Reservoirs: How they can negatively impact river ecosystems

Written by Emily K.


A practice that predates the Ancient Egyptians of around 2950-2750 B.C.E., dams have developed into a versatile tool for various societal needs. Reservoirs can be used recreationally, as a tool for flood control, and as source of irrigation, among other things. However, beyond the benefits of building reservoirs, dams also introduce variable environmental consequences, affecting many rivers and stream-side environments. 

Life in a riverside ecosystem evolves around the water itself and its properties: the timing and quantity of river flow, the concentration of chemicals in the water, the depth of the river floor, etc. Turning habitats from lotic, streaming bodies to lentic ponds, dams typically disrupt the equilibrium that has persisted for years, which in turn harms the ecosystem physically, biologically, and chemically. For example, dams hold back sediments that would naturally replenish downstream ecosystems.


When a river is deprived of its sediment load, it compensates by eroding the downstream river bed and banks (which can also erode bridges and other riverbank structures, as well as riverside woodlands). Riverbed deepening will also lower groundwater tables along a river, which plant roots need access to. Altering the riverbed also reduces habitat for invertebrates and fish that spawn in river bottoms. A study of the threatened fish in Oklahoma (Hubbs and Pigg 1976) suggested that 55% of the man-induced species depletions had been caused by the loss of free-flowing river habitat resulting from the construction of reservoirs, and a further 19% of the depletion was caused by dams acting as barriers to fish migration.

Reservoirs physically block migrating fish, who often times need to move from fresh water to sea water (or vice versa) in order to breed. With a dam delaying or completely disabling their upstream migration, we have seen a decline and even extinction of species that depend on movement along the stream during certain phases of their life cycle. The Rhine, once the most important salmon river in Germany, has only recently become home to salmon again after their extinction about 75 years ago. The estimate for the catch of salmon from the Rhine in 1880 was almost 250,000, but the building of weirs and dams served as the main factor that eventually resulted in the extinction of the Rhine salmon in the 1950s. Many fish had to sneak through the locks beneath the barges, heading upstream to Basel, inevitably being injured or dying. 

The damage due to spillways is also a great threat to river fish. Spillways can lead to death either directly or indirectly when fish are disoriented by the shock of impact and are more susceptible to predation. The mortality rate varies significantly from one location to another: between 0% and 4% for the Bonneville, McNary and John Day dams (about 30 m high spillways) on the Columbia River, 8% at the Glines dam (60 m high spillway) and 37% at the Lower Elwha dam (30 m high spillway) on the Elwha River for juvenile salmonids (Bell and Delacy, 1972; Ruggles and Murray, 1983). According to a study, fish are injured once the speed of the water of a downstream pool reaches 16 m/s (Bell & Delacy, 1972). This speed requires only a 13 meter high spillway. There is a 100% mortality rate for a drop of 50-60 m, with tens of thousands of dams exceeding that height.


Dam builders will justify these claims by carrying out environmental impact assessments (EIAs) and implementing mitigation measures such as fish passages. However, a study published in Science Magazine called “Balancing hydropower and biodiversity in the Amazon, Congo, and Mekong” has shown that many EIAs “have no actual influence on design parameters” of projects and that fish passages are often unsuccessful and even harmful. Though many produce hydropower, dams are not the clean, easy sources of electric power that they are made out to be. Hydropower is a great idea, but not at the expense of hundreds of river habitats and thousands of species. It is important that in the future we weigh out the benefits and the consequences of reservoir construction and ask ourselves, “is it worth it?”.