, as was reported in Giachetta and Willett (2018). Giachetta and Willett (2018) presented the same

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methodology for the upper Blue Nile catchment in the Ethiopian Plateau. They recalculated

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the Chi values after removing the drainage area of the supposed capture extension above the major capture knickpoint in the Jema catchment, a tributary of the Blue Nile. Giachetta and Willett (2018) show that after this removal area test, the Chi-plots collapse along the main

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capture.

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trend, confirming that the perturbation in the plots is consistent with the drainage area

The divide shared with the Cantabrian and Atlantic rivers obeys the same pattern as for the Ebro with the Mediterranean: high contrast in slopes and relief argue for a migration of the divide towards the inner part of the Ebro and Duero basins. A documented example of divide migration is the case of the Sil (Atlantic) and Luna (Duero) rivers by Santos-González and Fernández-Martínez (2011) and Pelaez González (2013). They suggest a fluvial capture of the headwaters of the Luna stream by the Sil River, and then, a movement of the divide to the east (star 6, Fig. 6A). The higher Chi values, steeper Chi-plot, and higher slopes in the Duero stream

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ACCEPTED MANUSCRIPT and the presence of a wind gap in the divide are all consistent with this study. An additional example of the importance of a slope gradient and different erosive capacity was documented in the Cantabrian-Atlantic divide (Ea River, in Lugo) by Llano Fúnez and López Fernández (2015). They reported a westward divide migration resulting from the shorter and steeper river from the Cantabrian Mountains relative to the Atlantic (star 7, Fig. 6A). The shared divide with

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the Tajo basin predicts a southward divide migration based on the higher value of Chi in the

Duero-Ebro divide migration

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5.2.

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Tajo side and the lower slope along the Tajo Chi profile relative to the Duero (see Fig. 8B).

The fluvial network history of the Duero and Ebro basins is a natural laboratory to study the

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dynamics of basin capture, dramatic base level changes and divide migration in absence of major tectonic deformation (e.g., Garcia-Castellanos et al., 2003; Benito-Calvo and Pérez-

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González, 2007; Antón et al, 2014; Garcia-Castellanos and Larrasoaña, 2015). Based on a preliminary statistical geochronological approach, Silva et al. (2016) obtained an Early

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Pleistocene age for general valley downcutting in Central Spain (c.a. 2.3 Ma) for Atlantic basins.

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That study includes terraces from the upper Arlanzón River (near the Hontomín and La Bureba region, Fig. 2) and the upper Tajo. Few authors studied the drainage divide between the Ebro

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and Duero and documented the fluvial captures in the La Bureba and Hontomín (Fig. 11) areas (Pineda, 2006; Benito-Calvo and Pérez-González, 2007; Mikes, 2009, 2010; Vacherat et al.,

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2018) shown in stars 8 and 9 in Fig. 6A and number 6 in Fig. 10. In agreement with the previous authors (Mikes, 2009, 2010; Vacherat et al., 2018), we can observe in the area near Hontomín (Fig. 11) an extremely recent fluvial capture: Middle Pleistocene to Holocene fluvial deposits of the Ubierna River continue to the south connecting with the Duero drainage network. Currently, the Ubierna River connects with the Homino River (knickpoint represented by a black star in Fig. 11) and flows to the Ebro side. In this area we can observe the process of fluvial capture leaving an abandoned valley through the current position of the drainage divide and preserving the initial drainage lines (Fig. 11).

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ACCEPTED MANUSCRIPT The reentrants of La Piedra and Fuencalenteja (star 10 in Fig. 6A and depression number 5 in Figs. 10 and 11B) coincide with depressions in the divide and leave some elbows in the streams. Further, as for the case of Hontomín, the wide valley connects the north and south with a paleoflow in this direction and now is captured by a tributary of the Ebro River trunk. The Almazán and Calatayud basins present a capture zone where lacustrine sediments were

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found and elongated depressions with flat valleys can be observed (star 11 in Fig. 6A). Vacherat et al. (2018), Giachetta et al. (2015) and Scotti et al. (2014) argue for the extended

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captured area of the Almazán basin from the Duero basin by the Jalón River, a tributtary of the

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Ebro River, following the line of Gutiérrez-Santolalla et al. (1996). In agreement with

