Pools, channel form, and sediment storage in wood-restored streams: Potential effects on downstream reservoirs

    Pools, channel form, and sediment storage in wood-restored streams: Potential effects on downstream reservoirs Arturo Elosegi, Joserra D´ıez, Lorea Flores, Jon Molinero PII: DOI: Reference:

S0169-555X(16)30006-X doi: 10.1016/j.geomorph.2016.01.007 GEOMOR 5488

To appear in:

Geomorphology

Received date: Revised date: Accepted date:

6 October 2015 29 December 2015 12 January 2016

Please cite this article as: Elosegi, Arturo, D´ıez, Joserra, Flores, Lorea, Molinero, Jon, Pools, channel form, and sediment storage in wood-restored streams: Potential effects on downstream reservoirs, Geomorphology (2016), doi: 10.1016/j.geomorph.2016.01.007

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ACCEPTED MANUSCRIPT Pools, channel form, and sediment storage in wood-restored streams:

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Arturo Elosegi , Joserra Díez , Lorea Flores and Jon Molinero

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potential effects on downstream reservoirs

1. Faculty of Science and Technology, University of the Basque Country. PO Box 644, 48080

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Bilbao, Spain

2. University College of Teacher Training, University of the Basque Country. Juan Ibañez de

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Sto. Domingo, 1. 01006 Vitoria-Gasteiz, Spain

3. INRA, UMR 1224, Ecologie Comportementale et Biologie des Populations de Poissons, Aquapôle, quartier Ibarron, 64310 Saint-Pée sur Nivelle, France

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4. Escuela de Gestión Ambiental PUCESE, Eugenio Espejo y subida a Santa Cruz, 080150,

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Ecuador

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*Corresponding author: Arturo Elosegi, Tel.: +34 946015514: E-mail: [email protected].

ACCEPTED MANUSCRIPT Abstract Large wood (LW, or pieces of dead wood longer than 1 m and thicker than 10 cm in diametre) is a key element in forested streams, but its abundance has decreased worldwide as

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a result of snagging and clearing of riparian forests. Therefore, many restoration projects

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introduce LW into stream channels to enhance geomorphology, biotic communities, and

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ecosystem functioning. Because LW enhances the retention of organic matter and sediments, its restoration can reduce siltation in receiving reservoirs, although so far little information on this subject is available. We studied the effects of restoring the natural loading of LW in four

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streams in the Aiako Harria Natural Park (the Basque Country, Spain) in pool abundance, channel form, and storage of organic matter and sediments. In all reaches log jams induced the

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formation of new geomorphic features and changes in physical habitat, especially an increase in the number and size of pools and in the formation of gravel bars and organic deposits. The

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storage of organic matter increased 5- to 88-fold and streambed level rose 7 ± 4 to 21 ± 4 cm on 3

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average, resulting in the storage of 35.2 ± 19.7 to 711 ± 375 m (733-1,400 m ha y ) of

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sediment per reach. Extrapolation of these results to the entire drainage basin suggests that

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basinwide restoration of LW loading would enhance the retention potential of stream channels 3

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by 66,817 ± 27,804 m (1075 m ha y ) of sediment and by 361 tonnes (5.32 T ha y ) of organic matter, which represents 60% of the estimated annual inputs of sediments to the

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downstream Añarbe Reservoir and almost twice as much as the annual input of organic matter to the entire river network. Therefore, basinwide restoration of LW loading is a potentially important tool to manage catchments that feed reservoirs, where retention of sediments and organic matter can be considered important ecosystem services as they reduce reservoir siltation.

Keywords: wood, channel accretion, stream, reservoir siltation, ecosystem service

ACCEPTED MANUSCRIPT 1. Introduction Large wood (LW) is a key element in forested stream channels. It influences the amount, type, and distribution of bed sediments (Keller and Swanson, 1979; Harmon et al.,

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1986; Lisle, 1995), enhances channel roughness (Manga and Kirchner, 2000), promotes pool

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formation (Richmond and Fausch, 1995; Abbe and Montgomery, 1996; Thompson, 2012), and

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shapes channel form (Nakamura and Swanson, 1992; Piégay and Gurnell, 1997; Collins et al., 2012). It also enhances habitat complexity and dynamics (Wohl, 2011) and affects the ecology of stream and river ecosystems (Gurnell, 2012). Additionally, LW promotes the retention of

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inorganic and organic sediments (Pretty and Dobson, 2004; Koljonen et al., 2012; Beckman and Wohl, 2014), and can thus exert a strong influence on stream ecosystem functioning (Flores et

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al., 2011; Elosegi and Sabater, 2013). Although the effects of LW depend on the disposition and size of the pieces and on stream size and slope (Díez et al., 2001; Klaar et al., 2009), in general

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river channels with abundant LW are more complex and store more sediments than wood-

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depleted rivers and streams (Montgomery et al., 2003; Sass, 2010). The abundance of LW in stream and river channels has decreased worldwide as a

