Introducing Geographic Information Systems with ArcGIS: A Workbook Approach to Learning GIS, 3rd Edition

Watersheds and Pour Points

A basin is an area that drains water and other substances carried by water to a common outlet as concentrated drainage. Other common terms for a basin are watershed, catchment, and contributing area. The contributing area is normally defined as the total area contributing water flow to a given outlet, also called a pour point.

A delineation of these areas is the output of the Basin tool calculation. The geographic line between two basins is referred to as a basin boundary or drainage divide. Such a line, as you might imagine, runs along ridge tops and other lines of relatively higher elevation.

An outlet, or pour point, is the point at which water flows out of an area. It is the lowest point along the boundary of the basin. The cells in the source raster are used as pour points above which the contributing area is determined. Source cells may be features such as dams or stream gauges for which you want to determine characteristics of the contributing area.

In Figure 8-36, the five watersheds correspond to five stream links. The arrows indicate the direction of flow from each cell; the cell shade indicates to which watershed the cell belongs.

FIGURE 8-36 The five watersheds correspond to the five stream links

Another ArcGIS tool that calculates areas that contribute water to a given point is the ArcToolbox Watershed tool. The distinction between them ...