When water flows over the land surface and through stream channels, it affects the landscape in significant ways. Shallow surface flow erodes soil and carries the material overland to headwater streams. Steep headwater streams have high flow velocity that leads to stream bed erosion. Eroded surface soil and bed material can be carried by the stream flow to lower reaches of the watershed. Sediment is often deposited in the lower reaches where the flow velocity slows. Reservoirs in the watershed will trap some or most of the sediment that enters with the inflow. Diversion structures can also affect the movements of sediment through the watershed. Erosion and Sediment Transport lead to a number of problems best evaluated in conjunction with hydrologic simulation.