Fluvial changes of the Guadalquivir river during the Holocene in Córdoba (Southern Spain)

Geomorphology 100 (2008) 14–31

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Fluvial changes of the Guadalquivir river during the Holocene in Córdoba (Southern Spain) David Uribelarrea a, Gerardo Benito b,⁎ a b

Dept. de Geodinámica, Facultad de CC. Geológicas, Universidad Complutense, 28040 Madrid, Spain CSIC-Centro de Ciencias Medioambientales, Instituto de Recursos Naturales, Serrano 115 bis, 28006 Madrid, Spain

A R T I C L E

I N F O

Article history: Received 13 January 2006 Received in revised form 23 November 2006 Accepted 15 April 2007 Available online 9 May 2008 Keywords: Channel migration Floodplain stratigraphy Holocene Geoarchaeology Medinat al-Zahira settlement Guadalquivir river Spain

A B S T R A C T Holocene fluvial changes of the Guadalquivir River at Córdoba City were studied with an emphasis on floodplain development, river migration rates, sedimentation rates and environmental history. During the Holocene, the Guadalquivir River has developed a large meander (El Arenal) with a general southwards lateral migration, undercutting Tertiary bedrock, and with a total incision of 9 m, which developed three alluvial surfaces: Fp1, Fp2 and Fp3. The oldest floodplain surface Fp1 (+7–9 m) was deposited during the early Holocene and reached its maximum extent around 1000 yr BP. The next floodplain surface Fp2 (+5 m) accumulated 500 to 1000 yr ago. Finally, the youngest floodplain surface (Fp3, +1–2 m) was developed in the last 500 yr. Migration rates and direction changed from 690–480 m2 yr− 1 in Fp1 (to the southeast), 2280 m2 yr− 1 in Fp2 and 620 m2 yr− 1 in Fp3 (to the west). The stratigraphical study of palaeomeanders and chute channel deposits show evidence of river position and dynamics through recent times: (1) “San Eduardo” was filled 4000 yr BP; (2) “Madre Vieja” has been active since 2100 yr BP to the present day; and (3) “El Cortijo” was formed and filled during historical times (the last 1000 yr). The chronology of the alluvial stratigraphy and fluvial dynamics are discussed within the context of historical hydrologic, climatic and anthropogenic changes. In addition, the geomorphological reconstruction of the riverine landscape in historical times provided some clue to the location of Medinat al-Zahira, a lost Muslim settlement built in the 10th century AD and believed to be situated at, or nearby, the Arenal meander. Paleogeographical analysis shows that the most suitable conditions for this medieval settlement were found on the northeast part of the Arenal meander. © 2008 Elsevier B.V. All rights reserved.

1. Introduction Alluvial environments are very sensitive to both hydroclimatic changes and variability in local intrinsic factors (e.g. geology, floodplain sedimentation). In the last 2000 yr, human activities such as land use changes have also altered river dynamics at the reach and catchment scales (Van Andel et al., 1990; Carrión et al., 2001; Macklin and Lewin, 2003). On the one hand, it is evident that fluvial geomorphology has influenced the location of human settlements (e.g. water availability, ford sites for crossings, bridge locations, etc.) and its activities (e.g. soil quality for agriculture). On the other hand, human activity in riverine areas (e.g. bridges, water mills, embankments, weirs, river channelisation and flood protection) has modified fluvial dynamics and introduced changes in floodplain sedimentation patterns and rates (Brown, 1997, p. 254). Floodplain dynamics near large settlements are, therefore, a major component of cultural history and a key element to understanding urban development and expansion through time (Butzer et al., 1983).

⁎ Corresponding author. E-mail addresses: [email protected] (D. Uribelarrea), [email protected] (G. Benito). 0169-555X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2007.04.037

Currently, there is a growing interest in environmental and landscape archaeology (e.g. palaeoenviromental reconstructions at prehistoric and historic human settlements, Hassan,1979; Goudie, 1987), since changes in the natural environment are of archaeological significance (Brown, 1997). In addition, geoarchaeology provides new tools in fluvial geomorphology enabling a better chronological understanding of fluvial changes and the intrinsic and extrinsic driving forces. In Spain, there is great potential for studies combining cultural and alluvial histories since many important historical human settlements were concentrated along the middle and lower reaches of major rivers. By the year 1000 AD, not only was there an increase on population density as a whole but urban centres were flourishing as well: Cordoba had more than 100,000 inhabitants; Valencia, Mérida and Toledo 37,000; Zaragoza 17,000; Seville and Málaga 20,000; Almería 27,000; and Granada 26,000 (García and González, 1994). During the past decades excavations conducted by archaeologists, with recent support by geoarchaeologists, at a number of those cities have provided detailed information and interpretation of on-site artifacts, but often failed to provide insight on long-term river dynamics, palaeohydrological changes and environmental history. An advantage of environmental reconstruction of river floodplains during historical times is that geomorphological and stratigraphical

