My research focuses on the complex interactions and constraints that govern the evolution of natural landscapes, including surfaces of other planets. This research combines field studies, theory, simulation modeling, and quantitative analysis. Field studies have included evolution of channels in badlands, the natural regime and man’s influence on the Colorado River in the Grand Canyon, and the role of groundwater sapping in erosion of sandstone canyons in the southwest U.S.. Simulation models have been developed for barchan dune equilibrium, stream network development, river meandering, drainage basin evolution, and scarp retreat by groundwater sapping. Theoretical modeling includes the role of equilibrium and thresholds in geomorphic systems and controls on drainage network geometry. Planetary research has included studies of eolian, polar, and fluvial processes on Mars. Recent student research on terrestrial landscapes focuses on landform evolution and river sedimentation on the Virginia Coastal Plain, thgeomorphic effects of large floods, hydrology and lakes of the U.S. Great Basin, and the history of debris flow erosion of the Virginia Blue Ridge. Planetary studies by students include erosion of Martian craters, deposition and erosion of thick sedimentary deposits on the rim of the Martian Hellas impact basin, and studies of drainage network evolution and lakes on Mars early in its history.