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result of snagging and clearing of riparian forests, thus causing changes in channel form and ecosystem functioning (Gregory et al., 2003). Today, LW is increasingly used in restoration projects to improve the hydromorphological and ecological status of streams (Gregory et al.,

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2003; Kail et al., 2007). Improving fish habitat is the main objective in many restoration projects (Roni et al., 2002; Nagayama and Nakamura, 2010), but other goals are also included, such as initiating natural channel dynamics (Kail et al., 2007), bank protection (Shields et al., 2004), enhancing invertebrate habitat (Hilderbrand et al., 1997), increasing physical complexity (Gerhard and Reich, 2000), and promoting stream ecosystem functioning (Hoellein et al., 2011; Elosegi et al., 2016). More recently, some projects have included sediment retention or streambed aggradation among their objectives (Kail et al., 2007), which could have important consequences in downstream ecosystems. More specifically, storage of sediments and organic matter in stream channels could reduce siltation in receiving reservoirs, which could be considered an important ecosystem service. In this paper we study the stability of LW introduced into four headwater streams that drain into a reservoir and its effects on channel

ACCEPTED MANUSCRIPT form and on the type and amount of sediments stored. We extrapolate our results to the entire catchment to calculate the potential effects of restoration of instream LW on reservoir siltation. Our hypotheses are that (1) the stability of introduced LW structures will be inversely related to

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stream size; introduction of LW will (2) trigger changes in channel morphology and (3) enhance

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the storage of organic matter and inorganic sediments; (4) the effects will be largest in small

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streams where LW will be more stable; and (5) addition of LW could significantly reduce the inputs of sediments and organic matter into a downstream reservoir.

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2. Methods

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2.1. Study area

The study was performed in the Aiako Harria Natural Park (Basque Country, north Spain), a rugged area over granite and schist lithology, with mountains ca. 1000 m high very

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close to the sea and extensive beech (Fagus sylvatica L.) and oak (Quercus robur L.) forests.

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The local climate is humid temperate with annual rainfall over 2500 mm, evenly distributed during the year. Streams in the area are flashy as a consequence of the rainy climate, steep

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topography, and impervious substrate. Stream channels are constrained by the steep slope of the surrounding land, but they are nevertheless very dynamic as a consequence of the relatively

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small sediment grain size, the large lateral inputs of sediments, and the torrential rainfall regime. This dynamism results in extensive movement of bed sediments during floods (personal observation). No major differences in channel planform or stream type can be seen in the area, apart from those associated with stream size. 3

A point of interest in the Natural Park is Añarbe, a 44-Hm reservoir built in 1976 that 2

drains a basin of 64 km and supplies drinking water to over 300,000 persons, roughly half the population

in

the

province

of

Guipuscoa.

According

to

the

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government

(http://www4.gipuzkoa.net), a continuous gauging station, located about 1 km upstream from 3 -1

the reservoir, registers an average discharge of over 2 m s , monthly averages ranging from 3 -1

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0.76 m s in September to 4.15 m s in February. Peak discharges only exceeded 40 m s on two occasions since the station started operating in 1999. Although the quality of the water stored in the reservoir is excellent, a point of concern are the large inputs of leaf litter and

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sediments to the reservoir, estimated at 123,000 m y (1,922 m km y , 0.28% of the reservoir capacity; CEDEX, 2005). These inputs seem to result partially from the low physical complexity of streams channels, caused by a very low standing stock of instream LW, a legacy of intensive

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snagging and harvesting of timber for fuel and charcoal until the 1960s. The streams were too

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narrow to float logs, and thus other actions to reduce channel complexity (such as removing

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boulders or rocks) were probably rare. Currently, LW recruitment is reduced by the young age of many forest stands, which burnt in 1956, and by the fact that many stands were pollarded in the past, resulting in short, thick trees that will hardly topple down into the stream. Therefore, as

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part of a project to enhance the conservation value of ecosystems in the Natural Park, a pilot action, based on restoring the loading of LW, was carried out in four streams flowing into the

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reservoir.

The four streams restored included one first-order stream (Atseginsoro), two second-

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order streams (Malbazar and Latxe), and a fourth-order stream (Añarbe) and are, in general, representative of the type of streams in the basin. They range from 125 to 5190 ha in drainage

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area, from 3.5 to 13 m in channel width, from 0.25 to 5% in channel slope, and from 26 to 2500 -1

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L s in average discharge (Table 1). From a geomorphic point of view, these are gravel-cobble bed, single-channel, riffle-pool streams, although runs dominate large parts of the reach studied in Añarbe Stream. The restored reaches were 110-115 m long in the three smallest streams

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and 400 m long in the largest one. They were located 200-700 m upstream from the Añarbe reservoir, surrounded by public land; the absence of nearby infrastructures or populated areas minimized the risks of such a restoration project.

