When the top layer of soil is moist, there will be more runoff then if the soil moisture content is low. Vegetation - Vegetative cover may slow the runoff from rainfall. As vegetation takes-in water, the runoff is retarded. Topography - Runoff varies as the terrain varies. A mountainous terrain will have a faster runoff rate than one of a flat terrain.
Frozen ground is more impervious to rainfall.
Flooding Flooding kills more people and causes more damage per year on average the any other natural disaster including hurricanes and tornadoes. There are many different forms of flooding, but only the most common and most destructive kinds will be covered. When it rains enough to cause runoff, the runoff flows into the rivers and the water levels begin to rise. The river will rise above its banks and flood the surrounding area when more water than the river can hold flows into it. The rain can fall over an area far upstream and be carried downstream, therefore flooding areas where no rain had fallen.