<P> If the soil is saturated, or if the rainfall rate is greater than the rate at which water can infiltrate into the soil, surface runoff occurs . If the runoff has sufficient flow energy, it will transport loosened soil particles (sediment) down the slope . Sheet erosion is the transport of loosened soil particles by overland flow . </P> <P> 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 hillslopes . Generally, where water erosion rates on disturbed upland areas are greatest, rills are active . Flow depths in rills are typically of the order of a few centimetres (about an inch) or less and along - channel slopes may be quite steep . This means that rills exhibit hydraulic physics very different from water flowing through the deeper, wider channels of streams and rivers . </P> <P> 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 . </P> <P> Valley or stream erosion occurs with continued water flow along a linear feature . The erosion is both downward, deepening the valley, and headward, extending the valley into the hillside, creating head cuts and steep banks . In the earliest stage of stream erosion, the erosive activity is dominantly vertical, the valleys have a typical V cross-section and the stream gradient is relatively steep . When some base level is reached, the erosive activity switches to lateral erosion, which widens the valley floor and creates a narrow floodplain . The stream gradient becomes nearly flat, and lateral deposition of sediments becomes important as the stream meanders across the valley floor . In all stages of stream erosion, by far the most erosion occurs during times of flood, when more and faster - moving water is available to carry a larger sediment load . In such processes, it is not the water alone that erodes: suspended abrasive particles, pebbles, and boulders can also act erosively as they traverse a surface, in a process known as traction . </P>

2. how did the rate of weathering and erosion change after the evolution of land plants