DIRECT AND INDIRECT MODIFICATION OF STREAM FLOW IN THE FLATHEAD RIVER BASIN IN NORTHWESTERN MONTANA: HYDROLOGIC PARAMETER DEVELOPMENT AND IMPLEMENTATION

by Bell, Angie Lynn

This thesis is comprised of two potential professional papers that were written to be independent of one another. Both papers were written in the context of the hydrology of the northwestern United States. The snowpack stores winter precipitation and releases it in the spring. This snowmelt-dominated streamflow is used by agriculture, municipalities and water-reliant ecosystems. Chapter 1 considered the response of unmodified snowmelt-dominated streamflow to climate change in the Flathead River basin in northwestern Montana from 1940 to 2006. A parameter to quantify annual flow regime components was developed. Drivers of natural variability of flow regime were also considered. A robust statistical analysis resulted in no significant trends in flow regime versus time, and significant trends in flow regime versus annual precipitation. There was no evidence for a linear response by flow regime to climate change. There was no significant linear trend in flow regime over the study period and flow regime was not significantly related to annual temperature in the Flathead River basin. In the upper reaches of the Flathead River precipitation is associated with flow regime variability. Precipitation, in the Pacific Northwest, is associated with natural climate oscillations. Therefore, flow regime variability may be associated with natural climate oscillations such as El Nino Southern Oscillation and Pacific Decadal Oscillation. Chapter 2 compared flow characteristics of dammed and undammed streams for the Flathead River Basin from 1954 to 2006. The quantile-derived flow characteristics were broken into a pre-dam and post-dam study period. Robust regression was used for trend analysis in the post-dam study period. Pearsons correlation analysis was used to compare variance of dammed and undammed streams to downstream streamflow for both the pre-dam and post-dam study periods. Trend analysis showed that the trend in the timing of dammed streamflow was larger and opposite in sign compared to the undammed streams. In general, the dammed streams showed increased variability of the flow characteristics compared to the undammed streams. The dammed stream influenced the downstream flow early in the water year (October to September), but the undammed streams influenced the flow characteristics once snowmelt-dominated streamflow began.