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stratigraphic data from Gutiérrez-Santolalla et al. (1996), Vacherat et al. (2018) proposed that during the Pleistocene-Holocene, the Jalón River (tributary of the Ebro River) reached the

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Paleozoic Aragonese branch (southwest of the Calatayud Basin) and then captured the Munébrega graben and the Almazán subbasin. However, we can find some portions that are

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still endorheic in the Iberian Chain, such as the Gallocanta half-graben or part of the Jiloca

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depression (Giachetta et al., 2015), arguing for a non-affection of the base level drop from the exorheism onset. This area was incision-limited for a general dome-like uplift of the Iberian Chain that started in the Late Pliocene that inhibited the headward incision in the interior of

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the Chain (Gutiérrez-Elorza and Gracia, 1997; Gutiérrez et al., 2008; Scotti et al., 2014 and

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references therein).

All these capture zones demonstrate the importance of the knickpoint location and lithology distribution throughout the basin in understanding large-scale catchment dynamics (Fig. 13). In our study region, the current knickpoint location strongly suggests two trends of erosion controlled by the pronounced erodibility contrast between the Cenozoic infill of the Duero Basin and its outlet through granitic bedrock (Fig. 13B). This erosive pattern is reflected in published incision rates. In the area above the major knickpoint wave, Silva et al. (2016) calculated an incision rate ranging from 0.1 to 0.2 mm/yr for the last 100 kyr for the Arlanzón

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ACCEPTED MANUSCRIPT River (upper Duero). For the Esla River, Schaller et al. (2016) obtained an incision rate of 0.1530.159 mm/yr for the same time period. In contrast, below the major knickpoint wave, the incision rate is completely different: the incision rate in the Arribes zone shows a range of 2-3 mm/yr (Antón et al., 2012) for terraces with an age of 100 kyr or older and an elevation difference of 200-300 m. Cunha (2008) identified a culminant fluvial erosive surface in Rego da

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Barca (see location in Fig. 2) interpreted as a sedimentary unit before the beginning of the incision stage in the Duero River. They obtained a mean incision rate of 0.2 mm/yr with OSL

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dating.

This separation of two areas of different incision rates is not observed in the Ebro drainage

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network, probably because all the tributaries experienced a faster base level fall and headward

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erosion (Fig. 13B).

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Fig. 13: (A) Simplified topography of the Ebro and Duero basins before the exorheism. The

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drainage divide is represented with a red and white line. Dark blue dashed lines indicate the interpreted evolution of the longitudinal profiles of the Ebro and Duero rivers and the blue lines are the current river profile. (B) Simplified topography of the current Duero and Ebro basins and the present longitudinal profiles of Duero and Ebro rivers. Note the incision decreasing in the Duero cover towards the east, in accordance with the incision propagation (see discussion). The complete incision in the Ebro basin resulted in the advance of the divide towards the west, evidenced by a fluvial capture in the northern part of the divide (as in Hontomín). (C) Sketch of the future topography of the basins highlighting the result of the

36

ACCEPTED MANUSCRIPT advance of the Duero major knickpoint wave. Grey dashed line indicates a next step of the river profile in order to reach a steady state, and then, an equilibrated drainage divide. SC is San Caprasio and TA is the Tarazona area, youngest sediment record in the Ebro endorheic

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basin (13.5 Ma, Pérez-Rivarés et al., 2002).

This difference of high erosion in the Ebro and lower in the Duero watersheds in the area of

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the divide is the reason why currently the Ebro is increasing its drainage area. Vacherat et al.

SC

(2018) argue that the westward Ebro-Duero divide migration results in the erosion capacity decreasing (based on a specific stream power calculation) in the Duero River and then, a

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preservation of the upstream part of this basin. However, the Chi-plot and longitudinal profile of the Ebro argues for a current equilibrated river network far from be very active to erode and

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make the divide migration to the east. The Chi-plots inform us about the longer-term drainage basin dynamics, with a steeper agressor plot for the Duero and a victim plot for the Ebro. We

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suggest that the erosion wave corresponding to the major Duero knickpoint set will eventually

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reach the Ebro-Duero divide in the future (Fig. 13C), and then the Ebro network will become victim to the aggressive Duero. Thus, it is important to differentiate the present local dynamics

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observed at the divide (geomorphological evidence for local captures; short-term drainage

term).

