Reach-Scale Channel Geometry of a Mountain River

N. St. Vrain Creek drains 250 km2 of the Colorado Front Range. The basin is underlain by granitic rocks, and the upper half was glaciated. We used 25 study reaches to examine controls on reach-scale channel geometry. Variables measured included channel geometry (width, depth, gradient, bedforms), grain size, and mean velocity. Drainage area at each study reach ranged from 2.2-245 km2, and gradient from 0.013-0.147 m/m. The increase in discharge with drainage area is strongly linear. Channel types included cascade, step-pool, plane-bed and pool-riffle. We examined correlations among (1) the reach-scale response variables bankfull width (w), hydraulic radius (R), mean velocity (v), Darcy-Weisbach ff, bedform wavelength (bw) and amplitude (a), grain size, relative roughness (R/D84) and shear stress (ss), and (2) potential control variables that change progressively downstream (drainage area, discharge) or that are reach-specific (bed gradient). Specifically, we tested the hypothesis that response variables correlate most strongly with local gradient because of the segmented nature of mountain channels. Results from linear regression analyses indicate that most response variables(R/D84, D50, D84, ff, ss) correlate best with gradient, although w, w/d ratio, and bw correlate best with discharge. Multiple regression analyses using Mallow's Cp selection criterion produced similar results in that most response variables correlate strongly with gradient, although the specific variables differ from those selected with linear regressions: w, a, v, ff and ss correlate with gradient, whereas R, bw and v correlate with discharge. These results suggest that the hypothesis is partially supported: channel bed gradient is likely to be a good predictor for many reach-scale response variables along mountain rivers, but discharge is also a good predictor for some response variables. Thus, although subject to strong constraints imposed by changes in gradient and grain size supplied by processes such as glaciation and rockfall, adjustments in the channel geometry and hydraulics of mountain rivers at the reach-scale produce discernible patterns analogous to those in fully alluvial rivers. Mountain rivers may differ in that imposed reach-scale gradient is an especially important control on reach-scale channel characteristics.