2.2. Restoration works The restoration consisted of introducing logs and branches into stream channels, mimicking the type of structures commonly found in streams in the region (Elosegi et al., 1999; Díez et al., 2001; Figs. 1-4). These included isolated logs in the channel, dams (logs or groups of logs spanning the entire stream channel), deflectors (diagonal logs blocking partially the channel), and V-shaped structures (double-side deflectors pointing downstream; Kail et al., 2007). The legacy of past activities in most of Europe makes it difficult to define the amount of

ACCEPTED MANUSCRIPT LW expected in natural reference streams (Kail, 2003). Therefore, we used a regression between channel size and LW abundance published by Bailey et al. (2008) in New Zealand streams under similar climate, topography, and vegetation as a rough guideline of the

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abundance of LW to be expected at our sites. In fact, the amount of LW in Olin, one of the least-

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managed streams in the Añarbe basin, almost exactly fit in the mentioned regression 3

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(unpublished data). Thus the volume of LW added ranged from 33 to 239 m ha (Table 1), and the number of structures created varied from 8 to 12 per reach. The logs were introduced manually by handheld motor winches into the channels in January 2008 and positioned forming

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perpendicular dams, deflectors, and isolated logs (Table 1). The lack of heavy machinery precluded moving very large logs. Therefore, the average log length was slightly larger than the

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channel width and, although some logs exceeded channel width in 4-5 m, others were shorter (Table 1). In contrast with other restoration works, no cables or other artificial devices were used to fix LW in place, so the logs were free to move with changes in discharge. Only three logs

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from three small ones.

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were cabled to each other, but neither to the banks or substrate, to create a large composite log

2.3. Field survey and streambed changes The streams were surveyed in the summers of 2006 and 2007, before adding LW to the

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channel (January 2008); in February 2008, immediately after restoration; and in the summers of 2008 and 2009, 6 and 18 months after the restoration, respectively. A network of reference points was established in both margins of the stream and used to define transverse and diagonal cross sections. Each cross section was equipped with a rope marked every 0.5 m (Atseginsoro, Malbazar, and Latxe) or every 1 m (Añarbe) locating the points at which the distance between the streambed and the rope was measured with a topographic rod. Given the limitations for using GPS or visual positioning equipment, a repeated survey of the same set of points seemed the best option to maximize the detection of temporal changes and to minimize the interferences caused by the irregular form of the streambed. At each mark we recorded streambed elevation and bed substrate composition following Díez et al. (2000): bedrock, coarse (boulder-rock > 25.6 cm), cobble (6.4-25.6 cm), gravel (0.2-6.4 cm), fine (sand-silt, < 0.2 cm), organic matter, roots, and LW. In addition, we measured habitat characteristics of the

ACCEPTED MANUSCRIPT reaches, including wetted perimeter, pool surface area, and pool depth. We focused on pools, paying less attention to other habitats (such as runs and riffles) because most of the reaches consisted of very shallow areas where flow characteristics were extremely variable with

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discharge and hence were difficult to assign to riffle or runs. Field survey data were processed

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with QGIS (Quantum GIS Project, 2010). Changes in the study reaches were further

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documented with photographs and handmade sketches of channel features on various occasions along the study period.

The survey data were used to construct digital elevation models (DEM) of the stream

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reaches. Survey data were interpolated by ordinary kriging with GSTAT (Pebesma, 1999) and the interpolated data converted to raster maps with a cell size of 0.5 m (Latxe, Atseginsoro,

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Malbazar) and 1.0 m (Añarbe). Changes in streambed elevation were determined by digital models of difference (DOD) following Fuller et al. (2003). Estimates of changes in sediment

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storage were calculated from the DODs. Following Webster and Oliver (2007), we performed cross-validations of all the generated DEMs and DODs (SM1). All the spatial interpolations were

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performed with GSTAT (Pebesma, 1999), and the resulting raster maps were stored and

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processed with PCRaster (van Deursen et al., 2010). Changes in sediment storage in cubic meters at each cell i of a raster (ΔVi) were

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calculated as:

∆Vi = (h2 - h1) A

where (h2 – h1) is the difference in bed elevation between two DEMs DOD value at the i 2

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cell (m), and A is cell area (0.25 m in Atseginsoro, Malbazar, and Latxe and 1 m in Añarbe). The total change in sediment storage was calculated as the sum of sediment storage in all the cells of a study reach.

2.4. Wood and benthic organic matter Coarse particulate organic matter (CPOM) was measured on one occasion before (July 2007) and on two occasions after LW introduction in early summer (July 2008 and July 2009). The CPOM in the entire channel (wetted and dry) was measured by collecting 10 random replicates with a Surber net (30 x 30 cm, 1-mm mesh size). The CPOM collected was washed

ACCEPTED MANUSCRIPT on a tray in the field and frozen in the laboratory (-20ºC). Samples were later thawed, dried (70ºC, 72 h), and ashed (500ºC, 8 h); and CPOM was expressed as ash-free dry mass (AFDM, -2

g m ). Flores et al. (2011) provided a detailed description of CPOM dynamics at the studied

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sites, so in this work we use these data to calculate organic matter retention over the stream

2.5. Statistical analyses and data extrapolation

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network and to obtain better estimates of inorganic sediment retention.

