a graph of the water level or rate of flow of a body of water as a function of time, showing the seasonal change.
Origin of hydrograph
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a graph showing the seasonal variation in the level of a body of water, from which its velocity and discharge can be calculated
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012