Developing hydraulic relationships at the riffle crest thalweg in gravel bed streams

The alluvial riffle crest is a recurring, geomorphic feature of gravel bed rivers that has a strong influence on low flow hydraulics and ecology in streams. Riffle crest depth has been used to quantify habitat availability in the upstream pool. Recently, a quantitative relationship between streamflow (Q) and median riffle crest thalweg depth (mRCT) has been observed by researchers in gravel bed streams throughout Northern California. The mRCT is the median from a data set of riffle crest thalweg (RCT) depths measured along a channel reach of known streamflow. This thesis was undertaken to: (1) determine the strength of correlation between mRCT and streamflow in gravel bed streams, and (2) if possible, to refine the RCT depth-Q relationship into a useful tool for ecological management and restoration. A field method was developed to reproducibly locate the RCT. Riffle crests were sampled in nineteen reaches from thirteen streams across Northern California. While RCT depth was moderately correlated to Q (R2 = 0.55), mRCT was strongly correlated to Q (R2 = 0.84). The regression equation from the mRCT-Q relationship was used to predict streamflows between 1 - 155 cfs, with the best prediction (5.1 cfs median error) in Median sediment size (D50) and wetted width of the channel (WWC) had an appreciable effect on RCT depth at a given flow. In addition, an inflection in the RCT depth-Q relationship was shown to occur when streamflow reached the active channel stage. The RCT depth-Q relationship appears not to be scaled to drainage area which stands in contrast of commonly used hydraulic geometry relationships. A dimensionless variable (D50/WWC) was developed to scale the RCT depth-Q relationship to local channel morphology. The (D50/WWC)/Q-RCT relationship produced a streamflow prediction error for individual riffle crests of 5.3 cfs based on a sample of 38 riffle crests. This thesis shows that both RCT and mRCT are correlated to Q, although the mRCT-Q correlation is much stronger. In addition, scatter in the RCT depth-Q and mRCT-Q relationships appear to be caused by variability in sediment size and wetted width of flow between riffle crests. The relationships developed in this thesis: RCT depth-Q, mRCT-Q and (D50/WWC)/Q-RCT, have potential to become tools in ecological management applications in which depth of flow at the riffle crest is an ecological control. Such applications include fish passage through natural riffles and developing streamflow-habitat relationships.