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The relationship between LW stability and stream size was assessed by computing the correlation between the proportion of logs drifting and mean annual discharge. Changes in the

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composition of the streambed and in the storage of COPM along time were compared by oneway (time as factor) and two-way (time and stream as factors) ANOVAs. Post hoc multiple comparisons were done with Tukey’s HSD test. All statistical analyses were performed with R

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(R Development Core Team, 2011) following Zar (1999).

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Finally, to put the pilot project in a wider context, the retention of sediments and organic matter measured in the restored reaches was extrapolated to calculate potential retention in the

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entire drainage basin. First, the length of the drainage network was measured from digital maps, stream sections classified according to their order, and the length of streams per order

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calculated. Second, the surface of channel per stream order was calculated multiplying stream length by the width of channels we measured for each stream order. Then, the changes in elevation measured in our reaches were extrapolated to the entire network (correcting by stream order), and so the total volume of sediments potentially stored was calculated and compared to the inputs of sediments estimated into the Añarbe Reservoir (CEDEX, 2005). For organic matter, instead of working in volume units, we first calculated the mass of CPOM stored in our reaches, then extrapolated it to the entire drainage network correcting for stream order, and the relevance of this amount was calculated by comparing it with the estimated inputs of CPOM to the streams. To estimate inputs we took the values for vertical (611 g CPOM m -2

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and lateral (149 g CPOM m y ) input rates measured by Pozo et al. (1997) in a Basque stream with similar vegetation and physiography and extrapolated them to the Añarbe drainage basin, taking into account the length and surface of channels per stream order.

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3. Results Years 2008 and 2009 could be considered hydrologically normal, with peak discharges -1

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of 19.4 and 34.8 m s , respectively (SM2). The stability of LW decreased with stream size, the 2

proportion of logs drifting being significantly correlated to mean annual discharge (r = 0.969; p

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< 0.005). In the smallest Atseginsoro Stream only a few logs were transported, and only a few meters; whereas in the largest Añarbe Stream most of the logs drifted downstream forming a massive jam in the downstream end of the reach (Figs. 1-4). In addition to movements occurring

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to the LW introduced by us, some LW entered the study reaches by upstream recruitment or by

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treefall.

As a consequence of the steep valley slopes, which laterally constrain stream channels, only minor changes occurred in channel planform (Figs. 1-4). The width of the active channel

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remained unaltered, except for the middle section of Malbazar reach, where the active channel

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expanded by a couple of metres as a consequence of the increased water level elevation produced by a log jam (Fig. 2). Therefore, a long-abandoned lateral terrace became part of the

flows.

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active channel; and a tree, which was formerly in one bank, formed a small island during high

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Despite the limited changes in planform, introduction of LW triggered important geomorphic changes, mainly an increase in the number and size of pools, and the formation of sediment bars. From 2006 to 2009, the average depth increased from 4.5 to 4.9 cm in Atseginsoro Stream, from 5.5 to 10.9 cm in Malbazar Stream and from 5.6 to 7.6 cm in Latxe Stream. The surveys from Añarbe Stream, cannot be compared as they occurred during different discharges. The number, size, and cover of pools also increased in all restored reaches (Table 2). Before restoration, pools averaged 2.7 pools/100 m in the study reaches, covering on average 11% of the reach surface; and right after the restoration (2008), these numbers increased to 3.9 pools/100 m and 27%, respectively. Afterward, the cover of pools kept increasing in Atseginsoro Stream but decreased in the rest of the reaches as they became filled with sediments or some LW structures failed (SM3). The surface of pools per unit of LW volume added decreased with stream size, as shown by the marginally significant negative

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correlation between increased pool surface and average discharge (r = 0.89, p = 0.057). The most extensive sediment bars were upstream from a large log jam in Añarbe Stream (Fig. 4) and upstream from an oak tree fallen in Atseginsoro Stream (Fig. 1); but smaller bars were

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formed upstream from most stable LW structures, as well as below some of the plunge pools

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created by LW dams. By the end of the study, bars formed after LW addition covered 18.8% of

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the channel in Atseginsoro Stream, 53.8% in Malbazar Stream, 19.8% in Latxe Stream, and 20% in Añarbe Stream.

Organic matter initially covered < 10% of the stream surface (Fig. 5). After LW

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introduction, the percentage of stream surface covered with organic substrates increased to 30% in Atseginsoro, to 40% in Malbazar, and to 20% in Latxe. Differences in organic cover

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were statistically significant between 2007 and 2009 (One-way ANOVA with time as factor, F3,12: 5.011, p = 0.018). In the case of inorganic sediments, although sediments tended to be finer right after LW introduction, differences were not statistically significant (One-way ANOVA with

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time as factor, F3,12: 1.143, p = 0.37). The addition of LW initially caused an increase in the

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cover of gravel in the bed surface, but this effect was temporary and the contribution of the

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various particle sizes to the streambed in 2009 was similar to that observed before LW introduction (Fig. 5). The amount of CPOM stored in stream channels also increased -2

significantly, from the initial range of 9.3 to 631.9 gm . Two years after the restoration, CPOM

