The Impact of Local Source Sediments on Bed Texture in the Fall River, Rocky Mountain National Park, USA

ABSTRACT

The bed texture of a gravel-bed river is related to the size distribution and quantity of source sediments, the routing of sediment through the reach, and the distribution of flow velocity. A reach morphology that is consistent in depth with little lateral topographic variation will typically have a bed texture that is characterized by a fairly uniform grain size distribution. However, spatial variations in source sediments within a given watershed may impact the distribution of gravel-bed river sediments, even at the reach scale, such that two proximal reaches of the same river having the same general morphology can exhibit contrasting distributions of surface sediments. We collected extensive topographic and sedimentological data from two reaches of the Fall River in Rocky Mountain National Park, Colorado. These were chosen for their simple morphology (both are straight reaches with fairly uniform depths) and contrasting location relative to alluvial fan deposits that were introduced into the valley in a dam-break event in 1982; the upstream reach was unaffected by the introduced sediments. Despite the long duration since this event, surveying in 2008 revealed that the fan sediment continues to coarsen the left and upstream portions of the affected reach relative to other regions of the channel. The persistent nonuniformity in bed texture in the downstream reach may eventually result in morphological adjustment by promoting differential routing of fine versus coarse bed load size fractions, which may induce meandering.

Keywords:

Bed texture , gravel-bed river , alluvial fan , channel morphology , spatial uniformity , fall river , rocky mountain national park. .