In recent geologic time, major river changes of different types occurred. These types are deep
incision and deposition as sea level fluctuated, changes of channel geometry as a result of
climatic and hydrologic changes, and obliteration or displacement of existing channels by
continental glaciation. Climatic change, sea level change, and glaciation, which are interesting
from an academic point of view, may also be causes of modern river instability, particularly
when the 50- 100-year design life of a bridge is considered. The movement of the earth's crust
is one geologic agent causing modern river instability.
The earth's surface in many parts of the world is undergoing continuous measurable change
by upwarping, subsidence or lateral displacement. As a result, the study of these ongoing
changes (called neotectonics) has become a field of major interest for many geologists and
geophysicists. Such gradual surface changes can affect stream channels dramatically. For
example, Wallace (1967) has shown that many small streams are clearly offset laterally along
the San Andreas Fault in California. Progressive lateral movement of this fault on the order of
25 mm (an inch) per year has been measured. The rates of movement of faults are highly
variable, but an average rate of mountain building has been estimated by Schumm (1963) to
be on the order of 7.6 m (25 ft) per 1,000 years. Seemingly insignificant in human terms, this
rate is actually 7.6 mm (0.3 in) per year or 76 mm (3 in) per decade. For many river systems, a
change of slope of 76 mm (3 in) would be significant. For example, the slope of the energy
gradient on the Lower Mississippi River is about 47 to 95 mm/km (3 to 6 in/mi).
Of course, the geologist is not surprised to see drainage patterns that have been disrupted by
uplift or some complex warping of the earth's surface. In fact, complete reversals of drainage
lines have been documented. In addition, convexities in the longitudinal profile of both rivers
and river terraces (these profiles are concave under normal development) have been detected
and attributed to upwarping. Further, the progressive shifting of a river toward one side of its
valley has resulted from lateral tilting. Major shifts in position of the Brahmaputra River toward
the west are attributed by Coleman (1969) to tectonic movements. Hence, neotectonics should
not be ignored as a possible cause of local river instability.
Long-term climatic fluctuations have caused major changes of river morphology. Floodplains
have been destroyed and reconstructed many times over. The history of semi-arid and arid
valleys of the western United States is one of alternating periods of channel incision and arroyo
formation followed by deposition and valley stability which have been attributed to climatic
fluctuations.
It is clear that rivers can display a remarkable propensity for change of position and
morphology in time periods of a century. Hence, rivers from the geomorphic point of view are
unquestionably dynamic, but does this apply to modern rivers? It is probable that during a
period of several years, neither neotectonics nor a progressive climate change will have a
detectable influence on river character and behavior. What then causes a river to appear
relatively unstable from the point of view of the highway engineer or the environmentalist? It is
the slow but implacable shift of a river channel through erosion and deposition at bends, the
shift of a channel to form chutes and islands, and the cutoff of a bend to form oxbow lakes.
Lateral migration rates are highly variable; that is, a river may maintain a stable position for
long periods and then experience rapid movement. Much, therefore, depends on flood events,
bank stability, permanence of vegetation on banks and the floodplain and watershed land use.
A compilation of data by Wolman and Leopold shows that rates of lateral migration for the Kosi
River of India range up to approximately 760 m/yr (2,500 ft/yr). Rates of lateral migration for
two major rivers in the United States are as follows: Colorado River near Needles, California, 3
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