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increased 7-fold in Atseginsoro and Malbazar, 88-fold in Latxe, and 5-fold in Añarbe (Table 3). Contrary to our hypothesis, although LW addition promoted CPOM storage, the effect did not depend on stream size (2-way ANOVA on log-transformed data with time and stream as factors, F1,152: 7.59, p < 0.01; F3,152: 10.12, p < 0.01; F3,152: 0.57, p = 0.63). All these changes in organic and inorganic sediments produced an elevation of the streambed. Prior to LW introduction (2006-2007), changes in streambed elevation were small and suggested a tendency to bed scouring in Malbazar and Latxe and little changes in Atseginsoro and Añarbe. However, in the first half year after restoration, bed elevation increased notably, especially in the smallest streams, Atseginsoro and Malbazar (SM4-7). This increase continued in the second year (2009) in all restored reaches except Malbazar, and by the end of the experiment bed elevation had increased between 7 ± 4 cm in Malbazar and 21 ± 4 cm in Atseginsoro (Table 4). These changes in streambed elevation caused by the addition of

ACCEPTED MANUSCRIPT LW were not related to stream size or slope (not shown) but were highly correlated with the bed changes observed during the design of the restoration project (Fig. 6), suggesting that LW produced the highest retention of sediments in those reaches that were initially most dynamic.

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This relationship was observed in the net elevation change, but also when areas of bed filling

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and bed scouring were considered separately. Bed elevation changes in Añarbe fit within the

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tendencies observed in the smaller streams only if the large plunge pool formed at the lower end of the study reach was not included in the calculations. As a consequence, the restored 3

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reaches retained 35.2 ± 19.7 to 711 ± 375 m (733-1400 m ha y ) of sediment between 2007 3

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and 2009, which amounts to 9.7-51.8 m of sediment/m of LW added. The efficiency of LW at 3

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retaining sediments ranged from 3.4 ± 1.9 to 52 ± 27 m of sediment stored by m of LW added, 2

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and the correlation between mean annual discharge and storage efficiency was positive (r = 0.979; p < 0.05).

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To put these data in a wider context, we calculated the retention potential if LW was added to the entire drainage network. The river network draining to the Añarbe Reservoir totals

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104 km of stream channels, of which 49% are first-order, 24% second-order, third-order, and

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17% fourth-order, with a total channel surface of about 45 ha (Table 5). Given that the bed elevation at Atseginsoro, our only first-order site, rose 21 cm, which would amount to a potential 3

storage of over 21,420 ± 4,080 m of sediments among the 51 km of first-order reaches (1394 3

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m ha y ). Similarly, second-order reaches have a retention potential of almost 9604 ± 410 m 3

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(654.2 m ha y ) third-order reaches 5184 ± 3240 (482.4 m ha y ), and fourth-order reaches 3

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30,609 ± 16,110 (1141 m ha y ), totalling 66,817 ± 27,804 m of sediment. The capacity to store OM, on the other hand, would amount to 110, 147, 57, and 46 tonnes, respectively, for -1 -1

streams of order 1 to 4, totalling 361 tonnes (5.32 Tha y ) of organic matter basinwide. These totals correspond to 59.1% of the estimated annual inputs of sediment into the reservoir and to 189% of the estimated annual inputs of organic matter into stream channels (Table 5).

4. Discussion The movement of LW pieces in streams is more complex than the movement of inorganic sediments (Braudrick and Grant, 2000). Size and density of LW, the parts of it that are

ACCEPTED MANUSCRIPT outside of the stream channel, log orientation, and the presence or absence of root wads are important factors to determine piece stability, as shown by field (Díez et al., 2001) and flume (Davidson and Eaton, 2012; Shields and Alonso, 2012) studies. In addition to the characteristics

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of LW pieces, channel size and form and hydrology are important factors controlling the

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transport distance of logs (Ehrman and Lamberti, 1992; Moulin and Piégay, 2004; Manners and

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Doyle, 2008). In our experiment, because all streams were of the same geomorphic type and subject to the same hydrologic regime, stream size was expected to determine LW stability. Results corroborated this hypothesis as the proportion of LW displaced was significantly

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correlated to average discharge, which in such a homogenous setting is a function of catchment area (i.e., stream size). Most of the branches and logs that moved in our experiment drifted for

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short distances and were stopped by other introduced LW structures, thus suggesting that restoring LW loading can reduce the travel distance of logs. The movement of LW is also affected by channel slope (Cadol and Wohl, 2010), but the low number of our sampling reaches

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precluded analysing this and other important factors.