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records can be completed with evidence derived from physical and biological remains of material culture, as well as with documentary records (e.g. written references on extreme events or landscape descriptions) of river dynamics (see Butzer et al., 1983; Benito et al., 2003a,b; Uribelarrea et al., 2003; Butzer, 2005). In this paper, a detailed study of the fluvial geomorphology and river dynamics of the Guadalquivir River at the Arenal meander, east of Cordoba, is presented. The specific objectives of this study comprise: (1) to study the geomorphological and sedimentological evolution of the Guadalquivir River at the Arenal meander, (2) to understand the spatial and temporal fluvial dynamics of this meander during the Mid- to Late Holocene, and (3) to establish links between environmental history and human occupation on the Guadalquivir floodplain with special emphasis at the Umayyad Period (AD 929– 1013). In this period, Cordoba was the most prosperous capital of alAndalus, and a worldwide reference in terms of culture, socioeconomic development and modernity (Levi-Provençal, 1950). Its wealthy economy favored the foundation of two cities Medinat alZahra and Medinat al-Zahira within a radius of 8 km from Cordoba (Arjona Castro, 1982; Vallejo-Triano, 2001). In written chronicles, Medinat al-Zahira is said to be located in eastern Cordoba, and likely within or nearby the Arenal meander, but the exact location remains unknown. The detailed geomorphological and stratigraphical analysis conducted at the Guadalquivir river upstream of Cordoba may provide insight into the most suitable sites for the location of Medinat alZahira, assuming the hypotheses of its location at the Arenal meander. 2. The study area The Guadalquivir River Basin is located in southern Spain covering a catchment area of approximately 60,000 km2, the fourth largest in Spain (Fig. 1). The river regime is greatly influenced by rains caused by Atlantic

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fronts that tend to cross the Iberian Peninsula between November and March, and is characterised by extreme seasonal and annual variability, with a present-day mean discharge at Cordoba (catchment area of 25,450 km2) of about 100 m3s− 1, and peak discharges over 3400 m3s− 1. The Guadalquivir River at Cordoba flows through an alluvial meandering section of medium sinuosity (1.31), within a channel gradient of 0.0008 m m− 1, and a floodplain width ranging between 250 and 2000 m. The bedload is composed of medium to coarse gravels and sand, with an average channel width of 80 m. The study area comprises a 5 km-long meander with a 1300 m radius of curvature, known as the Arenal meander. This meander is larger than the 2300 m and 1300 m lengths of the meanders located immediately upstream (Cañaveralejo Bajo) and downstream (Campo de la Verdad) (Fig. 1). The points of inflection of the Arenal meander have been traditionally used to wade through, and coincide within the location of historical water mills. Geologically the study zone lies at the contact between the passive southern margin of the Paleozoic Iberian Massif (Sierra Morena) and the Guadalquivir Neogene basin, a foreland basin connected directly to the Atlantic in the Gulf of Cádiz area, that developed and filled while the Betic Alpine mountains uplifted (Vera, 2000). In the Cordoba region, the Guadalquivir River cut through the contact between the highly deformed limestones and dolomites, Lower Cambrian in age, and the Miocene clay and marls with a subhorizontal structure (Sanz de Galdeano and Vera, 1992). As a result, the Quaternary river valley is highly asymmetric with most of the fluvial terraces being developed on the right-hand (northern) margin. The most complete sequence of fluvial terraces of the Guadalquivir River is found between Cordoba and Seville, where 14 terrace levels are distributed between +215 m above the thalweg and the present alluvial plain at +6 m. Baena Escudero (1993) and Baena Escudero and Díaz del Olmo (1994) grouped these terraces from the highest to

Fig. 1. Location of the study area and geomorphological map of the Arenal meander.

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Table 1 Numerical dates Site

Unit/depth (m)

Material dated

Analysis

Lab code

Radiocarbon age (yrs BP)

Calibrated agea

One sigma calibrated age rangea

San Eduardo San Eduardo San Rafael Madre Vieja Madre Vieja Madre Vieja

E-8/1.7 E-5/2.7 C8b-1/3.3 C10-1/3.8 C10-2/3.3 C17-2/3.7

Sg Ch Ch Ch Ch Ch

AMS AMS AMS AMS AMS AMS

UZ-5133/ETH-29101 UZ-5132/ETH-29100 UZ-5128/ETH-29096 UZ-5129/ETH-29097 UZ-5130/ETH-29098 UZ-5131/ETH-29099

3855 ± 55 3355 ± 55 2690 ± 50 2125 ± 50 1965 ± 55 2000 ± 50

2300 BC 1680, 1670, 1658, 1651, 1637 BC 828 BC 168 BC AD 30, 39, 52 AD 2, 14, 16

2458–2203 BC 1735–1529 BC 897–803 BC 336–56 BC 38 BC–AD 116 46 BC–AD 63

Site

Unit/depth (m)

Material dated

Analysis

Lab code

TL/OSL age (yr)

Equivalent dose (Gyr)

Dose rate (Gy kyr− 1)

Cortijo Huerta Vieja

CA-3/3.5 HV-1/3.7

P Sd

TL OSL

MAD-4003 MAD-3988

943 ± 84 8345 ± 743

15.22 43.23

16.13 5.18

Note. P, pottery; Ch, charcoal; Sg, gastropod shell, Sd medium to coarse sand, AMS, accelerator mass spectrometry; TL, thermoluminiscence. UZ, 14C laboratory of the Department of Geography at the University of Zurich (GUIZ) using the dating facilities of the AMS (accelerator mass spectrometry) with the tandem accelerator of the Institute of Particle Physics at the Swiss Federal Institute of Technology, Zurich (ETH). MAD, Laboratorio de Geología y Geoquímica, Universidad Autónoma de Madrid, Spain. a Radiocarbon dating samples and results, including calibrated ages calculated by the Radiocarbon Calibration Program Rev. 4.3 from University of Washington Quaternary Isotope Lab, based on Stuiver and Reimer (1993).

