Analysis of streamflow trends in the upper midwest using long-term flowrecords

This study examines trends in the longest-term streamflow records in the Upper Midwest, with periods of record in excess of 90 years. Results from long-term flow records from other locations in the eastern United States are also provided for comparison purposes. Emphasis is given to identifying comparative changes in the flow regime relative to total runoff (mean flow), rather than focusing solely on univariate trend analyses for individual flow parameters. Results indicate that significant trends in total runoff for the longest streamflow records in the eastern half of the United States are mostly limited to the Upper Midwest, defined primarily by the Upper Mississippi River Basin. This region also displays increases in median and low flows that are well correlated to coincident increases in total runoff. Two different patterns appear regarding trends in high flows. First, for roughly 60 percent of the region, changes in the magnitude of the annual maximum flows are accordant with coincident changes in total runoff. However, these trends in maximum flows typically have lower statistical significance as determined by the Kendall trend test, primarily because of the higher variance of the annual maximum flow series as compared to the annual mean flow series. For the remaining 40 percent of the Midwest, streamflow records have detectable decreases in the annual maximum flows relative to total runoff. For certain regions the reduction in the maximum flow appears to be related to seasonal changes in precipitation amount. Additional factors contributing to the decreases in high flows are being investigated. Copyright ASCE 2005.