Physical controls on fine-sediment infiltration into gravel beds.

English Abstract:

As humans continue the widespread modification of the Earth's terrestrial surface augmented fine sediment loading to river systems is likely to remain an ongoing issue. In gravel-bed streams the impact of increased fine sediment loads on streambed habitats is determined by factors controlling fines infiltration into channel substratum. Much of what we know about the infiltration processes is derived from laboratory studies. Few, if any, field studies have simultaneously investigated the multiple physical factors thought to control infiltration. Here we present the results of a field experiment conducted in a boreal forest stream in Quebec, Canada, that was designed to investigate the influence of the following factors on substrate fines content: (i) substrate pore constriction size, (ii) hyporheic flow, and (iii) the quantity of fines transported across the bed surface (i.e., exposure dose). Our results indicated that the ingress of fines into gravel-beds was primarily a gravity driven process influenced by size thresholds between the infiltrating fine sediment and the size of the substrate interstitial spaces. Results also indicated that fines content at depth was unrelated to fines exposure and that the estimated upward seepage rates at these study sites were well below the threshold velocities that would inhibit the percolation of medium-grained sand into the gravel bed. We discuss the implication of this research for stream habitat rehabilitation and suggest directions for future research.