Environmental Engineering Reference
In-Depth Information
Subdeltas
Subdeltas are extended into the water body by rivers crossing the parent delta. As the sub-
deltas extend seaward, the rivers build bar fingers along their routes. The bar fingers are
composed chiefly of sands. Between them are thick deposits of silts and clays often
interbedded or covered with thick organic formations. Considerable quantities of marsh
gas can be generated from the buried marsh or organic deposits.
New Orleans (an example)
The city of New Orleans is built over deltaic deposits. The complexity of the formations in
the area is illustrated in
Figure 7.35.
Estuaries
Occurrence
Streams whose cycle of erosion has been interrupted by a rise in sea level, resulting in the
drowning of the river valley, form estuaries. During the last glacial age of the Pleistocene,
the mean sea level was lowered by about 300 ft or more in the northeastern United States,
and river valleys, eroding downward, formed new profiles. When the glaciers began to
melt, the sea level rose to form drowned valleys, which are found in many seacoast
regions of the world. Along the east coast of the United States, for example, the Hudson,
Delaware, Susquehanna, and Potomac Rivers have drowned valleys at their mouths.
Estuarine conditions exist where the rivers enter the sea to form bays. The landforms of
the drowned valleys in the northeastern United States are shown in the LANDSAT image
mosaic
(Figure 7.36).
0
25
50
75
100
25
50
75
Legend
Organic deposits,humus,peat
Organic and extreme high
water content clays
Clay
Sand
Silt
Shell
FIGURE 7.35
Major environments of deposition and associated soil type in the vicinity of New Orleans, Louisiana.
(From Kolb, C. R. and Shockley, W. G.,
Proc. ASCE, J. Soil Mech. Found. Eng. Div
., July, 1289-1298, 1957.
With permission.)
