Laboratory Manual for Introductory Geology 4e

elevation all along its profile. Over time, the gradient lessens as erosion lowers
the headwaters area to elevations closer to that of the mouth. The stream still has
enough energy to alter the landscape, however, and that energy is expended on lateral
erosion. The valley then widens by a combination of mass wasting and meandering.
Even when headwaters are lowered to nearly the same elevation as the mouth, a
stream still has energy for geologic work, but because it cannot cut vertically below
its base level, most of its energy at this stage is expended on lateral erosion.
Initially, a stream meanders within narrow valley walls, but over time, it erodes
those walls farther and farther, eventually carving a very wide valley. As its gradient
decreases, a stream redistributes sediment that it has deposited, moving it back and
forth across the floodplain.
Although most streams follow this sequence of progressively lowering their gradients
and therefore changing their erosional behavior, not all do so. Some streams
have very gentle gradients and meander widely from the moment they begin to flow.
Streams on the Atlantic and Gulf coastal plains are good examples of this kind of
behavior.
As a stream cuts downward, it sometimes encounters rock that is much more resistant
to erosion than the rocks downstream. The stream will eventually cut through
the resistant rock, but until it does, there will be an irregularity (a nick point) in its
concave-up longitudinal profile. In small streams, nick points may appear as sets of
EXERCISE 13.3
Interpreting Stream Behavior
Name:
Course:
Section:
Date:
FIGURES 13.9, 13.10, and 13.11 on the following pages show three meandering streams, each of which balances energy
use differently between vertical and lateral erosion.
(a) From their valley width/channel width ratios alone, which stream would you expect to have the steepest gradient? The
gentlest gradient? Explain your reasoning. (You don’t have to calculate the gradients, but if it helps, feel free to do so.)
(b) Which stream do you think is doing the most vertical erosion? The least? Explain.
(c) For each of the three maps, describe features that indicate former positions of the stream channels. Do all three
maps show this information? If not, explain why.
(d) Label one example of each of the following stream erosional and depositional features on Figure 13.9: valley,
channel, meander, point bar, oxbow lake, meander scar.
(e) Indicate on Figure 13.11 where the velocity of the Arkansas River is the greatest and where it is the least.
338 CHAPTER 13 LANDSCAPES FORMED BY STREAMS