Elwha Dam - Effects of Dam On River Habitat

Effects of Dam On River Habitat

Since the construction of the dams, the lower reach of the river has seen a drastic decrease in sediment delivery. This has led to a net erosion of the river bed, including the gravels needed to create suitable habitats for spawning. Replacement gravel has been trapped in the sediment load behind the dams. These gravels are considered an essential resource for restoration of salmon habitats. The delta at the mouth of the river, where it enters the Straits of Juan de Fuca, has also seen a net erosion effect, confirmed by local tribal leaders.

Another effect of damming is the creation of Lake Aldwell and Lake Mills. These lakes have a multitude of impacts on the river beyond sediment retention. Because the water stays in the lake for so long, it warms up to approximately 16 °C. Normal water temperatures are around 0 °C. These higher temperatures are unnatural for spawning fish, which prefer colder ocean temperature waters. This increase in temperature also increases parasite populations. Disease has been known to wipe out two thirds of a spawning population, before it has a chance to spawn. The lakes also act as buffers for high and low flow conditions. While the dams are not actively used for flood control, the reservoirs behind them alter the natural flow patterns of the river. The normal seasonal high and low flow are evened out by the reservoirs and dams so that flow is relatively constant throughout the year. Natural flow patterns are known to promote the health of native species and to help eliminate non native species after dam removal.