Semi-arid continental climates in the northern hemisphere are experiencing large increases in surface temperatures due to climate change; however, to date there has been limited effort to evaluate stream flow trends in the Northern Rockies region. Cool in-stream temperatures are of great importance to the health and function of aquatic ecosystems during summer months. While historic stream temperature data is quite uncommon, stream discharge records are prolific throughout the Northern Rockies and are related to stream temperature, snow pack, and precipitation trends. The purpose of this study was to determine if mean August stream discharge values are decreasing over the last half-century, if low discharge values are occurring more frequently and if this trend can be attributed to climate change. Using historic discharge data from United States Geological Survey’s (USGS’s) national water information system (NWIS) web interface we analyzed data for trends of at least 20-50 years. Combining of these records along with aerial photos (with <10m resolution) and water rights records we selected gauging sites based on the length and continuity of discharge records, amount of diversion and location of water storage devices. In addition we used temperature records and precipitation records to perform a correlation analysis with discharge trends over the last half century.

Results/Conclusions

Preliminary analyses demonstrate that watersheds in select locations throughout the Northern Rockies are experiencing significant declines in stream discharge over the last half century. These results are further supported by recent research that shows decreasing mountain snowpack, earlier succession of snowmelt and more winter rain events. This regional assessment of stream discharge trends will provide invaluable information for local, county and state conservation plans and help to predict the severity of action needed to address decreasing discharge in the future.