Geology Reference
In-Depth Information
If a stream were at
equilibrium,
the stream would have the exact amount of energy
needed to transport all the sediment that is supplied to it. The streamflow would be
neither eroding nor depositing sediment, just moving supplied sediments to the ocean.
A stream in this state of equilibrium is called a
graded stream.
The changes in velocity and competence that take place when base level is raised or
lowered are a response of the stream seeking equilibrium; it wants to be a graded
stream. However, the idea of a graded stream is conceptual; it doesn't often occur in real
life. Streams seldom, if ever, become graded. And any stream that does become graded
does not remain at equilibrium for very long because too many other factors are con-
stantly changing, such as sediment input, water input, sea level changes, tectonic
changes, and manmade obstacles.
Leaving Their Mark: How Streams Create
Landforms
All this moving of sediment by streams leaves its mark on the landscape. Every time a
sediment particle is picked up or set down by a stream, the surface of the earth changes
just a little bit. In the previous sections I focus on flowing water and its characteristics.
In this section I describe the sediments that are left behind, including what types of
landforms streams leave on the landscape and how sediment erosion and deposition
create features and shape the surface of the earth.
Draining the basin
As flowing water moves toward lower elevations and removes sediments along the way,
it etches a pattern into the landscape. What the pattern looks like is determined by the
characteristics (such as composition and structure) of the rocks and sediments that the
stream flows over. These
drainage patterns
are described from a bird's-eye view: a view
from above the land surface. Figure 12-4 illustrates the four most common drainage pat-
terns:
Figure 12-4:
(a)
Dendritic drainage;
(b) rectilinear
drainage; (c) radial
drainage; and (d)
trellis drainage.
