Water and the Hydrogeology of Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 105

In the early 1600s, the United States had an estimated 89,435,527 ha (221 million ac) of wet-
lands, but by the mid-1980s their extent had been reduced to approximately 41,682,621 ha
(103 million ac)—a 54% reduction (Dahl and Allord 1994). Seven states including Indiana,
Illinois, Missouri, Kentucky, Iowa, California, and Ohio have lost more than 80% of their
wetlands. Although much of this wetland loss is attributed to agricultural land conver-
sion, other anthropogenic causes include
• Construction of dikes and levees along rivers to divert or minimize flooding
• Logging
• Mining
• Road construction
• Building construction
• Urban expansion
• Non-native plant invasion
• Overgrazing
As recently as the 1970s, destruction of wetlands in the United States was encouraged. It
was not until the passage of the Emergency Wetland Resources Act of 1986 that wetlands
became protected as a vital ecologic and hydrological resource.
Approximately 150 species of birds including the bald eagle, great blue heron, and
belted kingfisher and more than 200 species of fish depend on wetlands for their sur-
vival. Mammals such as the muskrat, white-tailed deer, and numerous species of reptiles,
amphibians, and invertebrates also rely on wetlands. Wetlands exceed all other land types
in wildlife productivity and diversity because they are ecotones—a region where two eco-
systems overlap and exhibit a synergistic (nonlinear) effect with respect to habitat quality.
In the case of wetlands, the overlapping ecosystems are an aquatic lowland area and the
adjacent drier upland terrain.
3.7.1.2 Dam Construction
Dams are constructed for hydroelectric power generation, flood control, improvement
of navigation, and water supply to provide a water source for irrigation and recreation
(Figure 3.48). Dams cause the water table to rise and may also increase head pressures in
confined aquifers if conditions are favorable. Two major concerns about dams are whether
the rise in groundwater levels will weaken the surrounding geologic material, and if this
rise will impact the natural groundwater flow patterns. These concerns were realized
during the Teton Dam collapse on June 5, 1976, when a leak appeared in the earthen dam
as it was being filled. The leak first appeared approximately 40 m (130 ft) below the crest
of the dam indicated by the arrow in Figure 3.49. The dam collapsed a couple hours later
(Figure 3.50) (Randle et al. 2000).
The arrow in Figure 3.49 shows the hole caused by seepage of water through the dam on
the left portion of the figure. A D-9 bulldozer is at work above the seepage point attempt-
ing to plug the hole. Figure 3.50 was taken a couple hours later and shows the nearly com-
plete collapse of the dam. Eleven people lost their lives in the collapse of the Teton Dam.
The collapse also caused more than 30 landslides, killed more than 13,000 cattle, incurred
more than $300 million in property damage, and flooded portions of Rexburg, Wilford,
Sugar City, Salem, and Hibbard, Idaho (Randle et al. 2000).