When dams are built they have an impact not only on the flow of water in the river, but also on the people who live downstream and on the surrounding ecosystems. By placing data from close to 6,500 existing large dams on a highly precise map of the world’s rivers, an international team led by McGill University researchers has created a new method to estimate the global impacts of dams on river flow and fragmentation.
The team combined two indices to provide a more comprehensive evaluation of the effect of dams:
1. Normally rivers and their watersheds function as integrated ecosystems. Upstream and downstream are connected in a continuous way by the way sediment is picked up and delivered, by the way stream temperature varies from source to outlet, through wetlands and floodplains, and through transfers between surface water and groundwater. Dams have a way of dicing up these relationships into more or less disconnected compartments. The team’s river fragmentation index is a way of measuring that effect.
2. The river regulation index is a measure of the proportion of the river water that can be stored in reservoirs, and thus affects the natural fluctuation and properties of river flow downstream.
By combining these two indices, the researchers have arrived at a way of assessing the impact of any existing or planned dam. Life cycles of freshwater species are often affected. Endemic species can be lost, exotics can invade, overall biodiversity can be reduced. Dams can have cumulative effects many hundreds of kilometers downstream and upstream of the barrier. Approaches that also take into account adjacent dams within the river system are therefore necessary but rarely performed.
Permanent dam disruption of river systems can have effects from species to ecosystem levels and from local to global scales. Most major global river basins are already impacted by large dams. In the future, dam development is expected to continue, with more than 3700 large hydropower dams alone currently planned or under construction worldwide. As more than one-sixth of the world’s population live in glacier- or snowmelt-fed river basins, dams are increasingly discussed as an option to buffer against climate-induced fluctuations in water availability. However, rapid proliferation of new dams may pose serious impacts on rivers, including those that support high levels of biodiversity or provide important sources of food from fisheries or flood-recession agriculture. Thus, the researchers say, it is of paramount importance to minimize the social and environmental impacts of new dams. For more information, see http://iopscience.iop.org/1748-9326/10/1/015001/article.