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