Effects of Variables on the Stream Channel

This section explains how each of the four variables affects the balance of forces in the stream channel. It is the interaction of all four variables that determines whether or not the stream channel will be in a stable dynamic equilibrium.

By making a selection in each of the drop down boxes you will initialize the model for the variables. A reminder of the relative effect of the options in each variable is indicated by the number of ">" symbols in the box.

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The balance between erosion and deposition depends in part on the energy the water possesses, energy which allows it to move sediments along the stream bed. It also depends on the resistance of the sediments to being moved. Anything that affects these variables can alter the erosion-depositon balance.
 

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Sediment Size:		As the size of the sediment particles gets larger it takes more energy to move them. The smallest clay and silt particles might easily be moved under conditons where the largest boulders would remain in place. If the other conditions remain stable, increasing the average size of particles in the sediments entering the stream segment will tip the balance toward deposition.
Clay and Silt   > Sand   >> Gravel   >>> Cobbles   >>>> Boulders   >>>>>
Sediment Load:		The stream has a limited amount of energy to move the load along. As the total amount of sediment entering the stream channel increases, the balance will tilt toward depositon because there is insufficient energy to move all of it along.
Very Light   > Light   >> Intermediate   >>> Heavy   >>>> Very Heavy   >>>>>
Stream Slope:		If the amount of energy in the stream channel changes, it can affect the ability of the stream to transport its load. As the slope of the channel increases the velocity of the water increases and it therefore has more energy. If the other conditions remain stable increasing the slope of the stream channel will tilt the balance toward erosion.
Almost Flat, < 0.5%   > Slight, ~ 1%   >> Moderate, ~ 2%   >>> Steep, ~ 4%   >>>> Very Steep, 5 to 10%   >>>>>
Stream Discharge:		Discharge is the amount of water flowing in the stream channel. The discharge affects the ability of the stream channel to transport its load. At a given velocity, the greater the discharge the greater the amount of sediment that can be transported. As the discharge increases the balance will tilt toward erosion.
Very Light   > Light   >> Intermediate   >>> Heavy   >>>> Very Heavy   >>>>>

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These are the fundamental variables that affect the balance between deposition and erosion. Of course there are other differences between streams which are eroding and depositing. For example, a stream which is depositing will usually be wider and shallower than an eroding stream. (After you finish working with the model you will be able to figure out why this is so.) However, differences such as this are the result of the erosion and/or deposition rather than the cause of it.

This completes the discussion of the effects of each of these variables. Now let's look at how to work the model.

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