Modeling incision of tributaries from Missouri River degradation: Gavins Point Dam to Platte River confluence

This research uses multivariate analysis to spatially assess channel network rejuvenation following a base-level drop. It develops and tests a GIS model to estimate the spatial distribution of channel incision with a focus on the loess-dominated tributary watersheds of the Missouri River in Nebraska and Iowa. In this homogenous-geologic setting, it was found that a multiplicative combination of base-level drop and a flow-length parameter (position ratio) performed better than any other tested combination of variables, explaining 73% of the variance in observed upstream streambed-elevation change and produced an overall predication sums of squares error of less than 0.4 meters. The position ratio method requires only a digital elevation model. This permits model extrapolation using map algebra to all stream sites responding to a base-level drop.; This study focused on the degraded tributaries of the Missouri River from Gavins Point Dam (RM 811.4) to the confluence with Platte River (RM 594.60). The 280 km study reach of the Missouri River is shown to have degraded its bed by 0 to 4 meters since the closure of the Gavins Point Dam in 1954. The tributary streams to the Missouri River were selected because they provided several fundamental constraints on the problem including: (1) these loess-dominated stream channels were free from aggradational feedbacks common to coarser-grained alluvial channels, (2) complete rejuvenation of fluvial systems was documented to occur in less than 30 years, (3) an extensive historical record of streambed elevations has been assembled for these streams, (4) known sources of base-level drop were identifiable both in time and location, and (5) varied environmental conditions exist related to basin size, slope, flow lengths and landuse patterns.; While the statistical linkage between base-level lowering and upstream-streambed lowering was an important finding, it was also expected. The two most significant contributions made by this study were (1) the creation of a new spatial model for the assessment of change in streambed elevation for networks responding to a base-level drop and (2) the production of a GIS layer (map) of expected tributary incision due to Middle Missouri River degradation.