Regulated dams in Missouri must have and maintain adequate freeboard, which is defined as the difference between the maximum water surface elevation (MWSEL) of the reservoir and the principal spillway crest. The MWSEL is determined for a dam by taking into account the hydrology of the land draining to the lake and the hydraulics of the spillway. As earthen dams settle over time, Dam and Reservoir Safety Program staff recommend that dams have more freeboard than is required (available freeboard). This is the difference between the low point on the dam crest and the principal spillway crest.