River source
Geographic point where a river's flow begins.
A river source, also called a headwater, is the geographical point where a river or stream begins. It is the location where surface runoff from rainwater, meltwater, or spring water first accumulates into a flowing channel of water. Each tributary of a river has its own headwater, and the most distant headwater along the longest watercourse from the mouth is commonly considered the true source of the main river.
- Definition
- The most distant point in a drainage basin from which water runs year-around (perennially) or could possibly flow ephemerally
- Alternative definition
- The headwaters of the stream commonly known as the source stream
- Key authority
- United States Geological Survey (USGS)
- Alternative authority
- National Geographic Society, Andrew Johnston (Smithsonian Institution)
- Common practice
- Use the longest tributary or stem as the source, regardless of local name
- Exception
- The Missouri River's source is sometimes given as the confluence of the Madison and Jefferson rivers, following Lewis and Clark's naming convention
Lore & Background
The United States Geological Survey (USGS) states that a river's length may be considered either as the distance from the mouth to the most distant headwater source (irrespective of stream name) or to the headwaters of the stream commonly known as the source stream. Under the first definition, the USGS uses the combined Missouri–lower Mississippi length in lists of world rivers, even though it sometimes considers the Missouri a tributary of the Mississippi. Most geographic authorities follow the longest tributary to identify the source, regardless of local map names.
Reader's Guide
The definition of a river source is significant because it determines the measured length of a river and its ranking among the world's rivers. For example, National Geographic and virtually every other geographic authority define the source of the Nile not as Lake Victoria's outlet but as the source of the Kagera River, the largest river flowing into the lake. This adds over 900 km to the Nile's length. Similarly, the Amazon's source is determined by the longest watercourse, even though the river changes names numerous times. However, alternative definitions exist: the Missouri River's source is sometimes given as the confluence of the Madison and Jefferson rivers, following Lewis and Clark's naming convention, rather than the source of its longest tributary. The US Army Corps of Engineers notes that geographers generally follow the longest tributary, which would place the Missouri's source well upstream of that confluence. A river source is never a confluence or a lake (except lakes with no inflows), though these often provide the starting point for a named river segment.
Did You Know?
- The source of the River Tees is marshland.
- Headwaters are often small streams with cool waters because of shade and recently melted ice or snow.
- The source of the Colorado River is at the Continental Divide separating the Atlantic Ocean and Pacific Ocean watersheds of North America.
- The verb 'rise' can be used to express the general region of a river's source, e.g., 'The River Thames rises in Gloucestershire.'
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