Line drawn on a map or chart in lieu of a mean high water line (MHWL) or the mean water level line (MWLL) in areas where either may be obscured by marsh, mangrove, cypress, or other type of marine vegetation. This line represents the intersection of the appropriate datum on the outer limits of vegetation and appears to the navigator as the shoreline. (Ellis 1978)
Members of the US Geological Survey were filmed while out on the Missouri River at Williston, North Dakota, USA, performing a hydrographic survey to monitor the state of riverbed erosion. They were using a multibeam echo sounder which transmits sound energy and analyses the return signal (echo) that has bounced off the riverbed or other objects. Multibeam sonars emit sound waves from directly beneath a ship's hull to produce fan-shaped coverage of the riverbed.