lowest into those of N800 ka (T1 to T3), 800-300 ka (T4 to T9), 30080 ka (T10 to T12), and a lowest group (T12 to T14) representing Late Glacial and Holocene times. However, along a 25-km studied reach near Cordoba, only four major terrace treads can be distinguished at the following heights above the present thalweg: T1 (+ 80 to 90 m), T2 (+25 m), T3 (+10 to 15 m) and the floodplain (+7 to 9 m). These stream terraces have been developed on the right side of the valley, and they are equivalent to terrace T8 (+90–95 m), T12 (+26–30 m), T13 (+13– 20 m) and T14 (+7–10 m) described by Baena Escudero (1993) and Baena Escudero and Díaz del Olmo (1994) for the middle and lower Guadalquivir river. The terrace fill deposits are composed of gravel and sand deposits with thicknesses varying between 3–4 m for T3 (+10– 15 m) and 8 m for the floodplain (+7 to 9 m). The present river valley configuration was reached after the Guadalquivir river's incision into T3 (+10–15 m). Upstream of the study area at Andújar, a terrace equivalent to T3 provided an OSL age of 55 ka (Pérez-González, unpublished data), whereas downstream at Palma del Río T3 it was dated between 180 to 80 ka (Baena Escudero and Díaz del Olmo, 1995). The floodplain has resulted mainly from the channel migration to the south during the Holocene. In the study reach, floodplain width varies between 2 km at the Arenal meander, where lateral accretion surfaces are well developed, and 250 m in Córdoba City. 3. Methodology and data sources In the 25 km study reach of the Guadalquivir valley, a general geomorphological map (1:50,000 in scale) was drawn based on aerial photographs (1:30,000) and the geological map (1:50,000, Copeiro de

Fig. 2. Flood frequency distribution and relative magnitude of the Guadalquivir river documentary floods at Seville and Cordoba.

Villar et al.,1973). In addition, a detailed geomorphological map (1:5,000 in scale) was drawn based on available aerial photographs (from 1956 at 1:30,000 in scale; 1996 at 1:18,000; and 1999 at 1:8000) and topographic maps 1:5000 in digital format (contour lines at 1 m intervals) and 1:10,000 in paper format (contour lines at 1 m intervals with additional elevation points). In the present study, a methodology based on a Geographical Information System (GIS) raster was developed to accurately measure the rate and direction of the channel migration through time. A new digital terrain model (DTM) was interpolated using a combination of elevation data from different topographic sources, namely the 1:5000 and 1:10,000 maps. This DTM was critical to (1) improve the geomorphological mapping of the floodplain, which is intensively used by human activity, and (2) to delineate the inundation areas at different flood stages knowing the water surface elevations from gauge records and historical data sources. Systematic recording at gauge stations began in 1911, although it is discontinuous and of poor quality. The available documented historical flood data for the Guadalquivir River at Córdoba and Seville from AD 700 to the present was compiled. The historical flood database contains 85 records derived from indirect sources (Borja Palomo, 1878; Vanney, 1970). Stratigraphical analysis of the floodplain was based on 16 boreholes and 11 trenches, located along three radial transects 250 m apart, which cover the main geomorphological features of the Arenal meander (Fig. 1). The boreholes reached a maximum of 8 m in depth, whereas the trenches reached up to 5 m in depth. The chronological constraints of the alluvial deposits were based on six radiocarbon datings and one thermoluminescence (TL) dating of a pottery fragment. TL dating was performed by the Universidad Autónoma de Madrid using a Risø TLDA-10 equipment and following the established methodology for the fine grains from pottery (Zimmerman, 1971). The annual dose was determined by combining two types of measures. On the one hand, the determination of the beta radioactivity coming from 40K present in the samples was measured by the Geiger-Müller counting system; on the other hand, the alpha activity coming from uranium- and thorium-series decay chains also present in the samples was measured using solid scintillation (SZn) counting. The calculated archaeological dose was 15.22 Gy and the annual dose was 16.13 mGy yr− 1. The resulting TL date for the pottery fragment was 943 ± 84 TL yr BP. In addition, the alluvial chronology was supported by visual identification of archaeological artifacts and pottery. Optically stimulated luminescence (OSL) dating of alluvial sand samples was also performed. Insufficient bleaching of the previous OSL signal during the last process of reworking was detected, which resulted in an overestimation of the burial age. Sand samples were collected in the field using PVC cylinders, from which feldspar particles with grain size of 8–10 μm were separated using routine procedures. The equivalent dose (De) was measured using the

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Fig. 3. 3-D diagram of the sedimentary environments found at the Arenal meander.

Multiple Aliquot Added Dose (MAAD) and the Thermoluminescence (TL) signal. Dose rates were calculated for radioisotope (U, Th, K) contents and the cosmic dose rate estimated from burial. Only one OSL sample was in agreement with the radiocarbon dates (Table 1), although it should also be interpreted as a maximum age. In the Guadalquivir River, there are abundant documentary sources recording extreme floods and droughts. In this study, historical data compilations based on public and ecclesiastic archives at Cordoba and Seville have been used (Borja Palomo, 1878). The documentary flood data show discontinuous information available between 700 and 1400 AD, and a nearly complete data series from 1400 AD to the

present day (Fig. 2). Most of the historical flood landmarks that provide water stage data are referenced to Seville (Vanney, 1970). Tentative flood discharge values were assigned to historical floods in Cordoba using the discharge relationship between the Alcalá del Río gauge station (located near Seville) and Cordoba which shows a correlation coefficient of 0.78. Therefore, the indicated discharge values aim to provide an idea of the severity and frequency of flooding during historical times. The most catastrophic flood in Medieval times occurred in winter of AD 992–993, and in December AD 1008. In the period AD 1000 to AD 1200, we should highlight the lack of

Fig. 4. Stratigraphic synthesis profile of the point bar environment.

Fig. 5. Stratigraphic synthesis profile of the chute channel environment.

