Laboratory Manual for Introductory Geology 4e

EXERCISE 13.7
Deducing the History of the Susquehanna River
Name:
Course:
Section:
Date:
(a) What is unusual about the relationship between the Susquehanna and Juniata rivers and the valley-and-ridge
topography shown in Figure 13.20?
Most of the small streams flow in the elongate valleys, but the Susquehanna and its tributary, the Juniata River, cut across the
ridges at nearly right angles. It is tempting to think that the big streams had enough energy to cut through the ridges, whereas
the small ones didn’t, but that is not the case. The answer lies in a multistage history of which only the last stage is visible today.
(b) Why do most of the smaller streams flow in the elongate valleys?
(c) Suggest as many hypotheses as you can to explain why the two larger rivers cut across the valley-and-ridge
topography. Hint: How might the landscape have been different at an earlier time?
Rivers with enough energy to cut through the ridges should certainly have been able to simply meander around them,
but the Susquehanna and Juniata rivers didn’t take the easy way out. It’s almost as if they didn’t even know the ridges and
valleys were there.
(d) With that clue, suggest a series of events that explains the behavior of the Susquehanna and Juniata rivers.
Hint: This type of stream is called a superposed stream.
EXERCISE 13.8
The Origin of Incised Meanders
Name:
Course:
Section:
Date:
The Green River is on the Colorado Plateau, an area where the meanders of many rivers are cut deeply (incised) into
the bedrock (see Fig. 13.21). The most famous is the Colorado River itself, particularly where it flows through the Grand
Canyon. This behavior is totally unlike that of the meandering streams you’ve encountered earlier in this chapter. The
Green River seems to violate the rules of stream behavior, but, like the Susquehanna, it is following them perfectly. Some
geologic detective work will let you figure out how.
(a) Describe the path of the Green River as shown in Figure 13.21.
(continued)
13.7 WHEN STREAMS DON’T SEEM TO FOLLOW THE RULES
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