MadSci Network: Earth Sciences
Query:

Re: How are the headwaters of a river determined or defined?

Date: Sat Jun 19 12:23:02 1999
Posted By: Diane Hanley, Staff, geologist , Science Center of NH
Area of science: Earth Sciences
ID: 923444935.Es
Message:

Dear Les,

Simply put, headwaters are defined as the furthest upstream tributaries of a river. Most rivers haver multiple tributaries which coalesce in the downstream direction becoming larger and larger tributaries. River systems like the Mississippi River usually have a tree-like branching pattern to them when seen from the air or on a map. Click here to see the upper portion of the Mississippi. Each branch of the tree pattern is a tributary with water flowing from the top 'branches' downstream to the 'trunk'. In this case, tributaries from Lake Itasca coalesce with the Schoolcraft River, in turn coalescing with the Turtle River, then Leech Lake River, etc. Ultimately all the tributaries shown flow into the main stem of the river and continue past St. Cloud located at the bottom of the map. (Each river shown on the map has its own headwaters but are not shown because the scale of the map can't accomodate that kind of detail. )

How do you know you are at the furthest upstream tributary or headwaters? They are the first collection points for rainwater flowing across the ground surface. Once rainwater can no longer soak into the soil, either because the soil is saturated or it is raining very hard, it flows downslope as a sheet of water. You can see this happen in parking lots during a heavy rain. The sheet of water flows downslope - geologists like to call this sheet flow - and is directed into small streams. These first, small streams are the upper tributaries, or headwaters, of a river. Therefore, headwaters are not the highest elevational point in a watershed; they are fed by sheet flow that originates further upslope!

Drainage divides are imaginary lines that define the surface area, or watershed, drained by a stream. The drainage divide indicates where sheet flow runs off in opposite directions, feeding separate streams. Divides are not always located on ridges. In fact, the divide often lies on a surface of low relief. Imagine a steep, roughly oval watershed with the main stem of a river flowing down its center and two tributaries, one on each side of the river valley. The drainage divide for the river and its tributaries is drawn along the high points of the bowl in the shape of an oval as we would expect. But what would the drainage divide look like for just the left tributary? Water from the right side of the watershed does not contribute to the left tributary (it would have to travel upslope to do so) so that portion of the watershed must be excluded. The result is a drainage divide that basically cuts the the original watershed in half and and is drawn down the center of the main stem of the river!

More information about the headwaters of the Mississippi River is available at the Headwaters' River Watch.


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