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information on extreme events, probably due to poor documentary records, since other Atlantic basins of the Iberian Peninsula (e.g. Tagus river) record peaks in the number of floods between AD 1160 and 1210 (Benito et al., 2003b). Unusually high flood frequencies were registered in the periods AD 1590–1650, AD 1775–1810 and AD 1850–1890. In the 20th century, flood frequency and magnitude decreases, with the exception of the time periods AD 1910–1925 and AD 1959–1963. 4. Results 4.1. Geomorphology and sedimentology of the Arenal meander The Arenal meander shows three main geomorphological and depositional environments (Fig. 3): (1) river channel, (2) coarsegrained point bar surfaces and (3) floodplain channels (chute and palaeochannels) and chute bar sediments. The river channel is characterised by pool and riffle morphology, concave banks undercutting the Neogene bedrock and convex bank migration due to point bar and lateral bar deposition. At present, river regulation by dams, bank protection with dykes and river engineering works since 1970s have destroyed most signs of recent fluvial activity. The Arenal meander comprises three major point bar surfaces which are structured at the following heights above the present thalweg: Fp1 (+7 to 9 m), Fp2 (+5 m) and Fp3 (+1 to 2 m) (Fig. 1). These point bar surfaces were developed by lateral accretion of coarsegrained bedload material overlain by vertical accretion facies dominated by settling of fine-grained suspended sediments (sand, silt and clay) (Fig. 3). The lateral accretion deposits are characterised by 0.5 to 1 m-thick beds of clast-supported coarse rounded quartz gravels with two gravel-size modes (4–5 cm and 10–12 cm), maximum size of 12– 30 cm, and a sediment matrix composed of coarse sand and pebbles (Fig. 4). The sequence contains intercalated layers of coarse to very coarse sand 25–30 cm in thickness, characterised by small foreset cross-bedding and trough-fill cross-bedding (3D) according to the classification recommended by the SEPM (Ashley et al., 1982), and occasional current ripples. The sand layers may indicate a discharge decrease and/or surface reactivation. Occasionally, the lateral accretion surface is composed of 20–30 cm-thick fining-upward coarse sand layers with massive structure. The sequence contains frequent subaerial exposure indicators, such as mud cracks, root cast, and infill of small channels. The vertical accretion sequence (floodplain s.e. deposits) is characterised by 10–30 cm-thick units of massive or structureless fine to very fine sand deposition, grading into finer sediments (silts) to the top of the unit, and with frequent dispersed clasts (b1 cm) (Fig. 4A). The total thickness of this vertical accretion sequence may reach in excess of 3 m. At the top of the sequence, subaerial exposure results in mud cracks and intense bioturbation (root marks) due to vegetation and burrows made by annelids and arthropods. Depositional differences may occur in this sedimentary environment depending on sedimentation chronology and distance from the present river channel. In Fp2 (+5 m) the sand facies are thicker than in Fp1 (+7 to 9 m), and the coarse-grained lateral accretion units contain gravel-size rounded pottery fragments (Fig. 4B). In addition, in Fp2 gastropod and bivalve shells as well as mammal bones and charcoal are more frequent than in the older fill terraces. Chute and chute bar sediments are well-developed on the floodplain surfaces. On Fp1 (+ 7 to 9 m), two chute channels have been described, namely San Eduardo at the meander's most inside edge, and Madre Vieja cutting through the southern part of Fp1 (Fig. 1). A third chute channel, Cortijo del Arenal, is located at the boundary between Fp1 (+7 to 9 m) and Fp2 (+5 m). These chute channels are characterised by relatively steep sides and flat bottoms with deepest channel morphology between their upstream and middle portions. Morpho-

logically, San Eduardo shows a sinuous planform whereas Madre Vieja and Cortijo del Arenal are convex channels following former river channels or ridge and swale topography. The chute channels show a complex sedimentology due to their exposure to a wide range of flow energy conditions (Fig. 3). During extreme floods, chute channels convey a relatively large discharge associated with high flow energy conditions where scours give way to gravel deposition in the form of chute bars. During annual floods, low energy currents flowing along chute channels cause only the deposition of suspended sediment. Accordingly, two depositional sequences infilling the chute channels are deposited, namely chute bars and low-energy floodplain deposition. Chute bars are composed of clast-supported quartzitic gravel, with two grain size modes of 1–2 cm and 4–5 cm, and D95 of 9–10 cm, with small to large foreset cross-bedding (Fig. 5A). The low energy facies (floodplain) infilling the chute channels comprises 20 to 40 cm-thick units of fine to very fine sands and silts with gradual fining-upward sequences (Fig. 5B). Gravel beds over 0.5 m in thickness can be intercalated within these low energy sequences. Post-medieval pottery was found up to 1.5 to 2 m in depth, and exceptionally was recorded up to 3.5 to 4 m deep. The frequency of flooding may be established according to the chute minimum elevation above the present channel, chute chronology, and the sediment storage rate on the floodplain channels. Minimum flood water surface elevation of 99–100 m, 97–98 m, and 95–96 m a.s.l. are required to overflow through San Eduardo, Madre Vieja and Cortijo del Arenal chute channels, respectively. The chute chronology and elevation influenced the distribution of sediment and stratigraphic development. In fact, the last overbank flood flowing through Madre Vieja occurred in 1963, although there is not an accurate discharge record for this event in Cordoba. Other gauged recorded floods overflowing Madre Vieja occurred in 1912 (3050 m3s− 1), 1915 (3400 m3s− 1) and 1925 (3350 m3s− 1). In the historical period, high flood frequencies likely overflowing Madre Vieja occurred in the periods AD 1590–1650, AD 1775–1810 and AD 1850–1890 (Fig. 2). 4.2. Fluvial dynamics during the Middle-Late Holocene Fluvial change interpretation was based upon the estimation of sedimentation rates, channel migration (extension, translation and rotation of meanders recorded as an average distance per year), and periods of river incision and chute channel activity. Floodplain incision on the Late Pleistocene fill terrace T3 (+15– 20 m above the present-day channel) occurred during the Late Pleistocene–Holocene transition. The base of the floodplain alluvial deposits (Fp1, +7 to 9 m) provided an OSL age of 8345 ± 743 yr ago (MAD-3988), which may be overestimated due to insufficient bleaching of the sample (Fig. 6). In Andújar, 66 km upstream of Cordoba, a similar fill terrace at +6 m above the present channel was dated using archaeological artifacts with an age older than 3000– 5000 yr BC (Santos García et al., 1991), whereas in Seville (130 km downstream) fill terraces at +9–10 and +7–9 m above the present channel were considered as Holocene (García Martínez et al., 1999). The field and chronological data confirm that during the early Holocene and until 5000 yr BP the Guadalquivir river channel was located next to the present T3 terrace scarp, in the northern sector of the Arenal meander. During the Holocene, the river channel has migrated southeastwards undercutting the Neogene bedrock in a movement that included extension (lateral channel shift) and rotation (downvalley migration). The main stages of this channel migration were obtained from the age dating of the chute channels. These chute channels have been functioning as high flow channels during different temporal periods. Therefore, the chute infill deposits post-date the active channel period and provide data on rates and direction of the river migration.

