Dams are ubiquitous features of most river systems whose construction and/ or removal can cause profound changes to the coupled hydrologic and geomorphologic watershed system. As a result of their intended function as water retention structures, dams disrupt the natural downstream flow of water and sediment in channels and the movement of aquatic species through the system. While the downstream impacts of dam construction on channel hydraulics and morphology have been investigated extensively, the upstream hydrologic and geomorphologic impacts are less well studied. Upstream dam impacts are caused by a rise in hydrologic base level and include (i) altering surface water-groundwater interactions, (ii) changing water table depths and soil-water content distributions, and (iii) a transition in the sediment regime from transport to deposition. Removal of aging dams is a growing practice whose impacts on the upstream watershed are largely unstudied. This dissertation employs numerical simulation to examine the effects of dam construction and removal. The hydrologic-response model used in this effort is the Integrated Hydrology Model (InHM), a fully-coupled physics-based algorithm for 3D variably-saturated flow in the subsurface and 2D depth-integrated flow over the surface. In the first phase of this study a process-based hydrologically-driven sedimenttransport algorithm was added to InHM and tested against two plot-scale datasets of artificial rainfall, runoff, and erosion. The rainsplash and hydraulic erosion components of the new algorithm are able to reproduce observed sediment discharge data. A relationship between groundcover percentage and the rainsplash erosion coefficient was established. The interplay between simulated hydraulic forces and erosion/ deposition processes along a longitudinal transect of the plot was examined. The second phase of this study focuses on R-5, a small experimental rangeland catchment in Oklahoma. First the extensive datasets of climate, spatially-variable infiltration and soil-water content, and catchment integrated runoff and sediment discharge were analyzed to gain a better understanding of how R-5 functions hydrologically. Factors influencing runoff generation at R-5 were identified, as were v possible long-term trends in catchment response behavior related to land management practices. Then, based on an improved conceptual model and boundary-value problem, InHM was used to simulate continuous hydrologic response and event-based sediment transport at R-5 for a six year period. These long-term simulations effectively demonstrate the application of the improved InHM, setting the stage for the third phase of the study. In the third phase of this study the physics-based simulation approach was extended to the watershed scale to examine in a concept-development mode the impacts of dam construction and removal at the Searsville Lake watershed. The available data used for boundary-value problem parameterization include digital topography, geologic and soils maps, and satellite-derived land cover. Additional field data on soil-water content, pressure head, sediment concentration, and hydraulic conductivity were collected over a one year period at various locations within the watershed. The results from the InHM simulations indicate that (i) dam impacts are largely confined to the area immediately beneath and surrounding the reservoir, (ii) impacts on peak discharge are substantial, (iii) evapotranspiration and total water discharge is only mildly impacted by the presence of the dam, (iv) sediment discharge is more severely impacted by the presence of the dam, and (v) climatic variability in general supersedes the impacts caused by the dam itself. This study demonstrates that an integrated, physics-based simulation approach can provide insight into the complex surface and subsurface hydrologic and geomorphologic processes that are impacted by dams and dam removal. Future studies on dammed systems can follow this approach provided that sufficient information is available for construction of the coupled surface-subsurface boundary-value problem.