Laboratory Manual for Introductory Geology 4e

13.6 Changes in Stream-Created Landscapes
over Time
Just as a single stream or entire drainage network changes over time, so too do
fluvial (stream-created) landscapes. Consider a large block of land uplifted to
form a plateau. Several things will change over time: its highest elevation, the
number of streams, the stream gradients, and the amount of flat land relative to
the amount of land that is part of valley walls. These changes reflect the different
ways in which stream energy is expended as the landscape is eroded closer to
base level. FIGURE 13.16 summarizes these changes in an idealized form. In the
real world, things rarely remain constant long enough for this cycle to reach its
end: sea level may rise or fall due to glaciation or tectonic activity, the land may
be uplifted tectonically, and so on. Nevertheless, the stages in Figure 13.16 are
typical of landscapes produced by stream erosion and can be recognized on maps
and other images.
FIGURE 13.16 Idealized stages in the development of a fluvial landscape.
Stage 1: A fluvial landscape is uplifted, raising stream
channels above base level.
Few streams, but with relatively steep gradients.
Broad, generally flat divides between streams.
Channels are relatively straight.
Relief is low to modest (not much elevation difference
between divides and channels).
Stage 2: Main streams cut channels downward, and
tributaries form drainage networks.
Numerous tributaries develop with moderate gradients,
dissecting most of the area.
Stream divides are narrow and sharp; most of the area is in
slope, with little flat ground.
Main streams and some tributaries meander moderately.
Relief is high, and little if any land is at the original uplifted
elevation.
Stage 3: Stream erosion has lowered land surface close to
base level.
Few streams, as in Stage 1, but with gentle gradients.
Stream divides are broad and flat; relief is low.
Streams meander broadly.
346 CHAPTER 13 LANDSCAPES FORMED BY STREAMS