Alterations of river flow regimes in the Northern Rocky Mountains, USA

I studied discharge data from stream gauges located in natural and anthropogenically-modified river basins of the Northern Rocky Mountains over 59 years. I applied linear and non-linear models to the data to determine what, if any, alterations have occurred in the annual flow regimes. By comparing the different results from the natural and anthropogenically-modified river basins, I was able to distinguish the impacts that climate change and anthropogenic modifications have had on flow regimes in the Northern Rocky Mountains over the period of record. I found that regional climate change has not significantly altered the natural flow regimes. However, I did find an underlying cyclical pattern in the total amount of annual discharge in both the natural and anthropogenically-modified river basins, suggesting a strong link between the quantity of runoff and a hydroclimatologic cycle. Conversely, direct basin anthropogenic modifications such as damming, irrigation, and urbanization have caused a decrease in the difference between the minimum and maximum annual discharges, a decrease in the daily variations of flows, and have changed the timing of flows to both earlier and later in the year across the Northern Rocky Mountains. In general, direct anthropogenic modifications of the river basins have altered the flow regimes to a much greater extent than climate change. More specifically, I found that extensive irrigation development over the last century in the region has severely altered the size of the annual extreme flows, often creating a more homogenous annual range of flows. The analysis of annual hydrographs from pre and post major irrigation development reveals that the timing and duration of flows has also been severely impacted. Specifically, the duration of the flood hydrograph has been extended to encompass the majority of the water year, with the number of days between the timing of the 25th and 75th quartiles of flow expanding by as much as 100 days in some basins. In addition, the variability of intra-annual flows has declined, contributing to a more homogenous annual hydrograph.