The results of a literature review on resistance to sheet flow are presented. The effects of surface roughness, rainfall, and vegetation are considered. At least in the case of laminar flow, it is found that the total flow resistance is the sum of the contributions of individual effects. The friction factor for the surface roughness effect in laminar flow is directly proportional to the relative roughness and varies inversely with the Reynolds number. A power function of rainfall intensity in laminar flow can represent the effect of rainfall on the product of friction factor and Reynolds number. For turbulent flow, however, the friction factor depends on the surface conditions which are partitioned into smooth, transition, and fully rough. The analysis of flow through vegetation is more complex and calls for further studies. For densely vegetated surfaces, the Darcy-Weisbach friction factor is shown to decrease significantly at Reynolds number well beyond the critical value of Re = 2000 for smooth surfaces. In some cases, the flow behaved as laminar flow at Re = 100,000.