August Stream Discharge Trends Portend Impacts of Climate Change in the Northern Rockies

     In the snowmelt dominated hydrology of arid western U.S. landscapes, late summer low streamflow is the most vulnerable period for aquatic ecosystem habitats and trout populations. This study analyzes mean August discharge at 153 streams throughout the Northern Rockies (NR) for changes in discharge from 1950-2008. The purpose of this study was to determine if (1) mean August stream discharge values have decreased over the last one-half century; (2) low discharge values are occurring more frequently; and (3) climatic variables are influencing August discharge trends. Here we use a strict selection process to characterize gauging stations based on amount of anthropogenic impact to tease out heavily impacted rivers and understand the relationship between climatic variables and discharge trends.  Using historic U. S. Geologic Survey (USGS) discharge data, we analyzed data for trends of 40-59 years. Combining of these records along with aerial photos and water rights records, we selected gauging stations based on the length and continuity of discharge records and categorized each based on the amount of diversion.  Our analyses indicate that non-regulated watersheds are experiencing substantial declines in stream discharge and we have found that 89% of all non-regulated stations exhibit a declining slope.  Additionally, our results indicate a significant (α ≤ 0.10) decline in discharge from 1951 to 2008 for the NR. Correlations at our pristine sites show a negative relationship between air temperatures and discharge, and these results coupled with increasing air temperature pose serious threat for aquatic ecosystems in the NR.