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The term "drainage area" is defined as the land area where precipitation falls off into creeks, streams, rivers, lakes, and reservoirs. It is a land feature that can be identified by tracing a line along the highest elevation between two areas on a map, often a ridge. Larger drainage basins, like the areas that drains into the Columbia River, contain many smaller drainage basins, often called watersheds. The drainage area for a river basin is measured in a horizontal plane enclosed by the drainage divide outlining the basin. In some cases river basins may have non-contributing sub-basins, or commonly called enclosed basins, where the runoff stays within the basin and not contributing to the larger basin surrounding the enclosed basin.
Historically drainage areas were demarcated using planimeters. More recently drainage areas are demarcated using Geographic Information System (GIS). The accuracy of the drainage area calculation depends on the map scale, technique, demarcation of the drainage divides, among other reasons. In addition the reporting of drainage areas may or may not include enclosed basins, and the use of the value in basin analysis should consider any diversion or return discharges within the larger basin or to/from the basin itself.
In some cases the drainage area will be reported as either "unknown" or "undetermined". This means either the drainage hasn't been determined, common with new stations where we will determine the area and report it future publications, or it means the drainage area has no relative meaning, where the station is a diversion, return, irrigation canal, lake, or similar hydrologic features where the streamflow or lake contents can not be related to a river basin and its drainage area.
Link to Web page on planimeters used with the permission of Dr. Robert Foote, Department of Mathematics and Computer Science, Wabash College.