Geology Reference
In-Depth Information
As a stream channel erodes, it deepens. But at some point, the channel cannot be any
deeper; this point is called the
ultimate baselevel
of the stream and is the same eleva-
tion as sea level. If the channel became any deeper than its ultimate base level, the
stream would need to flow uphill to continue toward the ocean, which just isn't pos-
sible.
Sometimes before a stream reaches its ultimate base level, it reaches a
temporary base
level
: a point at which it cannot erode the channel any deeper but is still above its ulti-
mate base level. A temporary base level is reached when a stream channel hits a layer of
rock that is too strong to be eroded. If you have ever seen a waterfall, you have seen a
stream flowing over the edge of its temporary base level. Why is it called
temporary
if
the stream can't erode the channel any further? Because if the resistant rock is removed,
the stream continues to try to erode the channel down to its ultimate base level.
Seeking Equilibrium after Changes in Base
Level
The surface of the earth is constantly moving up and down as a result of earthquakes or
continental plate collisions (which I describe in Chapter 9) or isostatic adjustment (see
Chapters 10 and 13). As a result, a stream's base level is constantly being raised or
lowered. The change in base level can happen suddenly, such as in response to an earth-
quake, or it may take hundreds or thousands of years.
When the base level of a stream is raised, the stream's flow slows down. Slower move-
ment (lower velocity) means that the stream can no longer carry particles as large as it
was carrying before. The result is
deposition
by the stream: The larger particles are left
behind. (I describe depositional landforms created by streams in the next section of this
chapter.) Raising the base level can happen when sea level changes, after tectonic uplift
(see Chapter 9), or even when a dam is built.
The opposite occurs if a stream's base level is lowered: The stream's velocity increases
as the water flows more quickly downslope. The faster movement means the stream can
pick up and carry larger particles than it was carrying before. The result is increased
erosion.
Raising a stream's base level leads to deposition, and lowering it leads to
erosion.
