Surface runoff occurs if the soil is saturated or if the rainfall rate is greater than the rate at which water can infiltrate into the soil. The runoff will transport loosened soil particles down the slope if it has sufficient flow energy. Sheet erosion is the transport of loosened soil particles by overland flow. Rill erosion refers to the development of small ephemeral concentrated flow paths which function as both sediment source and sediment delivery systems for erosion on hill slopes. Flow depths in rills are typically of the order of a few centimetres or less and along the channel slopes might be quite steep. This means that rills exhibit hydraulic physics very different from water flowing through the deeper wider channels of streams and rivers. Gully erosion occurs when runoff water accumulates and rapidly flows in narrow channels during or immediately after heavy rains or melting snow removing soil to a considerable depth. Valley or stream erosion occurs with continued water flow along with a linear feature. The erosion is both downward deepening the valley and head ward extending the valley into the hillside creating head cuts and steep banks.