Subsurface flow in lowland river gravel bars

Geomorphic and hydraulic processes, which form gravel bars in large lowland rivers, have distinctive characteristics that control the magnitude and spatial patterns of infiltration and exfiltration between rivers and their immediate subsurface environments. We present a bedform-infiltration relation together with a set of field measurements along two reaches of the San Joaquin River, CA to illustrate the conditions required for infiltration and exfiltration of flow between a stream and its undulating bed, and a numerical model to investigate the factors that affect paths and residence times of flow through barforms at different discharges. It is shown that asymmetry of bar morphology is a first-order control on the extent and location of infiltration, which would otherwise produce equal areas of infiltration and exfiltration under the assumption of sinusoidal bedforms. Hydraulic conductivity varies by orders of magnitude due to fine sediment accumulation and downstream coarsening related to the process of bar evolution. This systematic variability not only controls the magnitude of infiltration, but also the residence time of flow through the bed. The lowest hydraulic conductivity along the reach occurred where the difference between the topographic gradient and the water-surface gradient is at a maximum and thus where infiltration would be greatest into a homogeneous bar, indicating the importance of managing sand supply to maintain the ventilation and flow through salmon spawning riffles. Numerical simulations corroborate our interpretation that infiltration patterns and rates are controlled by distinctive features of bar morphology. 1. Geomorphic Dependence of Subsurface Flow Undulations in the topography of lowland gravel bed rivers arise due to interactions between the flow and sediment supply and commonly lead to bar and pool morphology variably mantled with gravel and sand [Leopold, 1992; Milne, 1982]. The flow of river water over an undulating bed creates spatial variations in hydraulic head at the bed surface that drive infiltration, throughflow, and exfiltration of surface water through the shallow subsurface [Savant et al., 1987; Vittal et al., 1977]. Thus, geomorphic processes influence hydrologic exchange along riverbeds, whose hyporheic processes are important for sustaining intragravel temperature [Alderdice and Velsen, 1978; Beacham and Murray, 1990; Geist et al., 2006], oxygen [Ingendahl, 2001; Wickett, 1954] and sediment [Greig et al., 2005; Hanrahan et al., 2005; Kondolf, 2000; Kondolf, 2000; Lapointe et al., 2004; Suttle et al., 2004] conditions required for spawning and embryo survival of anadromous and resident fish [Geist and Dauble, 1998; Tonina and Buffington, 2009b]. The spatial pattern and rate of infiltration are controlled by the spatially varying interactions of the water surface and the topography and sedimentology of the bed [Tonina and Buffington, 2009a]. Understanding these interactions facilitates prediction of hyporheic fluxes of water and ventilation of the subsurface [Edwards, 1998], and of whether management of streamflow or sediment supply might improve the quality of salmonid spawning and incubation habitats. While many studies of shallow subsurface or ‘‘hyporheic’’ flow at bedform-to-reach scales have focused in steep headwater river channels, lowland gravel bed reaches have received less attention and differ in several ways. First, they have larger drainage areas and gentler bed slopes, which together cause the ratio of the flow depth to the grain size and even to bedform amplitude to be large. Thus, transport capacities are usually high enough to entrain a portion of the bed material during bankfull events [Konrad et al., 2002; Leopold, 1992; Reid and Dunne, 2003] whereas bed material in headwater reaches may commonly be mobilized when debris jams fail [Mosley, 1981] or during large floods or debris flows [Yager et al., 2007]. Second, disruption of alongstream trends in sediment sorting and diminished sediment transport capacity Key Points: An analytical bedform-infiltration relation shows that infiltration areas are controlled by sinusoidal or irregular, asymmetric bed topography Streambed hydraulic conductivity varies by orders of magnitude along riffle-pool sequences related to bar evolution The range and spatial pattern in K relative to morphologic features strongly controls hyporheic residence times Supporting Information: Supporting Information S1 Correspondence to: E. Bray, [email protected] Citation: Bray, E. N. and T. Dunne (2017), Subsurface flow in lowland river gravel bars, Water Resour. Res., 53, doi:10.1002/2016WR019514. Received 18 JUL 2016 Accepted 24 JUL 2017 Accepted article online 1 AUG 2017 VC 2017. American Geophysical Union. All Rights Reserved. BRAY AND DUNNE SUBSURFACE FLOW GRAVEL BARS 1 Water Resources Research PUBLICATIONS