Page 249 - Fundamentals of Geomorphology

232 PROCESS AND FORM

which is approximately equal to the reciprocal of twice Hantke 1994). Erosive processes may have deepened and
the drainage density. And, importantly, it determines the widened them, but they are essentially endogenic features
distance from streams to valley divides, which strongly and not the product of antecedent rivers.
affects the general appearance of any landscape. Like the rivers that fashion them, valleys form
Early studies of stream networks indicated that purely networks of main valleys and tributaries. Valleys grow by
random processes could generate fluvial systems with becoming deeper, wider, and longer through the action of
topological properties similar to natural systems (Shreve running water. Valleys deepen by hydraulic action, cor-
1975; Smart 1978). Such random-model thinking has rasion, abrasion, potholing, corrosion, and weathering of
been extremely influential in channel network studies. the valley floor.They widen by lateral stream erosion and
However, later research has identified numerous regu- by weathering, mass movements, and fluvial processes on
larities in stream network topology. These systematic the valley sides. They lengthen by headward erosion, by
variations appear to be a result of various factors, includ- valley meandering, by extending over newly exposed land
ing the need for lower-order basins to fit together, the at their bottom ends, and by forming deltas.
sinuosity of valleys and the migration of valley bends Some valley systems are exceptionally old – the
downstream, and the length and steepness of valley sides. Kimberly area of Australia had been land throughout
These elements are more pronounced in large basins, but the Phanerozoic and was little affected by the ice ages
they are present in small catchments. (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
Valleys
been identified on the Australian continent.
Valleys are so common that geomorphologists seldom
defined them and, strangely, tended to overlook them as
landforms. True valleys are simply linear depressions on FLUVIAL DEPOSITIONAL LANDFORMS
the land surface that are almost invariably longer than
they are wide with floors that slope downwards. Under Alluvial bedforms
special circumstances, as in some overdeepened glaciated Riverbeds develop a variety of landforms generated by
valleys (p. 255), sections of a valley floor may be flat or turbulence associated with irregular cross-channel or
slope upwards. Valleys occur in a range of sizes and go vertical velocity distributions that erode and deposit allu-
by a welter of names, some of which refer to the specific vium. The forms are riffle–pool sequences (Box 9.1)
types of valley – gully, draw, defile, ravine, gulch, hollow, andripple–antidune sequences(Figure9.7).Step–pool
run, arroyo, gorge, canyon, dell, glen, dale, and vale. sequences are large-scale and created by, for example, the
As a rule, valleys are created by fluvial erosion, but dam-building activities of beavers.
often in conjunction with tectonic processes. Some land-
forms that are called ‘valleys’ are produced almost entirely Floodplains
by tectonic processes and are not true valleys – Death
Valley, California, which is a half-graben, is a case in Most rivers, save those in mountains, are flanked by an
point. Indeed, some seemingly archetypal fluvial land- area of moderately flat land called a floodplain, which is
forms, including river valleys, river benches, and river formed from debris deposited when the river is in flood.
gorges, appear to be basically structural landforms that Small floods that occur frequently cover a part of the
have been modified by weathering and erosion. The floodplain, while rare major floods submerge the entire
Aare Gorge in the Bernese Oberland, the Moutier– area. The width of floodplains is roughly proportional
Klus Gorge in the Swiss Jura, the Samaria Gorge in to river discharge. The active floodplain of the lower
Crete, hill-klamms in the Vienna Woods, Austria, and Mississippi River is some 15 km across. Adjacent flood-
the Niagara Gorge in Ontario and New York state all plains in regions of subdued topography may coalesce to
follow pre-existing faults and clefts (Scheidegger and form alluvial plains.