POOL AND RIFFLE CHARACTERISTICS IN RELATION TO CHANNEL GRADIENT

The depths of pools relative to the depths of runs and riffles were correlated with reach-scale channel gradient along three rivers in coastal northern California. The sample included 122 pools formed in channels with gradients from 0.172 to 0.002. Relative pool depth on these rivers, and relative distance between pools, increase as channel gradient decreases. Mean pool:riffle depth is 2.8:1 at the highest channel gradient, and 6.2:1 at the lowest gradient, while mean pool:riffle length is 1:0.8 at high channel gradient, and 1:1.8 at low channel gradient. We hypothesize that these trends reflect changes in energy expenditure with decreasing gradient, as a result of the flow's ability to erode its channel boundaries. Channel reaches with high gradients are characterized by resistant channel boundaries, coarse material, and relatively low discharge and total stream power. Channel reaches with low gradients have less resistant channel boundaries, finer-grained bed material, and higher values of discharge and total stream power. These changes in channel and flow characteristics with decreasing gradient result in flows in high-gradient reaches expending a greater proportion of their energy in overcoming boundary and internal resistance, with less energy available for channel-bed scour and the formation of pools in their relatively resistant channels. In contrast, flows in low-gradient reaches may more effectively scour the channel bed, creating deeper pools because the channel boundaries are less resistant, and the proportion of flow energy available for sediment entrainment and transport should be greater.