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It has been shown that LW is a key factor structuring stream channels, especially in lowland alluvial streams and rivers (Kail, 2003) where it controls channel migration and even

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island formation (Gurnell et al., 2005). Mountain streams tend to be constrained by the steep valley slopes, and thus LW there usually has a smaller effect on channel planform, although massive trees can accumulate and form erosion-resistant hard-points that evolve to complex

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river landscapes (Collins et al., 2012), even converting single-channel rivers into multithread channels (Polvi and Wohl, 2013). Regardless of the stream type, LW increases water stage (Collins and Montgomery, 2002) and promotes the retention of fine sediments, enhancing facies diversity (Davidson and Eaton, 2012). As hypothesized, LW introduction triggered important changes in channel form. The main changes consisted of an increase in water depth, retention of fine sediments and organic matter, and a general increase in the diversity of sediment types. These geomorphic changes can have important biological consequences, as biodiversity is tightly linked to the physical complexity of river channels (Gurnell et al., 2005), and many invertebrate species have very specific substrate demands (Imbert et al., 2005). In our reaches, trout (Salmo trutta L.) used newly formed gravel bars for spawning and log jams for refuge (Antón et al., 2011), thus confirming the finding of others (Roni and Quinn, 2001; Floyd et al.,

ACCEPTED MANUSCRIPT 2009; Nagayama and Nakamura, 2010). Another feature of biological importance could be the bank scouring associated with some LW structures, which can be used for some birds such as kingfishers (Alcedo atthis L.) for building their nesting burrows (personal observation).

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The most immediate change produced in our stream channels by LW addition was an

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increase in the number and cover of pools. The abundance of LW tends to be positively

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correlated with pool frequency (Montgomery et al., 1995) and log size with pool size (Keller and Swanson, 1979; Bilby and Ward, 1991), although some authors (Carlson et al., 1990; Inoue and Nakano, 1998) failed to find such a relationship in mountain streams. In our case, although the

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pools existing before restoration were not linked to LW, over 90% of new pools created were linked to LW added. As hypothesized, the effects were strongest in the smallest stream where

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LW was most stable.

Like some classic studies (e.g., Bilby and Likens, 1980) and following our expectations,

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our results showed a strong influence of LW on the channel area covered by organic substrate, especially in the smaller streams, as a result of the accumulation of organic debris in log jams.

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The response was fast during the first autumn, contrasting with the literature where commonly

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more than 2 years were necessary to detect significant increases in CPOM storage (Speaker et al., 1994; Negishi and Richardson, 2003; Lepori et al., 2005). The rapid accumulation of organic matter after LW introduction observed in this project is likely a consequence of the combined

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use of logs and branches, which result in very complex jams with a strong hydraulic control (Gerhard and Reich, 2000), as well as to the flashy regime and dynamic character of our streams. We must stress that the branches we introduced cannot account but for a minute fraction of the increase in organic matter measured and that most of the stored organic matter corresponded to riparian inputs of leaves and twigs, not to eroded sediment deposits. What is more interesting, although CPOM formed deposits up to 1 m thick, most of them did not become anoxic, suggesting good conditions for breakdown of organic material. Indeed, Flores et al. (2013) showed that alder (Alnus glutinosa (L.) Gaertner) leaf bags decomposed as fast inside these thick deposits as on the bottom of riffles and much faster than inside gravel bars. The breakdown rate of alder leaves cannot be extrapolated to the leaves of other species much less to other organic materials such as fruits or sticks, which can be more recalcitrant (Arroita et al., 2012). Nevertheless, the fact that the storage of CPOM increased so much as a consequence

ACCEPTED MANUSCRIPT of LW addition, that the main storage site was thick organic deposits, and that leaves there decomposed at a fast rate suggest that breakdown greatly increased at the reach scale. The storage of sediments (organic plus inorganic) increased in our reaches as a

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consequence of LW restoration, as was hypothesized. Several papers have previously shown

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LW removal to trigger sediment loss (Beschta, 1979; Smock et al., 1989, Smith et al., 1993),

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that in the case of Basque streams similar to those of Añarbe can result in 19 cm of incision in one year (Díez et al., 2000). The fact that introduced LW structures stored sediments at a similar rate suggests a similar delivery of sediments across Basque mountain streams. The 3

-1 -1

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sediment retention rate measured in our experiment (733-1400 m ha y ) is 2-3 times higher than that measured by Brooks et al. (2004) in a lowland Australian river, thus showing the

MA

strong dynamism of our streams. We must be stress that the retention efficiency of the 3

3

structures we built ranged from 3.4 to 51.8 m of sediment retained/m of LW added, which

D

compares and even surpasses the efficiencies reported in the literature (Megahan, 1982; Brooks et al., 2004; Davidson and Eaton, 2012). Sediment retention is, of course, context-

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dependent in the sense that the uppermost logs introduced in a basin with very low LW loading

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are likely to trap much more sediment than LW farther downstream in the same basin or LW introduced into basins where the loading of LW is already higher. Overall, our experiment enhanced the storage and breakdown of organic matter as well

AC

as the storage of inorganic sediments. Because LW introduction was limited to 700 m of stream channels, the project could not prevent the siltation of the Añarbe reservoir, but our calculations suggest that enhancing the load of LW in the entire basin could reduce the inputs of sediments and especially those of organic matter, thus confirming our hypothesis for the latter variable. We are well aware of the simplicity of our calculations and that many factors beyond stream order affect the role of LW (Cadol and Wohl, 2010), as evidenced for instance by the differences between Malbazar and Latxe despite the fact that both reaches belong to the same order. Nevertheless, as mentioned above, we consider streams in the Añarbe basin to be fairly homogenous in type and thus consider our estimates to be reasonably accurate. Sediment inputs reduce the life expectancy of reservoirs and degrade water quality (Detering and Schuettrumpf, 2014; Wang and Kondolf, 2014), and thus retention of sediments and organic matter in the stream network will benefit the long-term functioning of these key infrastructures.

