An assessment of retaining dams in hydropower reservoirs for enhancing bird habitat

The River Inna in the south Norwegian subalpine region was regulated in 1979-1982, creating a 6.7 km(2) reservoir with a 35 m range in water level. This reservoir inundated good habitat for wetland birds and, in general, provided poor habitat for birds. Along the main inflow river, but within the reservoir, a retaining dam was constructed after the breeding season in 1989 to create habitat for native ducks. The dam maintains a nearly constant water level about Im below the highest water level in the reservoir; this is especially important for ducks throughout the breeding season. The ducks showed an immediate response in the first season, increasing both in number of species and number of individuals of several species. To discern some factors responsible for attracting ducks to this pond, all ducks were censused at least 12 times each breeding season between 1990 and 1994. The numbers varied widely, both from one year to another and during each breeding season. Wigeon (Anas penelope) numbers after dam construction were highest early in the season. Tufted duck (Aythya fuligula) numbers mainly varied from one year to another, while long-tailed duck (Clangula hyemalis) numbers varied greatly through the season. In general, variations in water level had little effect on duck numbers. Four species established breeding populations in the pond. Such ponds may be valuable for establishing duck populations in many areas otherwise unsuitable for breeding water-birds. Appropriate management may also improve the chances of this being a rich habitat for several duck species in the future.