Catchment morphology and drainage density are strongly influenced by hillslope processes. The consequences of several different hillslope process laws are explored in a series of experiments with a numerical model of drainage basin evolution. Five different models are considered, including a simple diffusive-advective process transition, a runoff generation threshold, an erosion threshold, and two types of threshold-activated landsliding. These different hillslope processes alter both the visual appearance of the landscape and the predicted relationship between slope and contributing area. On the basis of the different threshold theories, we derive expressions for the relationships between drainage density and environmental factors such as rainfall, relief, and mean erosion rate. These relationships vary depending on the dominant hillslope threshold. In particular, the sign of the predicted relationship between drainage density and relief is positive in semiarid, low-relief landscapes and negative in humid landscapes dominated by a saturation threshold and/or in high-relief landscapes dominated by simple threshold landsliding.