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Fig. 6. Cross-sections and stratigraphic profiles and datings performed at the Arenal meander.

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The chute bar deposits in San Eduardo provided radiocarbon dates of 2458–2203 cal BC and 1735–1529 cal BC, whereas fine grained sediments in Madre Vieja were dated to 168 cal BC-52 cal AD (Fig. 6C; Table 1). In fact, at 2500 yr BP the river channel was placed half way between San Eduardo and Madre Vieja (from a radiocarbon date at San Rafael, Table 1). Considering San Eduardo and Madre Vieja as the Guadalquivir river channels at 5000 yr BP and 2000 yr BP, respectively, the rate of migration was 207 ha with a dominant direction southsoutheast (160°), corresponding to a mean lateral accretion rate of 690 m2 yr− 1, which is equivalent to 0.43 m yr− 1 (Fig. 7A). This lateral channel migration gave rise to a progressive river incision through time (see Fig. 6). Accordingly, the most internal part of the Arenal meander (San Eduardo) shows currently the highest

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elevation, particularly the northwest part of the Lope García water mill (Fig. 1) and, therefore, it probably has not been flooded in the last 2000 yr, even during extreme floods. However, Madre Vieja was flooded during extreme floods, and was inundated during the AD 1963 flood. In the period between 2000 and 1000 yr BP, the Guadalquivir river shifted towards the south-southeast (155°) at a lower rate than that of the previous period (480 m2 yr− 1 equivalent to 0.3 m yr− 1; Fig. 7B). After AD 1000, the river incised approximately 2 m and developed the alluvial surface Fp2. In fact the river channel at 1000 yr BP was placed at the boundary between Fp1 (+ 7 to 9 m) and Fp2 (+5m), as is indicated by a gravel deposit (channel facies) containing a rounded gravel-sized pottery TL-dated to 943 ± 84 yr ago (MAD-4003; Fig. 6A). In Fp2 (+5 m) and during the period 1000 to 500 yr BP, the river migrated 114 ha, most of them downstream (westwards at 260°) (Fig. 7C). The change in the lateral migration direction from southwards (during Fp1) to westwards (Fp2) may be related to the valley shape, and particularly with a higher coherence of the concave meander bankline formed by Neogene bedrock. The resistant bedrock on the left bank forced the river channel to migrate westwards (direction changed by 90°) where less resistant alluvial deposits (terrace fill T3; +15–20 m) were located. Subsequently, the mobility rate increased up to 2280 m2 yr− 1 (1.2 m yr− 1), three times higher than in the previous periods (Fig. 7C). Since the 14th Century AD, bank protection by dykes (e.g. San Julian dyke built in 15th Century; Fig. 1) and rip-rap protection works were undertaken to preserve channel location, by preventing lateral erosion of the left bank. These protection works were not effective in preventing bank erosion but slowed down the channel migration rate which decreased to 620 m2 yr− 1 (0.4 m yr− 1) (Fig. 7D). The San Julian dyke was built on the left bank of the Guadalquivir river and was destroyed by the AD 1554 flood (Ramirez de Arellano, 1874). The archaeological investigation carried out by the Cordoba Municipality found this dyke on the internal part of the Arenal meander buried by the lower floodplain surface (+1–2 m). Its elevation provides physical evidence of river incision in the last 500 yr, most probably related to an increase in fluvial activity associated with a higher frequency of damaging high floods during particular decades of the Little Ice Age (e.g. AD 1590–1650, AD 1775–1810 and AD 1850–1890; Fig. 2). Contemporaneous to the west-southwest migration of the Arenal meander, the upstream meander (Cañaveralejo; Fig. 1) shifted northwards, increasing the length and amplitude of the meander belt upstream of Cordoba. The point of inflection between these meanders (the river ford at the Lope García water mill) remained at the same position. In this period (~AD 1000–1500), the meander wavelength decreased from about 1300 m to about 900 m, and sinuosity increased from about 1.38 to 1.53, coinciding with a period of extreme floods in the initial decades of the Little Ice Age. A later channel avulsion affecting the Cañaveralejo meander, which occurred in a period postdating Fp2 (+5 m; most probably the last 500 yr), left a 2 km-long abandoned meander known as Rabanales (Fig. 1). 4.3. Potential location of Medinat Al Zahira Palace

Fig. 7. Upper: Map of subsurface exploration strategy, showing locations of boreholes and trenches. Below: Rose diagrams showing the migration direction, area and rate for different temporal periods.

