Environmental Engineering Reference
In-Depth Information
FIGURE 7.31
Topographic expression of a bajada dissected by the floodplain of a wash in an arid climate (Cañada del Oro,
Tucson, Arizona; scale 1:24,000). (Courtesy of USGS.)
FIGURE 7.32
(
Inset
) Terrace deposit of boulders, cobbles, gravel, and sand. (Cañada del Oro, Tucson, Arizona. Photo taken in
the area of Figure 7.31 by R. S. Woolworth.)
Deltaic Deposits
Deltaic deposits are characterized by a well-developed cross-bedding of mixtures
of sands, silts, and even clays and organic soils. Delta formation requires that the river
provide materials in such quantities that they are not removed by tides, waves, or
currents. This requires either a low-energy environment with little water movement,
or a river carrying tremendous quantities of material. The Mississippi delta
(
Figure 7.33
and
Figure 7.34)
and the Nile delta are located in low-energy environments. The locus of
active deposition occurs outward from the distributary mouths forming the delta
front complex. The front advances into the water body, resulting in a sheet of relatively
coarse detritus, which thickens locally in the vicinity of channels. Seaward of the
delta front is an area of fine clay accumulation, termed prodelta deposits, illustrated
in
Figure 9.58.
Over long periods of time, deltas will shift their locations as shown in
