Riparian forest vegetation patterns and historic channel dynamics of the Big Sioux River, South Dakota

The Big Sioux River, a major tributary to the Missouri River, drains approximately 22,550 km2 in South Dakota, Iowa, and Minnesota. Compared to many other Great Plains rivers, the Big Sioux is relatively natural and free-flowing, with a largely intact riparian corridor and little flow regulation. Along its course, the river flows through a largely agriculturally dominated matrix that contains several major population centers. The close proximity to these agricultural and urban land uses have contributed to a number of water quality problems during the last 50 years that have threatened the ecologically integrity of the Big Sioux. In order to conserve and properly manage the Big Sioux corridor, it will be important to gain an improved scientific understanding of ecological patterns and their environmental correlatives along the river. This thesis is an attempt to provide these data.;Using modern aerial imagery and ground-truthing techniques, I quantified the current extent of riparian forest communities along a 576 km stretch of the Big Sioux River. In total, there were 7 dominant riparian forest community types along the river that contributed to 6.85% of the total Big Sioux basin land cover. Riparian forest extent increased from north to south and was dominated by the Silver Maple-Elm-Cottonwood Forest community type (41% of total). In addition, 35 riparian forest sites and 5 upland forest sites were sampled to examine the typical vegetative characteristics of forest within the basin. In general, floristic quality and basal area decreased from south to north and % exotics increased. These findings provided data on riparian forest habitats along the river and can be used to assess future forest restoration efforts.;Streamflow has increased dramatically over the last 30 years in the Big Sioux basin (122% increase in mean annual flow) resulting in a highly significant, 62% increase in channel planform area, a 4% decrease in channel length and sinuosity, and a 69% increase in channel width. These channel adjustments were highly correlated with increasing mean annual flows and baseflow. To examine why these increases have occurred, the relative effects of land use change and climatic variation were examined, with findings from these analyses lending support towards land use change, specifically agricultural intensification, being a more important driver of increased streamflow and channel adjustments than changes in the climatic regime. These results are in line with findings from similar studies of agriculturally dominated basins throughout the Upper Midwest.;The final chapter examines potential pitfalls and uncertainties associated with using the Floristic Quality Assessment (FQA) across state boundaries. In total, results from 18 riparian and upland forest sites along the South Dakota/Iowa border were compared using Coefficients of Conservatism (C-values) from each state. Findings indicated that for the 141 observed species, South Dakota mean C-values were higher (3.57) than Iowa (2.50) and that there were 98 C-value divergences between states with 20 being equal to +/- 4. FQA values calculated using Iowa C-values were consistently lower than South Dakota and highlight the subjective nature of assigning C-values at the individual state level. This thesis provides several recommendations that would improve the utility of the FQA and allow it to become more widely standardized as a valuable ecological assessment tool.;This thesis provides baseline data critical to targeting current and future management efforts along the Big Sioux and provides an improved scientific understanding of ecological patterns in habitats along this valuable watershed in eastern South Dakota and northwestern Iowa.