OOS 49-5
Managed ecosystem flows reset the Truckee River's future: The Pyramid Lake and Truckee River case study
Starting in 1995, The US Fish and Wild Service (FWS) experimentally implemented ecologically-based flow regimes in the Truckee River and directed management of flows to encourage cottonwood recruitment through 2000. These experimental flows resulted in formulation of eight flow regimes that targeted overall ecosystem health. Each year, managers select a regime based on a matrix of factors which include available water from winter snow pack and stored water in Stampede Reservoir. The two highest flow regimes are only managed for cottonwood and willow recruitment, and the other six managed flow regimes range from above average conditions to extreme drought conditions. Starting in 2002 the FWS used these regimes to manage Truckee River flows for fish recovery and ecosystem health. In order to understand the influence of managed flows on cottonwood establishment after nineteen years of flow regime implementation, a dendrochronological study was paired with historic aerial imagery analyses. We harvested cottonwood individuals on randomly selected pointbars along transects perpendicular to the river and counted growth rings to determine establishment years. The aerial extent of riparian trees was digitized in GIS using a time series of imagery from 1938, 1954, 1974, 1994, and 2013 to quantify change in extent of riparian forests.
Results/Conclusions
Managed flows have reversed the steady decline of riparian and aquatic conditions in the Truckee River. Monitoring revealed that ecosystem flow management stimulated riparian tree recruitment, increased bird diversity, and enhanced fisheries. Currently, many new age classes of cottonwoods are growing along the Lower Truckee. Using GIS mapping on a time series of aerial imagery from 1938 to 1995, we documented over a 75% decline in the riparian cottonwood forest extent. However, from 1995 to 2013 we observed a 50% increase in cottonwood forest extent even with a measured 1%/year, on average, old growth decline. Tree ring analyses revealed that the majority of cottonwood establishment (40%) occurred from 1997-2000. Other years with large floods (e.g., 2006) were not accompanied by high establishment rates likely due to unsuitable flow conditions and habitat saturation. This study correlates new tree establishment with fluvial geomorphic work that created appropriate establishment surfaces and managed hydrologic conditions needed for seedling survival. In addition, several species of birds that disappeared from the Lower Truckee in the 1970’s have returned and a tremendous coldwater fishery has developed. These data demonstrate the ecological benefits of more than a decade of ecosystem managed flow regimes in the Truckee.