Features of landscape morphology—including slope, curvature, and drainage dissection—are important controls on runoff generation in upland landscapes. Over long timescales, runoff plays an essential role in shaping these same features through surface erosion. This feedback between erosion and runoff generation suggests that modeling long-term landscape evolution together with dynamic runoff generation could provide insight into hydrological function. Here we examine the emergence of variable source area runoff generation in a new coupled hydro-geomorphic model that accounts for water balance partitioning between surface flow, subsurface flow, and evapotranspiration as landscapes evolve over millions of years. We derive a minimal set of dimensionless numbers that provide insight into how hydrologic and geomorphic parameters together affect landscapes. We find an inverse relationship between the dimensionless local relief and the fraction of the landscape that produces saturation excess overland flow, in agreement with the synthesis described in the “Dunne Diagram.’ Furthermore, we find an inverse, nonlinear relationship between the Hillslope number, which describes topographic relief relative to aquifer thickness, and the proportion of the landscape that variably saturated. Certain parameter combinations produce features with wide valley bottom wetlands and nondendritic, diamond-shaped drainage networks, which cannot be produced by simple landscape evolution models alone. With these results, we demonstrate the power of coupled hydrogeomorphic models for generating new insights into hydrological processes, and also suggest that subsurface hydrology may be integral for modeling aspects of long-term landscape evolution.