the entire area of land that a river drains is called
http://water.usgs.gov/edu/watershed.html
The entire area of land that a river drains is called its "watershed" or "river basin." This includes all the land and water bodies that contribute water to the river, such as tributaries, streams, and other water sources. To determine the watershed of a specific river, you can use various methods:
1. Topographic maps: By examining the contours and elevation lines on topographic maps, you can identify the boundaries of a river's watershed. The highest points in the landscape are typically the boundaries, as water flows downhill from these points to form the river.
2. Satellite imagery: Using geographic information systems (GIS) and satellite imagery, you can visually analyze the flow of water, identify river channels, and determine the extent of the watershed.
3. Hydrological models: Computer models and hydrological tools can simulate water flow and predict the area that contributes to a specific river. These models take into account factors such as rainfall patterns, land cover, and soil permeability.
It's worth noting that watersheds can vary in size, from small streams to large river systems that span multiple states or even countries. They play a crucial role in the water cycle, as they collect rainfall and deliver it to the river, affecting the availability and quality of water resources in a given region.