FLOOD06421

• County: Statewide
• Basin: Statewide
• Title: Highways in the River Environment
• Date: 2/1/1990
• Prepared By: Federal Highway Administration
• Floodplain - Doc Type: Educational/Technical/Reference Information

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Rivers change position and morphology (dimensions, shape, pattern) as a result
of changes of hydrology. Hydrology can change as a result of climatic changes over
long periods of time, or as a result of natural stochastic climatic fluctuations
(droughts, floods), or by man's modification of the hydrologic regime. For example,
the major climatic changes of recent geological time (the last few million years of
earth history) have triggered dramatic changes in runoff and sediment loads with
corresponding channel alteration. Equally significant during this time were
fluctuations of sea level. During the last continental glaciation, sea level was on
the order of 400 feet lower than at present, and this reduction of base level caused
major incisions of river valleys near the coasts.
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 are interesting from an academic point of view but are
not considered as cause of modern river instability. The movement of the earth's
crust is one qeoloqic aqent causinq 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 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 25
feet per 1000 years. Seemingly insignificant in human terms, this rate is actually
0.3 inches per year or 3 inches per decade. For many river systems, a change of
slope of 3 inches would be significant. (The slope of the energy gradient on the
Lower Mississippi River is about 3 to 6 inches per mile).
Df 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