Downstream changes in the channel geometry of a large gravel bed river

Abstract

Field data obtained on a nearly contiguous segment of the Colorado River in western Colorado and eastern Utah are used to examine the mechanisms driving downstream changes in channel geometry. Measurements characterizing the bank‐full hydraulic geometry, bed material grain size, and average channel gradient were made at closely spaced intervals in 10 alluvial and quasi‐alluvial reaches covering 260 km of the river. These data indicate that changes in surface and subsurface grain sizes are small in relation to the change in channel slope: over the full length of the study area, the median grain size of the surface sediment decreases by a factor of a little more than 2, whereas the average channel slope decreases by a factor of about 5. The decreases in slope and median grain size are offset by a large increase in bank‐full depth relative to width, such that the bank‐full Shields stress, τ*b, is constant downstream. For the reach as a whole, τ*b averages 0.049, which is roughly 50% higher than the threshold for bed load transport.