The geomorphological and stratigraphical analysis provides arguments for reconstructing the palaeogeography and environmental history of the Arenal meander over the last 1000 yr, linking the riverine landscape during historical times with its contemporaneous fluvial infrastructures and human settlement described by historical chronicles to be located by the Guadalquivir River. Cordoba's golden age occurred during the Umayyad Period (AD 929–1013), when the city developed a high degree of cultural life, socio-economic wealth and modernity (Levi-Provençal, 1950). Its wealthy economy favoured the foundation of two cities Medinat al-Zahra and Medinat al-Zahira within a radius of 8 km from Cordoba (Vallejo-Triano, 2001). Chronologically, Medinat al-Zahra (AD 936–1010) was the first to be

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built under the orders of the Umayyad Caliph Abd al-Rahman III. As reported in the 12th century by Al Idrisi (1999) and later sources (see La première géographie de l'Occident, translation by P.A. Jaubert, 1836– 1840), the urbanised area was built on a three-terraced structure complex, the upper one dedicated to the Caliph's palace, the middle one for government buildings and reception halls and the lower one stood for the building of the mosque, simple houses, gardens and markets (Vallejo-Triano, 2001). The city went into decline soon after the Caliph's period and it was finally abandoned by the 15th century. Not until 1910, when the first archaeological excavations took place was Medinat al-Zahra rediscovered. The second city, Medinat alZahira (AD 978–1009) was founded by the Caliph's vizier al-Mansur to hold the government headquarters and Caliph's palace which replaced the former Caliphal center of Medinat al-Zahra (Murillo et al., 1997). The al-Mansur's city was destroyed during the fitna revolution (AD 1009 to 1013), meaning the end of the Caliphal Period in Cordoba and subsequent disappearance of the archaeological traces of its location, which is still unknown (Arjona Castro, 1980). A few significant clues

can be extracted from Arab chronicles (Ibn ‘Idari, in Huici Miranda, 1952). Medinat al-Zahira was located on the right bank of the Guadalquivir river, to the east, a short distance from Cordoba, since the extension of this settlement almost reached Cordoba's suburbs, and within a site known as Ramla or “sandy terrain” (in Spanish arenal). The historical descriptions support the hypothesis that Medinat alZahira is likely to be located on the Arenal meander or its surroundings. The geomorphological and chronological data indicate that the Guadalquivir River at AD 1000 was located at the Cortijo del Arenal chute channel (Fig. 1). The areas located on the left side of this chute can be disregarded as a potential site of Medinat al-Zahira. On the floodplain, the highest surface Fp1 (+7 to 9 m) is the most feasible surface for the settlement's location. The former medieval ground topography for Fp1 can be reconstructed based on the depth of the post-medieval pottery found within the alluvial sediments (Fig. 7). It is evident that Medinat al-Zahira remnants are buried on the alluvial plain, to at least 1 m depth, otherwise the farming would have

Fig. 8. Flooded area at different water stages from 95 m a.s.l. to 102 m a.s.l.

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exposed some at the surface. In the studied stratigraphical profiles in Fp1 (+7 to 9 m), pottery fragments are buried at less than 0.5 m, except in the Madre Vieja and San Eduardo palaeochannels. However, Madre Vieja channel is not the appropriate settlement site since it has been frequently flooded over the last 2000 yr. The existing geomorphological and stratigraphical data point out the northwest sector of the Arenal meander as the most feasible location of Medinat al-Zahira. In this area, the most remarkable geomorphic element is the San Eduardo chute that is infilled at the base with gravels (~ 4000 yr BP in age), which are then overlain by fine flood deposits containing post-medieval pottery buried up to 1.8 m (Fig. 7). In addition, it is the highest elevated sector of the Arenal meander which would be in good agreement with written chronicles describing extreme floods almost reaching Medina al-Zahira. The most catastrophic flood in the Medieval period occurred in winter of AD 992–993, where the “flood level covered the lower part of Cordoba and reached Medinat al-Zahra” (after the Arab reporter Ibn Abi Zar`, in the text Rawd al Qirtas, in Huici Miranda, 1963). In the Arab chronicle by Ibn ‘Idari in December AD 1008, “the Guadalquivir flooded the Ibn Galib orchards, next to al-Zahira, and it almost reached the Cadí (judge) Court, on top of the Zoco Grande, in the lower Cordoba….”. The document relates the flooding of both the orchards nearby Medinat alZahira and the Zoco Grande in the lower part of Cordoba at approximately 95–97 m a.s.l., indicating that al-Zahira was located between 98 and 100 m a.s.l. Note that Medinat al-Zahira may be buried (N1 m) and, therefore, the current ground surface covering the settlement would be located at a higher elevation, likely at 99–101 m a.s.l. The inundated areas for flood stages between 95 and 101 m were delineated using a detailed digital terrain model (Fig. 8). In the case of a flood stage of 100 m a.s.l., the greatest water depths are located at the west sector of the Arenal meander, whereas only the northeastern part of the Arenal meander (north of Lope García water mill) is emerged. The geomorphological data showed that this sector of Fp1 (+7 to 9 m) was partially eroded by the westward migration of the Cañaveralejo meander which probably occurred in the last 1000 yr. In fact the Fp1 alluvial outcrops nearby the Lope García water mill show abundant archaeological artefacts of undetermined age. In addition, the aerial photograph at 1:8000 scale shows different soil colour changes (soil and humidity subsurface variations) following geometric lineaments (Fig. 9).

Fig. 9. Non-flooded area during extreme flooding at the NE sector of the Arenal meander. Note the soil colour changes and geometric lineaments pointed by the arrows.

