Laboratory Manual for Introductory Geology 4e

FIGURE 13.12 The development of waterfalls.
rapids, but in large rivers they can be huge waterfalls, such
as Niagara, Victoria, and Iguaçu (FIG. 13.12). Over time,
the falls retreat as the stream carves through them.
(a) Iguaçu Falls on the Iguaçu River, the boundary between
Argentina and Brazil, is caused by a thick, highly resistant layer
of basalt.
Time 1
Time 2
Retreat
(b) The waterfall will retreat over time as the river cuts through
the basalt.
13.5.2 Drainage Patterns
13.5 Stream Networks
Streams are particularly effective agents of erosion because
they form networks that cover much of the Earth’s surface.
Rain falling on an area runs off into tiny channels that carry
water into bigger streams and eventually into large rivers.
Each stream—from tiniest to largest—expands headward
over time as water washes into its channel, increasing the
amount of land affected by stream erosion. Understanding
the geometric patterns of stream networks and the way they
affect the areas they drain is important in understanding
how to prevent or remedy stream pollution, soil erosion, and
flood damage.
13.5.1 Drainage Basins
The area drained by a stream is its drainage basin, which is
separated from adjacent drainage basins by highlands called
drainage divides. The drainage basin of a small tributary may
cover a few square miles, but that of the master (main) stream
in the region may be hundreds of thousands of square miles.
FIGURE 13.13 shows the six major drainage basins of North
America. Five deliver water to an ocean directly or indirectly,
but the Great Basin is bounded by mountains, and there is no
exit for the water. The yellow arrows show the dominant flow
direction for each basin—note that for some of them, that
direction is northward. The Gulf of Mexico drainage basin
is the largest and drains much of the interior of the United
States. The Continental Divide separates streams that flow
into the Atlantic Ocean from those that flow into the Pacific.
The Appalachian Mountains are the divide separating the
Gulf of Mexico and direct Atlantic Ocean basins; the Rocky
Mountains separate the Gulf of Mexico and Pacific basins. A
favorite tourist stop in Alberta, Canada, is a triple divide that
separates waters flowing north to the Arctic Ocean, west to
the Pacific, and northeast to Hudson Bay.
Master and tributary streams in a drainage network typically form recognizable patterns
(FIG. 13.14). Dendritic patterns (from the Greek dendron, meaning tree, as they
resemble branches from a trunk) develop where the materials at the Earth’s surface
are equally resistant to erosion. These materials may be horizontal sedimentary or
volcanic rocks; loose, unconsolidated sediment; or igneous and metamorphic rocks
that erode at the same rate. Trellis patterns form where ridges of resistant rock alternate
with valleys underlain by less resistant material. Rectangular patterns indicate
zones of weakness (faults, fractures) perpendicular to one another. In radial patterns,
streams flow either outward (in a centrifugal pattern) from a high point (e.g., a volcano)
or inward (in a centripetal pattern) toward the center of a large basin. Annular
342 CHAPTER 13 LANDSCAPES FORMED BY STREAMS