Spatial Organization of "Farmed" Wetlands in Iowa's Prairie Pothole Landscape: Geomorphic and Anthropogenic Controls

The Des Moines Lobe landform within North Central Iowa is the southernmost portion of the Prairie Pothole Region (PPR) that extends northwest into Canada. The PPR is a vast landscape dotted with thousands of glacially formed depressional wetlands known as prairie potholes. These potholes provide a wide range of ecological and hydrological services and are notable for their high waterfowl productivity. Within Iowa it is estimated that 95% of the wetlands in the Des Moines Lobe have been drained for agricultural purposes. Wetlands in this region are typically drained by subsurface tile drains in an attempt to lower the water table and increase agricultural productivity. Efforts are also underway in restoring some of these drained wetlands. In order to better understand the hydrological impacts of restoring drained wetlands at the watershed scale, it is important to understand how these depressions are distributed in space throughout the Des Moines Lobe. The overall objective of this study was to (1) understand the size-distribution and spatial organization of depressional features in the Des Moines Lobe as a function of watershed area and landform type; and (2) Explore the role of human impact on the size-distribution and spatial organization by comparing depressions based on 1m LIDAR DEM (surrogate for historic wetlands) with "farmed" wetlands based on National Wetlands Inventory (NWI ) data. It was found that the size-frequency relationship follows a power law regression that varies based on the landform type and the size of the study area. The power law function varies predictably with changes in area, suggesting fractal properties within the watersheds examined. Comparison between the National Wetlands Inventory (NWI) database and the LIDAR images was used to evaluate the effect of human disturbance on the landscape. The LIDAR captures the depressional areas that correspond to wetlands before the landscape was extensively tile-drained, while NWI captures the features of the existing wetlands in the area. The NWI size-frequency relationship and the LIDAR data showed existence of similar power function size-frequency relationships for depressional areas > 2000 m2. For the NWI data, the power function behavior broke down for smaller depressions, and fewer wetlands were detected than suggested by LIDAR. It is hypothesized that this alteration is indicative of human influence preferentially draining the smaller wetlands, while restoring the larger ones. Further investigation is required, including ground-truthing of the NWI information to prove this hypothesis. Metrics similar to the width function in stream networks was derived to describe the spatial distribution of the wetlands, of different size classes, over the landscape. Future work will involve applying these distribution relationships of depressions to develop watershed scale models of Prairie Pothole hydrology. Such models can be used for exploring the benefits of wetland restoration at the watershed scale, and determining the optimal spatial placement of the restored wetlands to maximize ecosystem benefits.