Numerical simulation of the impact of sediment supply and streamflow variations on channel grain sizes and Chinook salmon habitat in mountain drainage networks

Abstract

Climatically driven changes in streamflow and hillslope sediment supply could potentially alter stream surface grain size distribution patterns and thereby impact habitat for a number of threatened and endangered in-stream fish species. Relatively little is known about hydrograph (shape, peak flow) influence or the relative importance of chronic and episodic hillslope inputs on channel conditions. To better understand these external drivers, we calculated sediment routing through a gravel-bedded river network using a one-dimensional (1D) bedload transport model. We calculated changes in grain sizes and estimated Chinook salmon habitat suitability caused by a dry year and an extreme flood hydrograph, and chronic (diffusive, overland flow) or pulse (landslide, debris flow) hillslope sediment supplies. To obtain accurate channel conditions, a relatively high reference Shields stress, representative of steep mountain streams, was needed. An extreme event flood without any hillslope sediment inputs caused widespread bed coarsening and a decrease in aquatic habitat. Chronic sediment input combined with this hydrograph eliminated any changes in grain size and habitat, although when combined with a dry year flow, caused systematic bed fining. The influence of a given hydrograph therefore highly depends on the hillslope sediment supply. Regardless of the flow hydrograph or sediment pulse timing, grain size distribution or location, pulse sediment inputs did not cause widespread grain size changes despite being 100 times the total chronic input volume. Widespread and continuous hillslope sediment inputs may influence channel grain sizes and aquatic habitat more than a single discrete sediment pulse. Depending on the magnitudes of flow hydrograph and sediment supply alterations, climate change may induce no differences in grain sizes or very dramatic changes with significant consequences for longterm sustainability.

Tributary connectivity, confluence aggradation and network biodiversity

“…Numerous distributed numerical models of sediment transfer through river networks (Gasparini et al, 1999;Sklar et al, 2006;Lewicki et al, 2007;Neupane and Yager, 2013;Gran and Czuba, this volume) or across landscapes (Gasparini et al, 2004(Gasparini et al, , 2008van Balen et al, 2010;Coulthard and Van de Wiel, 2012) include spatially explicit, time-dependent adjustment of bed material grain size and bed elevation that could be used to detect confluence aggradation. Czuba and Foufoula-Georgiou (2015) have recently offered a numerical modelling framework for identifying places of excess sediment accumulation ("clusters") in river networks.…”