Page 245 - Fundamentals of Geomorphology

228 PROCESS AND FORM

River long profiles, baselevel, and grade
leads to an increased velocity. In turn, the increased veloc-
ity may then cause bank erosion, so widening the stream The longitudinal profile or long profile of a river
again and returning the system to a balance. The com- is the gradient of its water-surface line from source
pensating changes are conservative in that they operate to mouth. Streams with discharge increasing downstream
to achieve a roughly continuous and uniform rate of have concave long profiles. This is because the drag force
energy loss – a channel’s geometry is designed to keep of flowing water depends on the product of channel gra-
total energy expenditure to a minimum. Nonetheless, dient and water depth. Depth increases with increasing
the interactions of width, depth, and velocity are inde- discharge and so, in moving downstream, a progressively
terminate in the sense that it is difficult to predict an lower gradient is sufficient to transport the bed load.
increase of velocity in a particular stream channel. They Many river long profiles are not smoothly concave but
are also complicated by the fact that width, depth, veloc- contain flatter and steeper sections. The steeper sections,
ity, and other channel variables respond at different rates which start at knickpoints, may result from outcrops of
to changing discharge. Bedforms and the width–depth hard rock, the action of local tectonic movements, sud-
ratio are usually the most responsive, while the chan- den changes in discharge, or critical stages in valley devel-
nel slope is the least responsive. Another difficulty is opment such as active headward erosion.The long profile
knowing which stream discharge a channel adjusts to. of the River Rhine in Germany is shown in Figure 9.4.
Early work by M. Gordon Wolman and John P. Miller Notice that the river is 1,236 km long and falls about
(1960) suggested that the bankfull discharge, which has 3 km from source to mouth, so the vertical distance from
a 5-year recurrence interval, is the dominant discharge, source to mouth is just 0.24 per cent of the length. Knick-
but recent research shows that as hydrological variabil- points can be seen at the Rhine Falls near Schaffhausen
ity or channel boundary resistance (or both) becomes and just below Bingen. Most long profiles are difficult
greater, then channel form tends to adjust to the less to interpret solely in terms of fluvial processes, espe-
frequent floods. Such incertitude over the relationship cially in the case of big rivers, which are normally old
between channel form and discharge makes reconstruc- rivers with lengthy histories, unique tectonic and other
tions of past hydrological conditions from relict channels events in which may have influenced their development.
problematic. Even young rivers cutting into bedrock in the Swiss Alps
Changes in hydrological regimes may lead to a and the Southern Alps of New Zealand have knick-
complete alteration of alluvial channel form, or what points, which seem to result from large rock-slope failures
Stanley A. Schumm called a ‘river metamorphosis’. (Korup 2006).
Such a thoroughgoing reorganization of channels may Baselevel is the lowest elevation to which downcut-
take decades or centuries. Human interference within a ting by a stream is possible. The ultimate baselevel for
catchment often triggers it, but it may also occur owing to any stream is the water body into which it flows – sea,
internal thresholds within the fluvial system and happen lake, or, in the case of some enclosed basins, playa, or
independently of changes in discharge and sediment sup- salt lake (p. 234). Main channels also prevent further
ply. A good example of this comes from the western USA, downcutting by tributaries and so provide a baselevel.
where channels incised when aggradation caused the allu- Local baselevels arise from bands of resistant rock, dams
vial valley floor to exceed a threshold slope (Schumm of woody debris, beaver ponds, and human-made dams,
and Parker 1977). As the channels cut headwards, the weirs, and so on. The complex long profile of the River
increased sediment supply caused aggradation and braid- Rhine has three segments, each with a local baselevel.
ing in downstream reaches. When incision ceased, less The first is Lake Constance, the second lies below Basel,
sediment was produced at the stream head and inci- where the Upper Rhine Plain lies within the Rhine
sion began in the lower reaches. Two or three such Graben, and the third lies below Bonn, where the Lower
aggradation–incision cycles occurred before equilibrium Rhine embayment serves as a regional baselevel above the
was accomplished. mouth of the river at the North Sea (Figure 9.4).