When water is supplied by rainfall or snowmelt at a rate that is faster than it can either infiltrate or run off, it can cause floods in flat or low-lying areas. The excess builds up where it is, sometimes to dangerous depths. Surface soil can become immersed, which really stops invasion, where the water table is shallow, like a floodplain, or from extraordinary downpour from one or a progression of tempests.

Additionally, through frozen ground, rock, concrete, paving, and roofs, infiltration is sluggish to nonexistent. Due to the fact that the velocity of overland flow is dependent on the surface slope, areal flooding begins in flat areas like floodplains and local depressions that are not connected to a stream channel.

When precipitation exceeds evaporation, endorheic basins may experience areal flooding.
From the smallest ephemeral streams in humid regions to the world’s largest rivers and normally dry channels in arid climates, floods occur in all river and stream channels.

When overland flow occurs on tilled fields, it can cause a muddy flood in which sediments are carried as suspended matter or bed load by runoff and picked up by the water. Drainage obstructions like landslides, ice, debris, and beaver dams can either cause or exacerbate localized flooding.

The majority of the time, large rivers with large catchment areas experience slow-rising floods. Monsoons, prolonged rainfall, rapid snow melt, or tropical cyclones could all be the cause of the increase in flow. However, because large rivers may have large basins but small river channels, and rainfall can be extremely intense in smaller areas of those basins, they may experience rapid flooding events in dry climates.