Increase in mercury levels: In some cases, the construction of reservoirs has caused the mercury content of fish to increase rapidly to values that exceed guidelines for human consumption, as shown in the example in Figure 3-1 (Bodaly et al., 1984; Rosenberg et al., 1995). Increased mercury levels have led to losses of commercial, recreational, and subsistence fisheries. The increase appears to be largely the result of low oxygen concentrations caused by the decay of flooded vegetation. Such conditions promote the increased activity of bacterial species that transform inorganic mercury into the methylated form, which is greatly biomagnified (Rudd, 1995).
Release of greenhouse gases: The release of the greenhouse gases carbon dioxide (CO2) and methane (CH4) following the flooding of forests and peatlands is another major concern identified by limnologists and other environmental scientists. The total area of reservoir surface in North
BOX 3-1 BENNETT DAM AND THE DISAPPEARANCE OF THE PEACE-ATHABASCA DELTA
In the past, most dams and reservoirs were constructed without adequate study of their consequences for aquatic ecosystems, which has lead to irreparable or costly damage. One example is the installation of Bennett Dam on the Peace River. The Peace River flows from headwaters in northern British Columbia across Alberta to Lake Athabasca. Historically, the spring melt flood of the Peace backed up water into the lake and delta of the Athabasca River, flooding small, perched lakes and wetlands along the dendritic channels in the delta. The area was rich in wildlife and was home to more than 1,500 indigenous people.
Except in 1974, when an ice jam caused flooding, there has been no flooding of the Peace-Athabasca Delta since 1969, when Bennett Dam was constructed on the Peace River near the British Columbia-Alberta border. Muskrat, the staple of a thriving trapping industry, disappeared within a few years. Rich fisheries declined. Waterfowl numbers decreased dramatically as marshlands were invaded by willows and other trees. Many of the dendritic channels filled in, making boat travel impossible. Grazing lands for wood bison declined as range quality deteriorated after the annual deposition of rich sediments ceased (Carbyn et al., 1993).
Few indigenous people now live on the land. Most remain in the community of Fort Chipeweyan, where their use of natural foods is being replaced by less-nutritious alternatives (Wein et al., 1991). Damming of the river has resulted in the end of a traditional way of life for people in the area. Studies done to document the deleterious downstream impacts of the Bennett Dam provide resource managers and water resource planners with knowledge about the consequences of dam building so that these problems can be avoided in the future.