Geoscience Reference
In-Depth Information
coast of the United States, for instance, deep
parallel ditches were constructed in the early
twentieth century to lower water tables, avoid
the accumulation of standing surface water, and
thereby reduce mosquito breeding sites (Anon
1997; Resh 2001). Sediment removed in the
process was spread across the marsh surface to
create elevated areas, which were then used for
agriculture or cattle grazing.
Dredging and channeling are undertaken to
excavate bottom sediment of river banks and
l oodplains and deepen open standing water.
Dredging operations may be quite large in scale
and involve the use of underwater mechanical
excavators connected to pipes that transport
bottom sediment to barges or to shore banks.
Such operations are regularly undertaken
to ease navigation through major inland and
coastal waterways.
Biotic communities adapt to the natural dry
and wet cycles experienced by wetlands. Anthro-
pogenic interferences with these l ow cycles,
including the timing of peak l ows, continuity
of l ows and rapidity of change in l ow patterns,
have consequences on dependent communities
(Welcomme et al. 2006a). For wetlands whose
hydrologic l ows have been altered, mimicking
natural low- and high-water cycles through
controlling water levels could serve an impor-
tant function in supporting dependent commu-
nities and biotic integrity (Snodgrass and Burger
2001). Active water-level management through
a system of locks and sluices may inl uence the
composition and structure of wetland vegeta-
tion, and provide suitable breeding, foraging
and spawning habitat for bird, i sh, and other
animal populations (Fig. 12-3). Furthermore,
water-level management also serves to control
nutrient concentrations, dissolved oxygen, water
clarity and quality, and may help manage the
spread of invasive species (Laubhan and Roelle
2001; Snodgrass and Burger 2001).
The semi-natural pond wetland system
observed across parts of China and elsewhere
in Asia provides an example of managed water
levels (Yin, Shan and Mao 2006). The shallow
pond, small dam, weir-and-sluice landscape
found in agriculturally intensive parts of China
demonstrates an actively managed water irriga-
tion and distribution system. By providing
water-storage capacity, these maintained pond
wetlands act as buffers during heavy seasonal
precipitation events and provide an environ-
ment for i sh farming. A sluice system that con-
nects individual ponds to the regional drainage
system also channels water to i elds for
Figure 12-3.
A dam and outlet spillways at Little Salt Marsh function to store water and regulate downstream l ow
in the Quivira National Wildlife Refuge, south-central Kansas, United States. Twenty-one miles (34 km) of canals
along with many levees and other drainage-control structures are used to divert, hold, and distribute water
throughout the refuge. Kite aerial photo by J.S. Aber and S.W. Aber.
