By Nate Hough-Snee
When someone asks, “what is the signature of human beings on Earth?” what do you immediately think of? Does your mind wander to vast expanses of land converted to row-crop agriculture? Or do you think of a sea of urban pavement and rooftops? What about the earth’s brightened night sky, as captured from space? What systems have been ubiquitously changed, those large and small, across continents and cultures? Whatever your answer, many people can agree that the landscapes that humans have most heavily altered are those that they perhaps need most – water dependent ecosystems: streams, rivers and lakes.
The impoundment of rivers and streams has followed Europeans across the North American continent, from New England millponds to the locks that connect the Saint Lawrence Seaway to the Great Lakes, water, particularly running water, has been inextricably tied to commerce and settlement. The power of the mighty Ohio and Mississippi rivers was harnessed for flood control and shipping. The Tennessee Valley Authority, preceded by the Wilson Dam on the Tennessee River in 1924, turned the lights on for an entire region of America. Numerous and nameless small dams maintained water for agriculture across the rural U.S. for generations. Water was eventually impounded along the Colorado, Columbia, Snake, and Missouri rivers, harnessing the intermountain and Pacific Northwest snowpack for the generation of electricity and revenue. It is not too generous to say that the hydroelectric dam transformed America economically, socially, and ecologically during the 20th century.
The principle behind a dam is simply to hold water upstream of a given point. By preventing the evacuation of that water (and the energy it creates as it follows gravity towards the ocean), humans have been able to generate millions of megawatts of energy, create lakes in deserts, make inhospitable landscapes inhabitable, and make inhabitable places profitable. This miraculously simple principle interwoven with some miraculously well-executed engineering has set a lasting paradigm of impounded rivers and streams for better or worse.
The governing ecological principles of aquatic and riparian ecosystems are much more complex than the simple principle of a dam. Aquatic plants and animals have evolved over very long timeframes to adapt to the stresses of the riverine environment and perpetuate themselves. Anadromous species like salmon (Oncorhynchus spp.) have evolved entire life cycles that involve spawning in rivers, fertilized eggs emerging from riverbeds, and smolts growing in streams and estuaries. Adult fish eventually head out to the ocean to make a living for a few years before returning to the streams from which they originated. When someone puts a dam up, it becomes pretty difficult for a salmon to find his or her way back to the habitat that, as they used to say on MTV Cribs, is “where the magic happens.” Dozens of miles of spawning and rearing habitat can be lost behind a single dam. The implications of this for fish populations are generally ominous, especially when there are dozens of dams in many large watersheds. It doesn’t take many generations of unsuccessful spawning and/or rearing to really reduce a fish population from its historic levels.
In addition to limiting the movement of aquatic fauna, plants that have evolved life history strategies that require seasonal floods, don’t have an easy time reproducing below a dam. Many rivers of the American West have peak flows in the spring and early summer as mountain snows melt. These historic unregulated flows would periodically scour out new channels, create bars and eat into hillslopes, depositing sediment of all sizes across the floodplain. This deposited sediment and the predictable recession of floodwaters allowed plants like cottonwoods (Populus balsamifera), willows (Salix spp.) and box elder (Acer negundo) to drop their seeds each spring into a palette of hospitable, damp soil. The term for water-dispersed seeds is “hydrochory,” and wind-dispersed seeds use “anemchory” to find safe sites to germinate. In a riparian landscape, either strategy can work, as long as the seed is blown or washed into bare mineral substrate where it can germinate. With a large dam upstream from a riparian forest, the seasonal pulse of water is taken away, or at best muted. Mature trees can persist, but the recruitment of new individuals into the stand is much less certain. Landforms that were once fed by upstream sediment and shaped by water slowly erode under the dull, muted flows. Eventually channels can incise and mature trees become disconnected from groundwater or the hyporheic zone, where groundwater meets surface water. This dewatering of the floodplain leads to declines of mature forest stands. These stands may be replaced by other plant species, but the habitat and geomorphic potential of these species is not identical to those trees that evolved to live near the river.
Dams, like many things that forever changed the ways humans go about daily life, have their societal upsides and also their downsides. The global extent of the ecological downside to the damming of rivers is not entirely known, but we know they have deleterious effects on their surrounding systems. As a result, many unnecessary dams are being taken down brick by brick. Over the next day or two, a few of the biggest dam removal projects in American history will be put into context. Stay tuned…
Mims, MC and JD Olden. 2013. Fish assemblages respond to altered flow regimes via ecological filtering of life history strategies. Freshwater Biology 58, 50-62. PDF
Merritt, DM and DJ Cooper. 2000. Riparian vegetation and channel change in response to river regulation: a comparative study of regulated and unregulated streams in the Green River Basin, USA. Regulated Rivers: Research and Management 16, 543-564.
In addition to the more academic homework, Felt Soul media, a filmmaking business taken on by another ecologist, released the trailer for their impending documentary, DamNation last year: