The simultaneous withdrawal of water from streams for springtime frost protection of grapevines in the Russian River basin can coincide with the emergence of salmonid fry and the rearing of juveniles. These water diversions have contributed to water level declines, which in some instances, have resulted in the stranding mortality of fish. Endangered coho salmon and threatened steelhead trout can become stranded when water levels decrease abruptly and fish seek refuge in the rapidly dewatering gravel.
In response to this issue, the National Marine Fisheries Service (NMFS) has proposed a site-specific method to determine minimum flows to protect salmonids from these effects. This method seeks to identify “high risk” stranding surfaces and determine the stream stage at which they become exposed. In this study, we evaluated the ability of the NMFS protocol to accurately prescribe protective stages. To do this, we analyzed three components of the protocol: its stranding risk classification system, it’s sampling of stranding surfaces and its method of establishing protective stage recommendations.
We evaluated the risk classification system by comparing it to published literature values on salmonid stranding. We assessed the sampling of stranding surfaces by performing the protocol at two sites. NMFS developed the method based on data from a medium-sized drainage (12.6 mi2), so we selected a small drainage (4.6 mi2) and a large drainage (50.2 mi2) to evaluate how effectively the method characterized the variation in potential stranding surfaces in different watershed settings. We evaluated the protocol’s protective stage recommendation by comparing the protective stage from our two surveyed sites to stream stage data for the season of regulation.
Our assessment has led us to make several recommendations. First, the risk classification system would benefit from consideration of other factors influencing stranding risk and should adjust stranding risk thresholds to better fit the literature. Also, the protocol is weak in its ability to capture within-site variation. We therefore recommend increased sampling of stream reaches and scaled mapping of each site to better define stranding surfaces. These measures should result in improved protective stage recommendations but further studies may be necessary. With these changes, we believe that the NMFS protocol will be an effective tool for protecting fish from being stranded due to vineyard use of water during frost events in the Russian River Watershed.