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evolution) from the dynamics of the entire basin (Chi analysis and knickpoint distribution; long-

37

ACCEPTED MANUSCRIPT 6. 

Conclusions The Ebro drainage network is close to equilibrium based on the overall river profile

concavity, the location of the knickpoints in the upper reaches of the basin and the widespread incision since the drainage opening to the Mediterranean in the late Miocene between 12 and 7.5 Ma. The Duero drainage network is in disequilibrium, as shown by the concave Chi profile

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

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and the major bedrock knickpoint wave that has not yet reached the sedimentary Duero Basin since the change from endorheism to exorheism conditions between 9.7 and 2 Ma. The Ebro watershed presents a narrow barrier of folded cover rocks of the Catalan

SC



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Coastal Ranges in the lower part (~10% of the Ebro River path) and a homogeneous Cenozoic sedimentary infill in the upper part (~90% of the Ebro River path). This geologic configuration

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allowed the rapid incision wave propagation inside the sedimentary Ebro Basin, arriving to the upper parts of the Ebro tributaries. Topographic analysis suggests a homogeneous and evolved



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inherited from endorheic times.

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erosion pattern for the Ebro drainage network with no preservation of extended surfaces

The Duero catchment presents a wide barrier of low erodibility Paleozoic rocks in the

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lower part (~50% of the Duero River path) and a homogeneous Cenozoic infill in the upper part. The more extensive low-erodibility lithology in the sedimentary Duero Basin helps to

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inhibit rapid incision propagation and explains the observation of a completely different erosion pattern than in the Ebro one. This situation is supported with higher values for slopes and relief in the lower part (amounting 470 m in the Arribes Gorge) and lower slopes and relief (175 m in the Páramo Formation) in the upper part. This geologic configuration allows the preservation of the flat area inherited from the top of the Cenozoic endorheic basin infill (meseta). 

Two incision waves are interpreted in the Duero watershed related to the different

erodibilities: the bedrock wave located in the Arribes Gorge region, and the cover wave

38

ACCEPTED MANUSCRIPT located in the upper reaches of the Duero tributaries. A result of the cover incision wave is the limited incision in the meseta. 

From the location of the interpreted bedrock and cover knickpoint waves in the Duero

watershed, we obtained a first estimation of knickpoint migration velocity that is about two and a half times faster for the cover knickpoint than for the bedrock (Arribes Gorge). The Chi profile of the lower part of the present Duero River shows a steeper Chi-plot

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

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than the rest of the neighboring Atlantic rivers. The Chi profile of the upper part shows a low slope along the sedimentary Duero Basin. However, if we remove the upper basin of the

SC

profile we obtain the same trend of the neighbors, indicating and reinforcing the



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interpretation of a recent capture.

Both relief contrast and river captures argue for an ongoing westward migration of the

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Ebro-Duero divide, resulting from higher relief in the Ebro (aggressor) than in the Duero drainage network (victim). However, the major knickpoint wave and the Chi-plots describe an

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aggressive pattern for the Duero drainage network. We interpreted this seeming contradiction

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as the result of two synchronous processes occurring at different scales both spatially and temporally: ongoing short-term divide migration (aggressive Ebro drainage network) and longterm future catchment evolution (aggressive Duero drainage network). Our results, therefore,

AC

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warn against the use of Chi-plots as a self-sufficient technique to predict divide migration.

Acknowledgements This work is supported by MITE (CGL2014-59516), SUBTETIS (PIE-CSIC-201830E039) and ALPIMED (PIE-CSIC-201530E082) projects. We thank the three anonymous reviewers and the editor Scott A. Lecce whose constructive comments improved the original manuscript.

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The Ebro Basin is closer to equilibrium than the Duero Basin, which remains reorganizing and adapting to the newly-imposed Atlantic base-level. From the exorheism onset and base level fall in the Duero basin we suggest two knickpoint wave trains: a fast-propagating wave through the cover and a lowpropagating wave affecting the bedrock. The rate of knickpoint retreat on the cover is twice as high as the rate of retreat on the bedrock. Local divide dynamics argue for a victim Duero at short-term and, basin-scale dynamics argue for a victim Ebro at long-term.

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