A two-dimensional, axially symmetric, unsaturated flow model was developed to test hypotheses about geologic and hydrologic controls on the movement of water through the thick, heterogeneous, unsaturated zone underlying Oro Grande Wash in the Mojave Desert, California. Heterogeneity within the unsaturated zone was simulated with multiple realizations of subsurface geology estimated on the basis of transition probability/Markov chain statistics. Model results show lateral spreading of water away from the wash was best approximated by realizations that include thin, horizontally extensive clay layers that impede the downward movement of water. There was a wide range in model responses for these realizations, and the movement of water through unsaturated zones containing thin, horizontally extensive clay layers may be more difficult to predict than water movement through unsaturated zones where clay layers are less extensive. For realizations having less extensive clay layers, the range of model responses decreased with time, and model results became increasingly similar as water encountered larger volumes of material.