Fundamentals of Fluvial Geomorphology and Channel Processes
extends into graded time. From a geologists temporal point of view, engineers build major projects in an
instant of time, and expect the projects to last for a significant period.
In river related projects time is the enemy, time is our friend, and time is our teacher. We must
learn all we can by adopting a historical perspective for each project that we undertake.
The physical size of the stream may impose limits on the type of planned enhancements to the
stream. For example, many variations of anchoring trees along the bank have been successfully used along
small and moderate size streams to provide cover and to decrease erosion of the bank. Anchoring of trees
along the bank is a reasonable method of stabilization. However, for large rivers that may have bank
heights of 30 feet and a yearly water surface elevation fluctuation of 20 to 30 feet, the anchored tree may
be an unreasonable method for stabilization. Applications designed for a small stream may not be directly
transferrable to larger streams. If we are to transfer techniques for enhancement from stream to stream;
we must also understand the design principles of those techniques. Principles, such as increasing the cover
and decreasing the water velocity at the water-bank interface are transferable; however, the direct
technique may not be transferable.
Now it is time to give you a brief introduction into what you may see when you go to the field. The
following discussion will be confined primarily to depositional landforms along meandering rivers, and a little
information concerning terraces.
A floodplain is the alluvial surface adjacent to a channel that is frequently inundated (Figure 3.2).
Although much of the literature until the 1970s suggested that the mean annual flood was the bankfull
discharge, Williams (1978) clearly showed that out of thirty-five floodplains he studied in the U.S., the
bankfull discharge varied between the 1.01- and 32-year recurrence interval. Only about a third of those
streams had a bankfull discharge between the 1- and 5-year recurrence interval discharge. Knowledge
of alluvial landforms will allow a more informed determination of bankfull than depending solely on the
Table 3.1 and Figure 3.2 together provide a quick summary of some alluvial landforms found along
a meandering stream. From the perspective of a stream stabilization planner, it is extremely important to
know that all the materials along the bank and in the floodplain are not the same. The materials are
deposited under different flow conditions.
For example, backswamps and channel fills will usually be fine-grained and may be very
cohesive. This is because both landforms are deposited away from the main flow in the channel, in a lower
energy environment. Natural-levee deposits are coarser near the channel and become finer away from
the channel as the energy to transport the larger particles dissipates.