The Current Magazine Winter 2016/17 - Page 15

Photo by Mike Weir

The Eel River is the third largest river entirely contained in California, preceded only by the Sacramento and San Joaquin Rivers. Historically, during wet periods and productive ocean conditions, the Eel River likely supported adult salmon (Chinook and coho) and steelhead returns greater than a million fish annually (Yoshiyama and Moyle 2010). However, numbers of these salmonids declined drastically due to overharvest, logging, and grazing during the 19th century and the mega-floods of 1955 and 1964. While the Eel River is beginning to recover after decades of improved management and restoration, an increasing demand for water during summer and changes to the flow regime associated with climate change (Asarian 2015) may have potential severe consequences for salmonids. In the publication, ‘The thirsty Eel: summer and winter flow thresholds that tilt the Eel River of Northwestern California from salmon-supporting to cyanobacterially degraded states’, Power et al. (2015) discuss interactions between wet-winter flows and low summer flows, the diverse ecological outcomes produced by such flow sequences, and the potential implications for salmonids and other fishes.

Flow is often referred to as the ‘master variable’ because of its ability to broadly affect geomorphic and ecological processes in river ecosystems. For salmonids, flow affects physical habitat (e.g., pool depth, temperature), but can also indirectly affect river food webs, especially the quantity and quality of stream invertebrates consumed by juvenile salmonids. Not surprisingly, different sequences of winter and summer flows promote different physical habitat and ecological responses. For instance, large winter floods during wet years promote scour of fine sediments and large wood recruitment, but during dry years winter flows may not be of sufficient magnitude to elicit such responses. Conversely, summer flows during wet years are usually higher than those of dry years, promoting connectivity between surface and groundwater and between upstream and downstream reaches. Higher summer flows can also provide cooler over-summering water temperatures for juvenile salmonids.

The Eel How winter and summer flow sequences affect salmonid habitat

and food webs on the Eel River