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ERDC TR-04-1
3.2
PRINCIPLES OF FLUVIAL GEOMORPHOLOGY
Equilibrium and Dominant Discharge
A basic principle of fluvial geomorphology is the concept of equilibrium,
which holds that a river channel's form--its cross sectional shape, planform, and
gradient--is adjusted to the prevailing watershed conditions that control the
amount of sediment and water delivered to the channel (Leopold et al. 1964,
Leopold and Bull 1979). The concept of a graded stream was similarly defined
by earlier workers but focused more on the river's gradient, with less attention to
cross sectional shape and planform (e.g., Mackin 1948). Channels are self-
adjusting and will change their morphology in response to changing watershed
conditions (Dunne and Leopold 1978) but only if the threshold of critical power
necessary to precipitate change is crossed (Bull 1979).
The dominant, or effective, discharge in a particular region generally controls
the size and shape of the river channel. The effects of smaller flows that tend to
decrease the size of the channel are reworked with subsequent larger flows.
Conversely, larger flows that can increase the size of a channel generally occur
too infrequently for the impact on the river channel to persist on the landscape.
More frequent events, in concert with revegetation, tend to "heal" the impacts of
larger flows and return the channel to a size in equilibrium with the dominant
sediment and water discharge. The discharge at which channel maintenance is
most effective often corresponds to the bankfull stage where water completely
fills the channel and begins to spread out onto the floodplain (Dunne and
Leopold 1978). In other words, the bankfull discharge governs the shape and size
of the channel (Rosgen 1996). While the bankfull discharge has been shown to
have, on average, a recurrence interval of 1.5 years (Dunne and Leopold 1978,
Rosgen 1996), enough exceptions to this rule occur that one should not assume
that this average value holds for any given channel (Williams 1978).
While many of these basic concepts in fluvial geomorphology evolved, in
part, from early research in semi-arid climates (see, for example, Leopold et al.
1964), the general applicability of the equilibrium concept to desert regions has
been called into question (Tooth 2000). The effects of extreme events persist in
deserts for long periods because of the inability of the stream channel to recover
or "heal" from large floods, in part due to the absence of sufficient revegetation
(Baker 1977, Graf 1988a). A further discussion of equilibrium, dominant
discharge, and their application to desert regions is provided later in this chapter.
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