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Management of the Spring Snowmelt Recession

Publication Date
Jul 21, 2013
Center for Watershed Sciences John Muir Institute of the Environment


Over the past decade, the Natural Flow Regime paradigm has garnered widespread study, discussion and general acceptance in the scientific community as a guide for conservation, restoration and management of rivers. However, one fundamental aspect of the natural flow regime that has received little attention in both scientific study and resource management is the importance of the spring snowmelt recession to ecological stream processes. This study sought to improve understanding of both the ecology, through empirical field studies and hydrodynamic modeling, and water management, through hydropower optimization modeling, of the spring snowmelt flow regime in regulated and unregulated river systems. Substantial differences were observed between regulated and unregulated rivers in flow timing, temperature and spring recession rate of change and duration, particularly in relation to hydropower operation. These hydrologic differences were correlated with observed variation in frog breeding behavior as well as measures of primary and secondary productivity, as evidenced by quantified benthic macroinvertebrate indices. These results support the ecological basis for providing increased instream habitat diversity to promote increased biotic diversity. Using a daily percent decrease in flow methodology to quantify spring snowmelt recession flows, hydrodynamic modeling results showed that flow recessions with down ramping rates similar to those observed in unregulated systems (less than 10\% per day) provided the most diverse hydraulic habitat for an appropriate duration in spring to support native species and maximize aquatic biodiversity. Hydropower optimization modeling showed that projected regional climate warming did not necessarily decrease hydropower output in the Upper Yuba River in the near-term, while increased allocations to environmental flows through increased minimum instream flows and limited spring spill down-ramp rates decreased hydropower generation by less than 4\% in the near term. The combined results of this study provide resource managers not only with increased knowledge regarding the ecology of the spring snowmelt recession, but also with a series of methods that will help predict the impacts of various spring flow regimes on the diversity of aquatic and riparian species. As such, the results are directly applicable in current and future hydropower relicensing efforts where improved ecological knowledge and various modeling applications can be utilized to guide instream flow determinations.

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