Environmental and Wildlife Impacts of Dams and Levees

Background: Dams, Levees, and Flood Control

One of the issues resulting from civilization and urbanization is that most of the places humans chose to settle — for reasons of convenience, agriculture, transportation, and economic independence — have been near water. Dams provide hydroelectric power, help control floods, and make rivers navigable. Levees are quite similar to dams in their purpose, although they are primarily built to restrict water during periods of high flow and, for the majority of the time, are not submerged. Per capita, floods are the most destructive and frequent of nature's natural disasters, and in the last fifty to sixty years the number and severity of flooding events has worsened globally.

Several reasons have contributed to this trend: global warming and increasingly severe storm activity; the deforestation and paving of natural watersheds; and more people living and working on known floodplains. However, many scholars and engineers believe that it is not only external conditions that are causing greater flooding problems, but the very nature of the dam and levee approach to flood control. When engineering projects reduce the capacity of river channels, block natural drainage, and increase the speed of floodwater, flood damage becomes greater. Additionally, the "hard path" flood-control model based on a working levee system often ruins the ecological health of rivers and estuaries (McCully, 2007).

Environmental Impact: Upstream Effects

Despite the basic hydrological cycle — the recirculation of moisture from ground to air and back — many areas of the world are overdeveloped and face an extreme shortage of potable water. The environmental impact of dams and reservoirs is increasingly receiving attention as the global demand for water and energy grows, and as the number and size of reservoir and dam projects continues to increase. In general, the damming of a river creates a reservoir of water upstream from the dam. Dam projects produce four major categories of environmental impact: upstream impacts, downstream impacts, effects beyond the reservoir, and global or macro impacts.

Upstream impacts arise from the backflow of water that spreads from the area of construction into the surrounding environment, flooding natural habitats and creating a larger surface area than existed before the dam was built, and thus increasing evaporation. Scientists estimate some alarming statistics: over 400,000 km² of the earth have been flooded due to damming, and water loss to evaporation reaches up to 2.1 meters in some climates (Graham-Rowe, 2005). The initial filling of the reservoir submerges existing plant material, leading to decomposition and rotting that releases large amounts of carbon into the atmosphere. Because much of this decomposing material exists at the lower, non-oxygenated bottom of the reservoir, little oxygen circulates, resulting in the overproduction of dissolved methane (Marmulla, 2001).

Fragmentation of river ecosystems. A dam also acts as a barrier to the upstream and downstream movement of many migratory river animals and their spawning areas — for example, trout and salmon. This threatens to reduce species populations, and fish ladders are commonly used to mitigate the problem. Even with ladders or fish transport by barge, wildlife often has difficulty migrating through or around a dam. Combined with the permanent alteration of surrounding wetlands, this further disrupts ecosystems. Dams may reduce floodplains below the dam, benefiting nearby human populations, but the areas surrounding riverbanks are particularly rich in biodiversity, which is correspondingly reduced. Many endemic species will not survive these environmental changes, and new species often colonize the altered habitat, disrupting the indigenous ecosystem even further (Maser, 2009).

Reservoir sedimentation. Rivers typically carry several types of sediment along their courses, enabling the rich and diverse formation of river deltas, alluvial fans, braided rivers, riverbanks, oxbow lakes, and coastal shores. When a dam is constructed, it blocks the flow of sediment downstream, leading to serious erosion of depositional environments and increased sediment buildup within the reservoir. Eventually, most reservoirs develop a reduced water-storage capacity due to this buildup, which in turn decreases hydroelectric power output, available water for irrigation, and — if left unaddressed — can lead to the eventual failure of the dam and the degradation of the river itself (Morris and Fan, 1998).

Environmental Impact: Downstream Effects

Placing a barrier where none previously existed creates serious problems both upstream and downstream. As might be expected, the water that flows into a dam is not the same as the water that flows out.

River and coastline erosion. Dams reduce the sediment load downstream simply by their construction; a dammed river is often described as "hungry" for sediment. Ironically, because the rate of sediment deposition is drastically reduced — since less sediment is available — while the rate of erosion remains fairly constant, the continual water flow eats away at river shores and the riverbed. This continues to threaten shoreline ecosystems, deepens the riverbed, and, over time, narrows the river. The eventual consequences include a compromised water table, reduced water levels, a homogenization of river flow, and a reduction in the viability of its ecosystem (Berga et al., eds., 2006).

Water temperature. Without a dam, water temperature is seasonally variable but relatively stable over time. The water in a reservoir is typically warmer in winter and colder in summer than it would be naturally. As this water flows downstream, it alters the temperature of the river, impacting both flora and fauna and creating unnatural environments. Many fish will either not return to the river or experience a drastic reduction in breeding success (Wieland, Ren, and Tan, eds., 2004, p. 321).