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5. Discussion and conclusions Channel changes in the Arenal meander during the Holocene can be interpreted as a response to external variables (hydrological and sediment load changes), although at some periods intrinsic variables, mainly related to floodplain shape, geology or chute cut-offs, have influenced the channel migration rate. In last 1000 yr, human environmental modifications have become more important, especially since the 15th century AD when bank protection structures, as well as bridges and water mills, were built on the inflexion points. The contribution of geomorphology, stratigraphy, documentary and archaeological data provides an understanding of changes on the Guadalquivir river dynamics and fluvial landscape as well as on impacts of the human settlement since medieval times. Fluvial dynamics can be interpreted in the context of local and regional environmental and hydroclimatic changes, as follows. (a) The Arenal has been a single meander, shifting during the Holocene by extension and rotation. Through the Middle-Late Holocene, three floodplain surfaces (Fp1 to Fp 3) have been developed, incising its base level by 9 m. The radiocarbon and thermoluminescence dating of the alluvial deposits (Table 1) provided ages of 2300-1637 cal BC, 828 cal BC, 168 cal BC-AD52 and 950 TL yr. Fp1 (+7–9 m) was developed since the Early Holocene (as San Eduardo chute infill predates the alluvial surface) and reached its maximum extension about 2000 yrs ago. Fp2 (+5 m) was formed between 1000 and 500 yrs BP, and Fp3 is presently the most active part of the floodplain (+1–2 m) dating to the last 500 yrs. The meander upstream of the inflexion point (eastern riffle near the Lope Garcia water mill) has remained at a similar position during the Late Holocene. The chronological data are in good agreement with the existing alluvial data of the Iberian Peninsula that shows peaks in flood frequency and alluviation phases at ca. 2750–2150 cal BP and 930–520 cal BP, with the earliest post-Roman date for alluviation at 1352–1170 cal BP (Thorndycraft and Benito, 2006). (b) The floodplain channels (chute channels) are critical in understanding the Holocene fluvial dynamics as they are snapshots of the channel changes that occurred at different periods. Two chute channels (San Eduardo and Madre Vieja) post-date the Fp1 floodplain surface with radiocarbon ages of 2458–2203 cal BC and 168 cal BC–52 cal AD. At AD 1000, the Guadalquivir river was located between Fp1 and Fp2 (Cortijo del Arenal chute). In San Eduardo there is not evidence of flooding over the last 2000 yr, whereas Madre Vieja was frequently flooded during historical flood events, the last one occurred in 1963. (c) The average channel migration rate has changed during the study period: in Fp1 the average channel migration rate was 690 m2 yr− 1 (0.43 m yr− 1) between 5000–2000 yr BP and 480 m2 yr− 1 (0.30 m yr− 1) between 2000-1000 yr BP; in Fp2 2280 m2 yr− 1 (1.26 m yr− 1) between 1000 BP and 500; and in Fp3 620 m2 yr− 1 (0.40 m yr− 1) between 500 BP and the present (Fig. 7). The estimated migration rates may be biased since lateral migration on the meandering belts may reduce the preservation potential of older channels or self-removal of alluvial units by fluvial dynamics (Everitt, 1968; Lewin and Macklin, 2003). River lateral migration coupled with slow incision appears particularly favourable to the preservation of more complete alluvial records (Brackenridge, 1984; Lewin and Macklin, 2003). This seems to be the case of the Arenal meander as the radiocarbon and thermoluminescence datings of coarse gravel deposits (channel or point bar deposits facies) shows consistently younger ages towards the present location of the Guadalquivir River (Table 1), indicating dominant southwards channel migration at least since 5000 yr BP. The relative change in river mobility is also confirmed by other

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fluvial indicators, such as the vertical accretion rate inferred from radiocarbon dating of the overbank deposits. At San Eduardo (Fp1), the vertical sedimentation was estimated at 0.07–0.08 cm yr− 1, in the central part of the meander 0.11– 0.12 cm yr− 1, and in the southern sector of Fp1 (+7 to 9 m) 0.18– 0.20 cm yr− 1. In the last 900 yr (Fp2 and Fp3) the vertical accretion rate reached 0.36 cm yr− 1. Fluvial changes between 5000 to 2000 yr BP seem to be controlled mainly by hydroclimatic controls, probably enhanced by slight settlement impacts during proto-Early Bronze and Late Bronze settlement (post-1650 BC) (Butzer, 2005). The evidence, from pollen records, of human impact on the Spanish landscape indicates that there has been significantly increased deforestation over the last 2000 yr (Allen et al., 1998; Burjachs et al., 1997; Santos et al., 2000) and since 2000– 1700 BP in south-central Spain (Carrión et al., 2001). In the present study, a reduction in the channel shifting rate has been identified, from 690 m2 yr− 1 (ca. 5000–2000 yr BP) to 480 m2 yr− 1 (ca. 2000–1000 yr BP). The Guadalquivir alluvial chronology indicates either that (1) deforestation or its impacts were not significant at the basin scale until late medieval times, or that (2) the hydroclimatic conditions were not optimal in terms of river transport capacity and river mobility until approximately AD 1000. This late medieval alluviation phase accords with available radiocarbon fluvial records in Spain as described by Thorndycraft and Benito (2006) with a cluster from 930 to 520 cal BP (AD 1020–1440), which includes a peak of alluviation at 790–520 BP (AD 1160–1440). There are remarkably few radiocarbon dates in Spanish alluvial records between 2000–1000 yr BP, a period of low river migration rates (480 m2 yr− 1) and relative stability at the Arenal meander. The available geoarchaeological and bioarchaeological data in Spain supports this picture of Roman-era stability, extending until AD 1000 with associated soil formation on floodplains and forest regeneration (Butzer, 2005). A major break in increasing fluvial activity and vertical alluviation rates was found at AD 1000 which coincides with higher flood frequency (Benito et al., 1996, 2003b; Fig. 2) and high available sediment load due to deforestation and expansion of the agricultural land to marginal areas. The environmental disequilibrium and accelerated alluviation in the second half of the Muslim settlement record (~ AD 1100) have been recorded in radiocarbon-dated floodplain sediments in other Spanish rivers (Thorndycraft and Benito, 2006) and in the burial of irrigation devices (Butzer et al., 1985) in response to land-use changes and to periodic clusters of extreme precipitation events until the 1500s (Butzer et al., 1983; Benito et al., 1996; Butzer, 2005). Over the last 500 yr, channel migration and sedimentation rates resulted from the combination of episodic flooding enhanced at particular decades of the Little Ice Age (Fig. 2), and successive bank protection works near Cordoba. The climate changes of the Late Medieval period and early modern period are now considered to have probably been the most dramatic in the Holocene (Rumsby and Macklin, 1996). (d) The meander shift changed direction through time: (1) in Fp1 the channel shifted to the S–SE (N-160°), (2) in Fp2 to the W and SW (N-260° and N-200°), and (3) recently in Fp3 it shifted to the W. Some geomorphologic features must be pointed out to explain the change on shifting direction, namely the stable location of inflexion points, the scarp in cohesive Tertiary clays on the meander's external margin, and the narrowing of the floodplain in Cordoba city confined by alluvial terrace T3. Several authors underline the role of margin resistance in meandering rivers (e.g. Hickin and Nanson 1975, 1984; Thorne 1991; Richard et al., 2005), which can explain the westward change in the orientation shifting over the last 1000 yr. The

