A study conducted by Oregon State University makes the case that floods are not just an inconvenient force of nature to be reckoned with — they are also a key component to the long term health of entire river ecosystems.
That relationship between natural flooding events and a fully developed and thriving river system means that any human intervention through impediments such as dams, dredging or diversions can have untold repercussions in the future, according to the study. Natural variations in river flow caused by changes in precipitation can also impact the vitality of streams and rivers, but the study indicates that the most severe consequences occur when “cyclical flooding” is eliminated altogether.
“We think of floods as being these damaging forces because of what they can do to human infrastructure,” said Jonathan Tonkin, a postdoctoral scholar in integrative biology at the OSU College of Science, in a press release. “But flooding has benefits across the board, for both organisms and habitats in and around rivers.”
During the study, OSU researchers used models to identify an array of alternative flow patterns along major rivers and then mapped the long-term effect on five tree and shrub guilds common to rivers around the world. Those plant groupings were selected based on their similar responses to water availability and river flow fluctuations. In one case, scientists used 83 years of flow data from the Yampa River in Colorado as well as detailed species biology in order to build a computer model to predict future flows and the effects of future flow changes on aquatic plants. The Yampa River is a free flowing 250-mile waterway.
Results from that Yampa River model showed that modest alterations in historic patterns of flood and drought can negatively impact ecological networks. Over time, plant guilds were forced to compete for resources and became less connected throughout the river system. Similarly, the study showed that the “homogenization” of river flow through damming can be just as harmful to riparian systems.
“Connectance plays a fundamental role in maintaining biodiversity,” Tonkin said in a press release. “Evidence suggests that highly connected communities are better able to deal with species losses in food webs and are more resistant to invasion by non-native species. The simplification of these networks, including because of drought conditions that are predicted to increase widely over the next century, may predispose networks to collapse.”
According to Tonkin, the preservation of natural flow should be a priority for river managers in order to promote connectance throughout watersheds.
“River-dependent communities have evolved over millennia and have been tailored by natural selection to the volume and seasonal variability of the flows,” Tonkin said in the release. “Maintenance of flooding is fundamentally important for ecosystem health. Flooding is a vital driver of the ecology of rivers.”
When river flow is altered, the “keystone” riparian plant for that particular river can change. The term keystone indicates which riparian plant guild plays the most influential role in the overall ecology of the river. When the dominant plant guilds change the entire composition of the river is susceptible to follow suit.
“Removing floods, in particular, led to a loss of keystone status of hydroriparian pioneer trees, which are species like cottonwoods, alders, and river red gum,” Tonkin explained in the press release. “Loss of keystone guilds leads to changes in fundamentally important ecosystem services.”
According to Tonkin, those changes in river composition can include wildlife habitat, flood mitigation, bank stability, microclimate regulation and nutrient cycling.
“Because different guilds have different soil requirements and ecological roles, it is important to predict which ones will function in a keystone role under future flow regime scenarios,” Tonkin noted.
The OSU study was supported by the U.S. Department of Defense, the U.S. Forest Service and Dinosaur National Monument. Collaborators on the project include David Lytle of the OSU College of Science and researchers from the University of Washington and the U.S.F.S. Complete findings from the OSU study are published online at http://dx.doi.org/10.1038/s41559-017-0379-0.
