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

[1] 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, tau(b)*, is constant downstream. For the reach as a whole, t(b)* averages 0.049, which is roughly 50% higher than the threshold for bed load transport.