Tens of thousands of dams on U.S. rivers are having a detrimental effect on the biodiversity of aquatic and riparian ecosystems across the nation, according to a Colorado State University report published this week in the journal, "Proceedings of the National Academy of Sciences."
Understanding the cumulative effects of dams in modifying regionally distinct ecosystems is critical to conservation on a national scale, said LeRoy Poff, associate professor in biology and the graduate degree program in ecology at Colorado State. Poff and his research team examined years of U.S. Geological Survey stream-flow records for periods before and after the damming of 186 medium-sized rivers. The sample included 16 hydrologically-distinctive regions across the country over the course of the project, and used stream-flow records of undammed rivers as a control. They found that dams have caused historically-distinct regions to become much more similar, creating more homogeneous habitat conditions across the country.
"From an ecological perspective, there is a substantial risk of eliminating the diversity of regionally-unique habitats that have existed for thousands of years," Poff said.
Dams can modify the timing of ecologically critical periods of high- and low-river flows, Poff said. Native aquatic and plant species have adapted to these long-standing patterns of natural variation of high- and low-flow; eliminating them creates conditions that puts native species at a disadvantage and favors the spread of invasive species. The study found one dam about every 30 miles on rivers that were part of the research data.
"We are having a very large-scale effect on the landscape, often with unintended consequences" Poff said. "From a conservation standpoint, we need to take into account the natural dynamics of these river systems if we want to preserve their regional character."
Poff said one way to mitigate the homogenizing effects of dams is to manage the volume and timing of water released from dams to better mirror natural patterns, a process known as "environmental flow management."
He also stressed that another important consideration for conservation is to preserve some of the free-flowing rivers that remain, as they represent the strongholds of natural river processes and native biodiversity.