WATERSHED LAND USE CHANGES AND THEIR EFFECTS ON HYDROLOGY AND SEDIMENT YIELD IN THE LAKE PEPIN BASIN

Sediment loading derived from erosion of streambanks and bluffs to Lake Pepin has increased in the past 70 years. One hypothesis is that tributaries in the Lake Pepin basin have become more erosive. Increasing erosivity of rivers should be accompanied by changes in hydrology. Recent study of 21 tributaries in the Lake Pepin basin has showed that flow and runoff ratio (flow/precipitation) has increased significantly in many agricultural watersheds during the period May-June, 1940-2009. May-June contribute a substantial proportion of the annual sediment load to Lake Pepin. This study evaluated possible factors in the runoff ratio increases by investigating temporal changes in artificial drainage and crop type as well as total precipitation volume, precipitation intensity and indicators of antecedent soil moisture over the same period. Mann-Whitney median and Kendall-tau tests were used to evaluate trends. Results showed that watersheds with the greatest increases in flow and runoff ratio are those that have been substantially altered by artificial drainage and increased soybean agriculture. Potential evapotranspiration (PET) in May-June was shown to decrease significantly in watersheds where soybean agriculture has increased. Conversely, total precipitation volume and intensity, and antecedent soil moisture showed no significant increase over the same period. Results suggest that increases in artificial drainage and decreased PET due to soybean agriculture are the primary factors in the flow and runoff ratio increases. Others have measured channel widening in many of the watersheds in this study. The degree of hydrologic change presented in this study correlates well with the degree of channel widening, suggesting changing hydrology has increased erosivity of rivers.