The most obvious way to identify nickpoints or headcuts is by the use of aerial photographs.
Particularly in arid and semi-arid regions, headcuts are very easily recognizable because the
upstream valley floor or channel is essentially undisturbed, whereas the channel below shows
significant erosion. On topographic maps of large scale the presence of a nickpoint or headcut
is indicated by closely-spaced contours. This can be verified with field surveys which show the
break in the longitudinal profile of the stream. A change in the dimension of the channel and a
change in the character of the bank line may indicate the presence of nickpoint or headcut
migration. A low width-depth ratio below the break in slope is an indication of scour and
deepening of the channel. Bank erosion is also a possible consequence and a sharp change
in the bankline characteristics representing a change from stability to instability may identify the
presence of a nickpoint or headcut.
5.2.5 Geomorphic Threshold
Evolution of a drainage network and sediment production from drainage basins are very
complex processes. Geomorphic history and climatic changes introduce a new degree of
complexity into the response of watersheds and fluvial systems. Experimental studies
demonstrate that within a complex natural system, one event can trigger a complex reaction as
the components of the system respond to change. The magnitude of this complex response is
likely to appear during early stages of an erosion cycle or during rejuvenation of high sediment
producing areas. Nevertheless, these areas produce major land management and
conservation problems, hence their practical interest.
It is possible to explain variations in sediment yield and channel adjustment by using the
concept of geomorphic thresholds described by Schumm (1977). For example, in a given
drainage area, it is possible to define a valley slope above which the valley floor is unstable.
Stable valley floors above the threshold line are incipiently unstable and any major flood may
eventually cause erosion and trenching of the alluvium stored in these valleys (Figure 5.1
illustrates the concept of geomorphic threshold). Another very common example of a
geomorphic threshold is the progressive increase in channel sinuosity and meander amplitude
until a cutoff or channel avulsion results on alluvial plains and deltas. This is due to channel
lengthening and gradient reduction accompanying increases in sinuosity and delta size. Since
permanent changes result only when a geomorphic threshold has been exceeded, events with
landscape, but at other times may produce seemingly "instantaneous" change with potentially
catastrophic consequences.
5.3 VARIABILITY AND CHANGE IN ALLUVIAL RIVERS
Those who work with rivers are aware of the great variability that exists among rivers and
between river reaches. In addition, the impact of human activities can greatly alter the
behavior, dimensions, and general morphology of a river. Large alluvial rivers have always
played an important role in human affairs. All of the early great civilizations rose on the
banks of large alluvial rivers such as the Nile, Indus, Yellow, and Euphrates. River
engineering began early in human history to minimize the effects of floods and channel
changes. Today, engineers face the same problems and they have been successful in
developing flood control, navigation, and channel stabilization programs, but often at great
cost and with the need to continually maintain and repair structures and channels (Schumm
and Winkley 1994). In this section, historic change and channel response on the Mississippi
River and the Nile are surveyed to illustrate the range of variability and change to be
expected on alluvial rivers.
5.5
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