Flushing flow recommendations for maintenance of salmonid spawning gravels in a steep, regulated stream

A study was conducted to develop flushing flow recommendations for maintaining the quantity of salmonid spawning gravels in the North Fork of the Feather River, a regulated stream in California, U.S.A. This required the development of a technique which would prescribe flows to remove sediments from the gravels without removing the gravels themselves, which are in finite supply in the river. Field studies involved the use of a ‘two-point-are’ procedure which allowed depth, velocity, and substrate measurements to be concentrated over spawning gravels at each site. Measurements were repeated at each site at three different flows ranging from 1.7 to 11.3 m3 s−1. The local velocity and depth measurements were used to evaluate hydraulic conditions within areas containing spawning gravels. Four flow levels were derived which would provide varying degrees of sediment transport and flushing; (1) surficial flushing of gravels; (2) mobilization of gravels; (3) surficial flushing of cobbles; and (4) mobilization of cobbles. Results of the study indicated a flushing flow of 56.6 m3 s−1 occurring as a planned release or natural flow for from 1–3 days would be sufficient to transport sediment from spawning gravels. This flow was intermediate to levels which maximize surficial gravel flushing and levels which maximize surficial cobble flushing.