According to the National Weather Service, a hydrograph is a plot of the water discharge over time. The overall purpose is to chart the flow of a body of water past a particular physical point over a period of time.
Water discharge and stage are two ways of measuring the way water flows in a river or other body of water. A single location is chosen and the rate at which water flows, typically expressed in cubic feet or cubic meters per second, is measured. Over time, information regarding patterns, changes in speed or the effect on the location or overall system can be graphed. These graphs are important for a number of reasons, including understanding urban runoff and sewer systems.