Naturalists at Large: Rivers watersheds

The streams found in

mountains are likely to have

formed relatively recently.

These young streams

typically have V-shaped valleys.

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.

How long do you think it took to form the Grand Canyon?

Most scientists believe it took 1 to 3 million

years!

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.