Rivers & Watersheds
What is a watershed? A watershed is an area of land from which all runoff drains, or 'sheds' to the same river, lake, or other body of water.
A divide, areas of high elevation, separate watersheds. • A watershed is the land that water flows across or through on its way to a stream, lake or wetland. • Areas of higher elevation, called divides, separate watersheds
The streams found in mountains are likely to have formed relatively recently.
The rivers tend to move fast in their upstream portions, although, some are slowed by the large rocks and boulders in the river channel.
The young mountain streams tend to have a steep slope or gradient. Slope or Gradient
Rivers move a great deal of material. The faster the water moves, the larger material that can be carried down stream.
Fast moving upstream portions can often move rocks and boulders. Downstream portions usually only transport small silts and clays.
This process tends to sort materials by size. Have you ever wondered why we have so much silt and clay in our soil?
Valleys with very steep, almost vertical sides are called canyons or gorges.
A stream or river can not cut its bed any lower than the body of water into which it flows. As the stream approaches this base level, the slope and speed of the stream decrease.
The river channel becomes wider, deeper, and the volume is bigger as there are more tributaries contributing to the volume.
The river valley also changes at this point. It becomes more open, more of a U shape with a wider river floor.
The lower or downstream portion of the river is at its deepest, widest and slowest speed. It also has the largest volume and a very gentle gradient.
The river valley tends to be broad, flat and bounded by bluffs. These bottom lands are often areas that flood.
Lower portions of rivers often start to meander or form a s-shape river pattern.
Erosion is greater on the outside of the bend, deposition more on the inside.
The river can eventually cut through the meander, leaving a straighter section and an ox-bow lake.
To view an animation of this process click on this web site. http://www.school-portal.co.uk/GroupDownloadFile.asp?file=21606
Rivers deposit the sediment they have carried as they slow down. This sediment load will often form deltas as the river flows into the quiet waters of a bay or gulf.
The river tends to be split into channels by its own deposits as it drops more sediment. As the deposits grow, they resemble the Greek letter ▲(delta).
Alluvial Fans An alluvial fan differs from a delta in several ways. The deposit is formed on land, not in water.
Alluvial Fans Also, the sediments of these deposits are coarse sands and gravels, rather than fine silts and clays of the deltas.
Watershed A watershed, or drainage basin includes all of the land that drains into a river or bay either directly or through its tributaries.
Watershed The high land that separates one river valley or watershed from the next, is called the divide.
Texas river basins • We live in the San Jacinto river basin. • Our field trip will be to the Brazos river.