Stream Flow Response to Climate in Minnesota

The variability of stream flows in Minnesota, and the relationship between stream flows and climate are the focus of this report. We analyze historical flow records of Minnesota streams to determine how much frequency and magnitude of flows have been affected by climate and land use changes. Flow duration analysis, high and low flow ranking, and flood frequency analysis were applied to recorded mean daily stream flows, 7-day average low flows, and annual peak flows. Data from 36 gauging stations located in five river basins of Minnesota (Minnesota River, Rainy River, Red River of the North, Lake Superior, and Upper Mississippi River Basins) covering the 1946-2005 period were used. To detect any changes that have occurred over time, data from the 1986-2005 and the 1946-1965 periods of record were analyzed separately. Flow duration curves were prepared for all gauging stations, and low flows (Q90, Q95), medium flows (Q50), and high flows (Q5, Q10) in the two time periods were examined. Multiple stream gauging stations in the same river basin generally showed consistent changes in stream flows, although deviations from a typical river basin pattern were noted at a few gauging stations. The Minnesota River Basin has experienced the largest stream flow changes compared to the other four basins. High, medium, and low flows have increased significantly from the 1946-1965 to the 1986-2005 period in the Minnesota River basin. The increases in medium to low flows were larger than the increases in high flows. Considerable changes in flows were also observed in the Upper Mississippi River Basin and the Red River of the North Basin. Streams in the Rainy River Basin and tributaries to Lake Superior showed little or no change in stream flow between the 1946-1965 and 1986-2005 periods. The changes observed in these river basins were also variable. In two tributaries to Lake Superior, average flows seem to have increased on the order of 10%, 7-day low flows seem to have decreased, and annual peak flows seem to be unchanged.