Geography Reference
In-Depth Information
versions (an inch or so deep) of the Grand Canyon. From the perspective of geography, however, it is
large-scale phenomena, like rivers, waves, and ocean currents, that are of greatest interest.
Rivers and streams
The characteristics of individual rivers and streams vary greatly. In very general terms, however, one
may think of a river system as originating in highlands, gathering the waters of tributaries as it snakes
through foothills, and then flowing through a low-lying coastal plain as it approaches the seas (illus-
trated in Figure 7-3). The nature and effects of mass wasting tend to be very different in each of these
settings. Here is what happens in each setting:
Highlands setting:
By their nature, highlands are high above sea level. Steep gradients are
most likely to occur there, resulting in rapidly flowing streams that erode their beds and carry
away weathered material with comparative ease. This process creates valleys. In highlands,
V-shaped valleys are fairly commonplace, and a sure sign that erosion rather than deposition
is playing the largest (in fact, almost exclusive) role in changing the landscape.
Foothills setting:
In foothills, river gradients tend to be much less steep, resulting in de-
creased velocity and, therefore, decreased erosion of streambeds. In fact, erosion of valley
walls may exceed erosion of the riverbed. As a result, the profiles of foothills valleys may as-
sume the shape of a somewhat flat-bottomed V — or rather \_\ — with much of their floors
being occupied by land rather than river.
While the fringing slopes — that is, the lines of the V — provide evidence of the continued
presence and power of erosion, the appearance of a flat and comparatively (versus the high-
lands) expansive valley floor is testimony that deposition has been an active player in shaping
the landscape. Not uncommonly, the width of these valley floors is sufficient to accommodate
modest-size towns and substantial agricultural activity.
Coastal plain setting:
In the coastal plain, rivers are at their peak volume by virtue of
so many tributaries having added their waters to the combined flow. Velocity is fairly slow,
however,sincebydefinitioncoastplainsarelow-lyingandflat.Heretheindividualriversflow
through broad
floodplains
bound by diminutive valley walls. The rivers themselves are bound
by even less diminutive natural
levees
that consist of sediments deposited during past episodes
when the river overtopped its banks.
Indeed, “floodplains” aren't so-named for nothing. Because the landscape is so flat and ex-
pansive, flooding tends to affect a very wide area. Such events are beneficial to the extent that
silt and other sediments in floodwaters are deposited and enrich the soils. On the other hand,
flatandproductivelandattractspeopleandenterprise,andthusvirtuallyguaranteesthatfloods
have a major impact on life, property, and infrastructure (see sidebar).
