Dams

The construction of dams on rivers became a major human impact on river systems during the twentieth century. Most notably in the southern Appalachians, the Tennessee Valley Authority created a series of dams and reservoirs for power generation.

(Table: Water Surface Area of Flooded Rivers and Lakes)

Dams are constructed because the human and economic benefits can be considerable. Different types of dams serve different purposes. For some people, the aquatic system is improved by the recreational opportunities in the reservoir and in the regulated river section downstream of the dam.

Dams differ in whether water is released from the surface of the reservoir, near the bottom, or both. Water near the bottom of a reservoir is generally colder than that at the surface. In summer, surface water in mountain reservoirs can be much warmer than in the flooded river. Thus, releases from the surface can result in higher temperatures in the river downstream. On the other hand, a number of successful tailwater trout fisheries have been created downstream from bottom-release reservoirs.

Early research on dammed rivers focused mainly on reservoir limnology. More recently, scientific and public concern has generated research on the effects of dams on the river itself (for example, see Hynes 1970, Baxter 1977, Petts 1984) or river regulation (Lillehammer and Saltveit 1984, Craig and Kemper 1987, Gore and Petts 1989). Dams cause fundamental changes in community structure and ecosystem function as a naturally free-flowing and continuous river course is transformed into river segments interrupted by impoundments (Allan 1995).

In addition to hydrologic disruption, dams also affect water quality, and may cause permanent biological change. The regulated flows change the discharge and current velocity of the downstream river, altering habitat for native species. The loss of upstream-downstream connectivity halts genetic exchange among populations. Access of migratory species to former habitats often is blocked.

Management can be used to mitigate the negative effects of dams and regulated rivers. One suggested management paradigm for regulated rivers has been termed the natural flow regime (Poff and others 1997), which seeks to preserve the natural variability of flow regimes within a river system. This approach involves:

• Managing dam releases to simulate seasonal cycles of high and low flows.
• Safeguarding against upstream river development and damaging land uses that modify runoff and sediment supply in the watershed.