CHANGES IN RIVER ECOLOGICAL FUNCTIONALITY DUE TO RAPID ALTERATIONS OF CHANNEL MORPHOLOGY IN TWO CALIFORNIA RIVERS

Anadromous fish populations along the Pacific coast have been declining for decades, partly due to river regulation. This study presents an application of the “functional flows model” that integrates the role of hydrogeomorphic processes and ecological functions in stream physical habitat evaluations. Functional flows are discharges that serve ecological functions. In this study, the key question was how flow functionality for the spawning life stage of fall-run Chinook salmon changes in response to rapid alterations of channel morphology. The assessment of functional flows involved evaluation of shear stress dynamics that determine sediment transport stages favorable to ecological functions that require either channel stability or change. Ecological functions studied were bed occupation- spawning, incubation, and emergence- and bed preparation- periods when the river bed is reworked. Model inputs were discharge, cross section geometry, water surface slope, and median bed-material grain size. Model outputs were the ranges of streamflow that are functional, and the number of days within a water year that present functional flows. The model was applied to evaluate the ecological functionality of riffles undergoing rapid hydrogeomorphic changes in two different rivers supporting fall-run Chinook salmon: the narrow, sediment starved, low-flow Mokelumne River (MR) which has undergone river habitat rehabilitation through gravel augmentation, and the wide Yuba River (YR), with abundance of hydraulic mining sediment in floodplain and diverse flow regime which has undergone rapid morphologic changes due to floods. Each river was analyzed at three cross sections and at three water years. Results indicate that river rehabilitation on the MR has increased the number of days with functional flows by an average of 47 days and that natural floods on the YR has maintained the number of days with functional flows even as the channel incises. A comparison between the rivers shows that the YR presents a larger range of functional flows and a better balance of number of days with bed occupation and bed preparation flows than the MR. Functional flows analysis provided an objective comparative tool to assess changes in ecological functionality at distinct hydrogeomorphically dynamic sites.