Geology Reference
In-Depth Information
which is approximately equal to the reciprocal of twice
the drainage density. And, importantly, it determines the
distance from streams to valley divides, which strongly
affects the general appearance of any landscape.
Early studies of stream networks indicated that purely
random processes could generate fluvial systems with
topological properties similar to natural systems (Shreve
1975; Smart 1978). Such random-model thinking has
been extremely influential in channel network studies.
However, later research has identified numerous regu-
larities in stream network topology. These systematic
variations appear to be a result of various factors, includ-
ing the need for lower-order basins to fit together, the
sinuosity of valleys and the migration of valley bends
downstream, and the length and steepness of valley sides.
These elements are more pronounced in large basins, but
they are present in small catchments.
Hantke 1994). Erosive processes may have deepened and
widened them, but they are essentially endogenic features
and not the product of antecedent rivers.
Like the rivers that fashion them, valleys form
networks
of main valleys and tributaries. Valleys grow by
becoming deeper, wider, and longer through the action of
running water. Valleys deepen by hydraulic action, cor-
rasion, abrasion, potholing, corrosion, and weathering of
the valley floor. They widen by lateral stream erosion and
by weathering, mass movements, and fluvial processes on
the valley sides. They lengthen by headward erosion, by
valley meandering, by extending over newly exposed land
at their bottom ends, and by forming deltas.
Some valley systems are exceptionally old - the
Kimberly area of Australia had been land throughout
the Phanerozoic and was little affected by the ice ages
(Ollier 1991, 99). The drainage system in the area is
at least 500 million years old. Permian, Mesozoic, Mid-
to Late Cretaceous, and Early Tertiary drainage has also
been identified on the Australian continent.
Valleys
Valleys are so common that geomorphologists seldom
defined them and, strangely, tended to overlook them as
landforms.
True valleys
are simply linear depressions on
the land surface that are almost invariably longer than
they are wide with floors that slope downwards. Under
special circumstances, as in some overdeepened glaciated
valleys (p. 255), sections of a valley floor may be flat or
slope upwards. Valleys occur in a range of sizes and go
by a welter of names, some of which refer to the specific
types of valley - gully, draw, defile, ravine, gulch, hollow,
run, arroyo, gorge, canyon, dell, glen, dale, and vale.
As a rule, valleys are created by fluvial erosion, but
often in conjunction with tectonic processes. Some land-
forms that are called 'valleys' are produced almost entirely
by tectonic processes and are not true valleys - Death
Valley, California, which is a half-graben, is a case in
point. Indeed, some seemingly archetypal fluvial land-
forms, including river valleys, river benches, and river
gorges, appear to be basically structural landforms that
have been modified by weathering and erosion. The
Aare Gorge in the Bernese Oberland, the Moutier-
Klus Gorge in the Swiss Jura, the Samaria Gorge in
Crete, hill-klamms in the Vienna Woods, Austria, and
the Niagara Gorge in Ontario and New York state all
follow pre-existing faults and clefts (Scheidegger and
FLUVIAL DEPOSITIONAL LANDFORMS
Alluvial bedforms
Riverbeds develop a variety of landforms generated by
turbulence associated with irregular cross-channel or
vertical velocity distributions that erode and deposit allu-
vium. The forms are
riffle-pool sequences
(Box 9.1)
and
ripple-antidune sequences
(Figure 9.7).
Step-pool
sequences
are large-scale and created by, for example, the
dam-building activities of beavers.
Floodplains
Most rivers, save those in mountains, are flanked by an
area of moderately flat land called a
floodplain
, which is
formed from debris deposited when the river is in flood.
Small floods that occur frequently cover a part of the
floodplain, while rare major floods submerge the entire
area. The width of floodplains is roughly proportional
to river discharge. The active floodplain of the lower
Mississippi River is some 15 km across. Adjacent flood-
plains in regions of subdued topography may coalesce to
form
alluvial plains
.
