Recall that most of the precipitation that falls on land either entens the soil (infiltration) or remains at the surface, moving , downslope as runoff The amount of water that runs off rather than soaking into ground depends on several factors(1) intensiry and duration of rainfall. (2) amount of water already in the soil(3)nature of the surface material(4)slope of the land, and(5)the extent and type of the vegetation. When the surface material is highly impermeable, or when it becomes saturated, runoff is the dominant process. Runoff is also high in urban areas because large areas are covered by roads and parking lots. Runoff initially flows in broad, thin sheets. This unconfined flow eventually develops threads of current that form tiny channels called rills. Rills meet to form gullies, which join to form streams. At first streams are small, but as one intersects another, larger and larger ones form. Eventually rivers develop that carry water from a broad region.