The distance, measured along the axis or centerline crest of the dam at the top level of the main body of the dam or of the roadway surface on the crest, from abutment contact to abutment contact exclusive of an abutment spillway; provided that, if the spillway lies wholly within the dam and not in any area especially excavated for the spillway, the length includes the spillway. The length along the top of a dam. This also includes the spillway, powerplant, navigation lock, fish pass, etc., where these form part of the length of the dam. If detached from the dam, these structures should not be included.