What The Science Says
While wet winters followed by drought like summers appear to be contradictory, such a flow sequence may be becoming more frequent in the Eel River. Climate change is shifting historical flow regimes to a higher frequency and magnitude of winter floods and prolonged periods of summer base flow (Asarian 2015). Power et al. (2015) also suggest that summer water extraction associated with a recent boom in marijuana cultivation in the region is further contributing to summer base flow reductions (Bauer et al. 2015). The key to minimizing such flow sequences and the subsequent habitat effects is to improve summer base flow conditions.
Power et al. (2015) suggest several important actions which could improve summer flow conditions on the Eel River, returning it to a more resilient and productive salmon and steelhead river. First, the authors suggest that timely upstream flow releases from Lake Pillsbury during critical low flow periods could promote hydrologic connectivity and improve over-summering habitat conditions on the mainstem Eel. Secondly, winter diversions and subsequent storage of water in small tanks would enable irrigators to rely less on summer diversions. Third, the authors recommend improving geomorphic complexity through the introduction of additional large wood (dead trees) to the Eel River. Large wood promotes pool formation and scour, providing cold deep pools for over-summering fish. Finally, encouraging re-establishment of mature forests will reduce evapotranspiration, improve hillslope stability (reducing fine sediment influx), and harvest fog as a valuable water source. The interplay between wet-winter and low-summer flows has
important implications (and possible consequences) for salmonid habitat on the Eel River and in other coastal rivers. Minimizing extreme low flow conditions during the summer ensures a robust and healthy aquatic ecosystem and will help promote salmonid recovery.
Dr. Robert Lusardi is the California Trout-UC Davis Wild and Coldwater Fish Scientist.
Power et al. (2015) first appeared in the scientific journal Copeia.
Power, M. E., K. Bouma-Gregson, P. Higgins, and S. M. Carlson. 2015. The thirsty Eel: summer and winter flow thresholds that tilt the Eel River of Northwestern California from salmon supporting to cyanobacterially degraded states. Copeia 103(1): 200-211.
Other literature cited:
Asarian, J. E. 2015. Long-term streamflow and precipitation trends in the Eel River basin. Prepared by Riverbend Sciences for Friends of the Eel River, Arcata, CA. 30 p.
Bauer, S. B., J. Olson, A. Cockrill, M. van Hattem, L. Miller, M. Tauzer, and G. Leppig. 2015. Impacts of surface water diversions for marijuana cultivation on aquatic habitat in four northwestern California watersheds. PLoS ONE 10(3): e0120016.
Yoshiyama, R. M. and P. Moyle. 2010. Historical review of Eel River anadromous salmonids, with emphasis on Chinook salmon, coho salmon and steelhead. Report for California Trout, Center for Watershed Sciences, University of California, Davis.