Erosion by overbank runoff is caused when runoff from the flood plain becomes concentrated and enters the channel by running down a bank. Rills or even deep gullies often occur in this situation, particularly if the main stream channel bed has been lowered by general bed degradation or headcutting. Overbank runoff erosion is often triggered when artificial flow concentrations, such as the discharge from roadside ditches or parking lots, enter the channel. Overbank runoff gullies will dissect the bank face, and deposits of eroded material will create a delta or fan along the bank toe at the gully mouth.