0 An Historical Perspective of Hydrologic Changes

This study documented hydrologic changes, specifically wetland losses, in Seven Mile Creek Watershed. Historical aerial photos along with a Geographic Information System (GIS) were used to assess these changes as it relates to water resources management. The 95.3 km2 (36.8 mi.2) study area is a small, agricultural watershed located in south-central Minnesota. More than 130 aerial photographs from seven different periods dating back to 1938 were scanned and rectified for use in a GIS. Wetland areas converted to cropland were then interpreted and digitized. In addition, other land use changes, such as surface and sub-surface drainage modifications and cropping system shifts, were mapped and documented. Results from the study indicate significant hydrologic changes have occurred in the watershed. Analysis of pre-settlement maps and survey notes indicate that about 50 % of the watershed was once covered by wetlands. Of those wetlands, it is estimated that 88 % of the natural wetlands have been converted to cropland. About 47 % of those losses occurred from early settlement (late 1800’s) to 1938. From 1938 to 1985, an additional 41 % of the wetlands were drained and converted to cropland. This translates to an average annual net wetland loss of 40 hectares (100 ac.) per year. During this same period (1938-1985), 40 km (25 mi.) of drainage ditches were constructed, more than 966 km (600 mi.) of public and private sub-surface drainage systems were installed, and it is estimated that total corn and soybean acreage increased from 30 % to 96 % within the watershed. The most rapid percent change, a 50 % wetland decrease, occurred between 1955 and 1961. The construction of two county drainage ditch systems in 1955 accounts for this change. After 1985 the wetland loss trend has decreased. Wetland increases are a direct result of conservation programs combined with grants from private and state water resource protection programs