ACCEPTED MANUSCRIPT With the data available, forecasting the long-term effects of such a restoration on the amount and quality of water stored in the Añarbe Reservoir would be highly speculative. Among other important variables, we lack information on the relative contribution of organic versus inorganic

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materials to the annual sediment inputs. The larger the contribution of organic material, the

IP

more effective will instream retention be, as leaves and twigs (the main components of CPOM in

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our streams, Pozo et al., 1997) are readily decomposed in the Añarbe stream channels (Flores et al., 2013) and thus would likely strongly reduce their inputs into the reservoir. On the other hand, LW can retain and temporarily store inorganic sediments; but provided that their delivery

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rate in the basin is not greatly reduced, they would eventually reach the reservoir. The Añarbe Reservoir is considered to be in good ecological potential, although oxygen saturation in the

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bottom can drop to values close to 30% (Fraile, 2011). Anoxia, which can become an important problem in reservoirs, is promoted by increased concentration of OM and by reduced volume of hypolimnetic water, i.e., by sedimentation (Beutel, 2003), Sustainable reservoir management

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should aim at reducing the impact of current reservoir exploitation to future generations by

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maintaining their water storage capacity (Palmieri et al., 2003; Kondolf et al., 2014). Our results highlight the potentially important effect of restoring instream LW in catchments feeding

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reservoirs, where the retention of organic matter and sediments can be seen as important ecosystem services and where the restoration of LW can be beneficial not only in biophysical

AC

but also in economical terms (Acuña et al., 2013).

Acknowledgements The authors wish to acknowledge the assistance of the students of the University of the Basque Country that helped in field work. This paper was supported by the Project ‘Complextream: effects of channel complexity on stream communities and ecosystem functioning’, funded by the Spanish Ministry of Science and Innovation (project CGL2007-65176/ HID). Lorea Flores enjoyed a predoctoral grant by the Spanish Ministry of Education and Science. Jon Molinero is currently a PROMETEO researcher for the Secretaría de Educación Superior, Ciencia, Tecnología e Innovación (SENESCYT) of Ecuador. We appreciate the constructive comments of two unknown reviewers and of the Geomorphology editing team, which greatly improved the manuscript.

ACCEPTED MANUSCRIPT

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Table 1

Channel slope (%)

Average discharge (m3s-1)

Reach length (m)

Logs before restoration (#)

Atseginsoro

1

125

3.5

5.00

0.026

110

8

Malbazar

2

157

4.5

3.50

0.075

115

4

Latxe

2

233

3.5

4.00

0.110

115

Añarbe

4

5190

13.0

0.25

2.500

400

Logs added (#)

LW volume added (m3ha-1)

Lw length (m, mean, range)

81

216

5.3 (1-8)

74

239

5.4 (2-9)

LW structure s created (#) 12

Dams created (#)

Deflectors created (#)

Isolated logs introduced (#)

V-shape structures created (#)

9

3

0

0

8

4

1

2

1

2

53

144

5.5 (3-7)

11

8

2

0

1

4

72

33

13.6 (7-18)

11

4

4

1

2

PT ED

Channel width (m)

NU

Drained area (ha)

MA

Order

AC CE

Stream

SC RI

Main characteristics of the study sites

ACCEPTED MANUSCRIPT Table 2 Number of pools/100 m of channel length and areal cover at the study sites before LW introduction (July 2007), just following LW introduction (February 2008), and a year and a half

July 2009

Number of

% Cover

Number of

% Cover

Number of

% Cover

pools/100 m

of pools

pools/100 m

of pools

pools/100 m

of pools

3.6

6.5

3.6

Malbazar Latxe

2.6

9.4

5.2

4.3

22.3

6.0

Añarbe

0.5

5.5

1.0

AC

CE P

TE

D

MA

NU

Atseginsoro

SC R

Stream

February 2008

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July 2007

T

later (July 2009)

11.6

8.1

18.8

27.9

5.2

15.1

62.0

3.5

27.8

7.5

0.7

4.7

ACCEPTED MANUSCRIPT

Table 3 -2

Average amount of organic matter stored (g m ± S.E) in the four streams before and after LW

After

Summer 2007

Summer 2009

Atseginsoro

279.4 ± 88.4

1832.3 ± 579.4

Malbazar

524.5 ± 165.9

3418.7 ± 1081.1

9.3 ±

2.9

1985.0 ± 627.7 46.1 ±

14.6

TE

D

MA

NU

7.1

CE P

Añarbe

22.6 ±

AC

Latxe

SC R

Before

IP

T

introduction

RI PT

ACCEPTED MANUSCRIPT

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Table 4 3

Changes in average stream bed elevation (Δh, m) and sediment storage (ΔV, m ) at the study sites; errors in the estimates of bed elevation changes and

