A. Surface water erodes through river and stream processes.

B. Coastal ocean waters erode through wave processes.

C. Wind erodes through wind processes (dunes).

D. Gravity erodes through mass wasting processes.

E. Ice erodes through glacial processes.

A. Most water enters the atmosphere by evaporating from oceans. Some water evaporates from the land.

B. The water vapor eventually forms clouds which results in precipitation.

C. More than half the water that falls to the Earth quickly returns to the atmosphere through evaporation.

D. What's left either seeps into the ground to become ground water or flows over the land's surface as streams that eventually flow to lakes or oceans.

III. What affects the velocity of water in a river?

A. Gradient. Gradient is the downhill slope of a river. You can think of a gradient in terms of the number of feet a river drops in a mile along its course. Gradients can best be seen along the profile of a river. A profile is a cross section along the path of a river, from its source to the river delta.

1. Gradients are highest nearer the river source and lowest nearest the delta.

2. There is a theoretical limit to how low a river will erode. This theoretical limit is called the base level. A river will not erode below the base level. At base level, the gradient is zero, or horizontal.

3. At higher gradients erosion is dominant. At the lowest gradients, deposition, rather than erosion is dominant.

B. Channel shape and roughness affect a river's velocity.

1. Generally a narrower, more circular river channel allows faster flow of water. Broader flat channels tend to slow a river down.

2. Smoother channels also allow faster flow of water; rougher channels slow water flow.

3. Generally, anything that increases the surface area of the channel, against which the water flows will tend to slow the flow because of the increase in friction. Anything that reduces surface area also reduces the amount of friction and causes faster flow.

C. At different areas in a channel, water flows at different speeds.

1. Toward the middle of a river, water tends to flow fastest; toward the margins of the river it tends to flow slowest.

2. In a meandering river, water will tend to flow fastest along the outside bend of a meander, and slowest on the inside bend.

IV. River erosion

A. Rivers move material down stream in one of three ways.

1. Water may dissolve rock as a part of chemical weathering and carry the material downstream. This is the dissolved load.

2. Material may be suspended in the water due to the continuous turbulent motion of the water. This is the suspended load. It typically includes silt and clay.

3. Material may be moved downstream incrementally, in short spurts along the bottom of the river bed. This is called the bed load.

B. Styles of erosion.

1. At higher gradients, downward, vertical erosion is more dominant. This produces V-shaped valleys.

2. As gradients decrease, downward erosion is less dominant, and lateral erosion begins to dominate, forming meanders.

3. Gullies can erode and extend back toward the headwaters overtime due to the process of headward erosion. If two streams on opposite sides of a ridge or divide erode toward each other, one stream may end up "capturing" the other. This is called stream piracy.

4. A graded stream is one in which a stream profile has a smooth, concave shape. This can be thought of as an equilibrium condition. An ungraded stream has an uneven, "bumpy" profile.

C. Erosional features -- the further above base level that a river is, the more likely to find one or more of the following features:

1. Slot canyons are narrow canyons that are usually preserved only in dry climates.

2. Incised meanders form when a mature stream with developed meanders is either uplifted, or the base level drops down further. Meanders deepen and become entrenched.

3. Stream terraces form in a similar fashion. Rivers with floodplains are either uplifted or have their base level droped so that the river cuts down further, leaving a set of shelves or terraces rising above the newer river and floodplain.

V. River features

A. Meanders are the s-like bends in a river that's eroding and depositing more laterally (see river evolution).

1. As suggested above, in response to a streams velocity, a river will tend to erode along the outside of a meander, and deposit on the inside. The feature produced on the inside of a meander is a point bar. The eroding outer side of a bend is called a cut bank.

2. This causes the meander to become more exaggerated and pronounced. Eventually, the meander will get cut off, isolating the meander from the river. This isolated meander is called an oxbow lake.

B. Flood plains are the broad, lowing-lying plains that lie on either side of a river.

1. During flood stage, the river rises out of its banks, and into the flood plain. This results in a broad, flat channel for the flooded river.

2. Remember the effect of channel shape on river velocity? Well, the decreased velocity of the river in flood stage allows river sediment to be deposited in the flood plains.

3. The deposits are greatest nearest to the normal river channel. This build-up of sediments adjacent to the river creates natural levees.

4. Natural and artificial levees may cause standing water to be left behind after a river flooding because the water does not drain back to the original river channel very easily. This creates backswamps. Backswamps are a type of natural wetlands area that has disappeared rapidly as civil engineering projects have sought to avoid flooding of the flood plains.

5. A tributary may run for miles in the flood plain along side the main river, separated from it by a natural levee. This is called a yazoo tributary.

A. Various features have been built to protect communities from flooding. These include

1. Dams

2. Levees

3. Flood walls

4. Bypasses - this opens up the flood plain to the river.

B. Urbanization tends to increase the amount of water runoff, which intensifies and amplifies flooding.

1. Because urbanization tends to reduce the density of vegetation, fewer plants are available to take up watr.

2. Pavement, sidewalks, and building footprints usually form a barrier to the percolation of water into the subsurface.

Last update 11/15/2000
Webpage designed by Hiram Jackson.
Contact Geology webmanager, Hiram Jackson, at jacksoh@crc.losrios.edu