recent channel shifting produced bank erosion of non-cohesive Late Pleistocene alluvial deposits of terrace T3 located on the left margin at the Campo de la Verdad (Fig. 1). Since the 15th century, there are on-site archaeological remains (San Julian dyke; Fig. 1) and archive descriptions of protection walls built to prevent bank erosion on the left (south) river margin. Over the last 500 yr the terrace scarp retreat was estimated at 350400 m, which can be related to extreme floods occurring during the 16th–17th and 19th centuries. (e) The area's proximity to Cordoba, capital of the Umayyad alAndalus, provides a unique opportunity to bring together geomorphology, archaeology and history, to obtain insights into the potential location of a lost 10th century settlement, Medinat al Zahira, known to be placed east of Córdoba and near the Guadalquivir river. At the time of the settlement of Medinat al-Zahira (AD 978–1009), the rates of the channel migration were the lowest of the Middle-Late Holocene period, and, according to the available geomorphological data, a period of relative fluvial stability. The geomorphological and stratigraphic data pointed out the NE sector of the meander as the most likely emplacement for Medinat al-Zahira. The Palace complex was probably placed at an elevation of 98–100 m a.s.l. (on Fp1), as inferred from archive descriptions on water stages and flooded areas with references to Cordoba and Medinat AlZahira during the AD 992–993 and AD 1008 extreme floods. In fact, north of Lope García water mill, the aerial photograph shows traces of lineaments which we speculate to correspond to traces of buried walls. This NE sector was eroded by Guadalquivir channel migration at Cañaveralejo over the last 1000 yr, and it is expected that part of the archaeological remnants have already disappeared. Acknowledgements This research was funded by a grant from Proyectos de Córdoba Siglo XXI coordinated by Prof R. José Roldán Cañas (Universidad de Córdoba). The authors thanks the historical comments and archaeological identification of artefacts by Juan F. Murillo Redondo (Gerencia de Urbanismo de Córdoba) and Maudilio Moreno Almenara (Universidad de Córdoba). We are very grateful to Varyl Thorndycraft (Royal Holloway, University of London) for the review of the original manuscript, and to Louise Bracken (Durham University), Adrian Harvey and Andy Platter (University of Liverpool) for the very useful comments and suggestions. References Al Idrisi, 1999. La première géographie de l'Occident (1154-1157). Translation by Jaubert, P.A., 1836-1840. 1999 edition introduced by Bresc, H., Nef, A., Garnier F. Flammarion, Paris. Allen, J.R.M., Huntley, B., Watts, W.A., 1998. The vegetation and climate of northwest Iberia over the last 14,000 years. Journal of Quaternary Science 11, 125–147. Arjona Castro, A., 1980. Andalucía musulmana: estructura político-administrativa. Publicaciones del Monte de Piedad y Caja de Ahorros de Córdoba. Arjona Castro, A., 1982. Anales de Córdoba musulmana: (711-1008). Published by Monte de Piedad y Caja de Ahorros, Córdoba. Ashley, G.M., Southard, J.B., Boothoroyd, J.C., 1982. Deposition of climbing-ripples beds: a flume simulation. Sedimentology 29, 67–79. Baena Escudero, R., 1993. Evolución Cuaternaria de la Depresión del Medio-Bajo Guadalquivir y sus Márgenes. PhD Thesis, University of Seville. Baena Escudero, R., Díaz del Olmo, F., 1994. Cuaternario Aluvial de la Depresión del Guadalquivir: Episodios Geomorfológicos y Cronología Paleomagnética. Geogaceta 15, 102–104. Baena Escudero, R., Díaz del Olmo, F., 1995. Confluencia Genil-Guadalquivir (Córdoba): Cuaternario fluvial y localizaciones del Paleolítico. Geogaceta 18, 97–100. Benito, G., Machado, M.J., Pérez-González, A., 1996. Climate change and flood sensitivity in Spain. In: Branson, J., Brown, A.G., Gregory, K.J. (Eds.), Global continental changes: The context of palaeohydrology. Geological Society Special Publication, vol. 115, pp. 95–98. Benito, G., Sopeña, A., Sánchez-Moya, Y., Machado, M.J., Pérez-Gonzalez, A., 2003a. Palaeoflood record of the Tagus River (Central Spain) during the Late Pleistocene and Holocene. Quaternary Science Reviews 22, 1737–1756. Benito, G., Diez-Herrero, A., de Villalta, M.F., 2003b. Magnitude and frequency of flooding in the Tagus basin (Central Spain) over the last millennium. Climatic Change 58, 171–192.

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