Fundamentals of Fluvial Geomorphology and Channel Processes
If we change a river we usually do some good somewhere and "good" in quotation
marks. That means we achieve some kind of a result that we are aiming at but
sometimes forget that the same change which we are introducing may have
widespread influences somewhere else. I think if, out of today's emphasis of the
environment, anything results for us it is that it emphasizes the fact that we must
look at a river or a drainage basin or whatever we are talking about as a big unit
with many facets. We should not concentrate only on a little piece of that river
unless we have some good reason to decide that we can do that.
188.8.131.52 The System is Dynamic
In each of the idealized zones described above, a primary function is listed. Zone 1 is the sediment
source that implies that erosion of sediment occurs. Zone 2 is the transfer zone that implies that as rainfall
increases soil erosion from the watershed, some change must result in the stream to enable transfer of the
increased sediment supply. Zone 3 is the zone of deposition and change must occur as sediment builds in
this zone, perhaps the emergence of wetland habitat in a lake, then a change to a floodplain as a drier
habitat evolves. The function of each zone implies that change is occurring in the system, and that the
system is dynamic.
From an engineering viewpoint some of these changes may be very significant. For example, loss
of 100 feet of stream bank may endanger a home or take valuable agricultural land. From a geomorphic
viewpoint, these changes are expected in a dynamic system and change does not necessarily represent a
departure from a natural equilibrium system. In planning stabilization measures, we must realize that we are
forced to work in a dynamic system and we must be try to avoid disrupting the system while we are
accomplishing our task.
Landscape changes are usually complex (Schumm and Parker, 1973). We are working in a system
and we have defined a system as an arrangement of things to form a whole. Change to one portion of the
system may result in complex changes throughout the system.
When the fluvial system is subjected to an external influence such as channelization of part of a
stream, we can expect change to occur throughout the system. Channelization usually increases stream
velocity and this would allow the stream to transfer more sediment, resulting in erosion upstream and
deposition downstream of the portion of the stream channelized. For example, some Yazoo Basin streams
in north Mississippi that were channelized in the 1960s responded initially, but an equilibrium has not yet
been reestablished as repeated waves of degradation, erosion, and aggradation have occurred.