Δh

Δh

ΔV 3

(m)

(m)

(m)

(m )

2006-2007

2007-2008

2008-2009

2007-2009

2006-2007

Atseginsoro

0.04 ± 0.03

0.12 ± 0.04

0.09 ± 0.04

0.21 ± 0.04

Malbazar

-0.12 ± 0.03

0.19 ± 0.04

-0.12 ± 0.04

0.07 ± 0.04

Latxe

-0.03 ± 0.03

0.10 ± 0.04

0.06 ± 0.03

0.15 ± 0.05

Añarbe

0.02 ± 0.04

0.09 ± 0.04

0.08 ± 0.04

0.17 ± 0.09

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(m)

ΔV

3

ΔV 3

ΔV 3

Storage efficiency 3

3

(m )

(m )

(m )

(m sed/m LW)

2007-2008

2008-2009

2007-2009

2007-2009

16.2 ± 10.9

46.3 ± 13.5

34.8 ± 14.0

81.1 ± 14.3

9.7 ± 1.7

- 56.3 ± 14.7

92.5 ± 17.8

-57.3 ± 17.3

35.2 ± 19.7

3.4 ± 1.9

- 12.3 ± 14.4

40.1 ± 18.0

23.4 ± 12.5

63.5 ± 20.2

10.6 ± 3.4

105.2 ± 156.9 391.9 ± 160.9

319.1 ± 177.2

711.0 ± 375.2

51.8 ± 27.3

EP

AC C

Site

Δh

MA

Δh

NU

sediment storage were calculated as the halved interquartile range of the interpolation errors obtained by cross-validation

AC C

EP

TE D

MA

NU

SC

RI PT

ACCEPTED MANUSCRIPT

SC RI

PT

ACCEPTED MANUSCRIPT

Table 5

Extrapolation of the volume of sediment and mass of CPOM stored in the study reaches to the entire drainage network, and comparison with vertical and

Channel length (m)

Channel width (m)

Channel 2

area (m )

Sediment

Potential sediment 3

2

stored (m /m )

MA

Order

NU

lateral CPOM inputs calculated from data published by Pozo et al. (1997) in similar streams

3

storage (m )

CPOM

Potential

Vertical CPOM

Lateral CPOM

retention

CPOM

inputs (T)

inputs (T)

2

retention (T)

1078

110.4

62.6

15.3

51,219

2

102,438

0.21 ± 0.04

2

24,466

4

97,865

0.11 ± 0.04

9604

± 4010

1497

146.5

44.9

10.9

3

10,235

7

71,648

0.14 ± 0.04

5184

± 3240

800

57.3

18.8

4.6

4

17,880

10

178,797

30,609 ± 16110

259

46.3

27.3

6.7

360.5

153.6

37.5

103,800

450,748

AC CE

Total

PT ED

1

0.17 ± 0.09

21,420 ± 4080

(g/m )

66,817 ±

27,840

ACCEPTED MANUSCRIPT Figure legends: Fig. 1. Changes in physical habitat as a result of LW introduction in Atseginsoro stream (reach

T

length: 110 m). LW, deposits of inorganic and organic material, and large rocks are shown

IP

before LW introduction (July 2007), directly after LW introduction (February 2008), and a year

SC R

and a half later (July 2009). Water flow from top to bottom as indicated by an arrow.

Fig. 2. Changes in physical habitat as a result of LW introduction in Malbazar stream (reach

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length: 115 m). LW, deposits of inorganic and organic material, and large rocks are shown before LW introduction (July 2007), directly after LW introduction (February 2008), and a year

MA

and a half later (July 2009). Water flow from left to right as indicated by an arrow.

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Fig. 3. Changes in physical habitat as a result of LW introduction in Latxe stream (reach length:

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115 m). LW, deposits of inorganic and organic material, and large rocks are shown before LW introduction (July 2007), directly after LW introduction (February 2008), and a year and a half

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later (July 2009). Water flow from top to bottom as indicated by an arrow.

Fig. 4. Changes in physical habitat as a result of LW introduction in Añarbe stream (reach

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length: 400 m). LW, deposits of inorganic and organic material, and large rocks are shown before LW introduction (July 2007), directly after LW introduction (February 2008), and a year and a half later (July 2009). Water flow from top to bottom as indicated by an arrow.

Fig. 5. Bed substrate composition (%) at the study sites.

Fig. 6. Correlation between bed elevation changes observed before (July 2006-July 2007) and after the addition of LW to the study reaches (between July 2007 and July 2009). The arrow indicates the position of the Añarbe reach when the plunge pool located downstream from the only remaining structure is included in the calculation of the mean bed elevation.

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Highlights  Large wood (LW) loading was restored in 4 mountain streams  It promoted pool formation and deposition of sediment bars  Organic matter storage increased 5- to 88-fold  Restoring LW in the entire river network would reduce export of sediments and OM  This could greatly benefit water stored in a downstream reservoir