Laboratory Manual for Introductory Geology 4e

EXERCISE 13.1
Differences between Streams
Name:
Course:
Section:
Date:
The basic principles listed above control how all streams work, but those principles may be displayed in different ways
by different streams. The result is that streams like those in Figure 13.1a and Figure 13.1b may appear to be acting quite
differently from one another. Look carefully at the streams in Figure 13.1 and describe as many differences as you can.
The following questions will help, but do not necessarily cover all the ways in which the Yellowstone River and River
Cuckmere differ.
(a) Which stream has the wider channel?
(b) Which stream has the broader valley?
(c) Which stream has the more clearly developed valley walls?
(d) Describe the relationship between valley width and channel width for both streams.
(e) Which stream has the straighter channel?
Which has a more sinuous (meandering) channel?
(f) Which stream appears to be flowing faster?
determine this?
What evidence did you use to
(g) Which stream appears to be flowing more steeply downhill?
A brief lesson in stream anatomy will help to explain stream erosion and deposition.
A stream begins at its headwaters (or head), and the point at which it ends—by
flowing into another stream, the ocean, or a topographic low point—is called its
mouth. The headwaters of the Mississippi River, for example, are in Lake Itasca in
Minnesota, and its mouth is in the Gulf of Mexico in Louisiana. The longitudinal profile
of a stream from headwaters to mouth is generally a smooth, concave-up curve
(FIG. 13.2). The gradient (steepness) of a stream may vary from a few inches to
hundreds of feet of vertical drop per mile and is typically steeper at the head than
at the mouth.
A stream can erode its channel only as low as the elevation at its mouth, because
if it cut any deeper, it would have to flow uphill to get to the mouth. The elevation
at the mouth, called the base level, thus controls erosion along the entire stream.
13.2 HOW DO STREAMS WORK?
329