Using their tails the way gardeners use hoes, salmon dredge up as much sediment from stream bottoms as the current itself does--even including erosion from spring floods--according to new research. The activity can dramatically alter the shape of streambeds and the health of stream ecosystems. By taking this influence into account, researchers might be able to design better stream-restoration projects.
Salmon are the kamikaze of fish. Spending most of their lives at sea, they end their lives with a splash. Adults make a grueling and ultimately fatal journey to reach their freshwater spawning grounds. Once there, the female salmon lay their eggs, or roe. But first, they must dig out a wide and shallow hole, called a redd. In the process, a single fish will excavate several cubic meters of sand and gravel. The smaller bits of that material stay suspended in the water long enough to be swept downstream a short distance by the current. Multiplied by millions of salmon, and repeated year after year, the dredging can have dramatic consequences.
How dramatic? Reporting in an upcoming issue of Geophysical Research Letters, a team led by geomorphologist Marwan Hassan of the University of British Columbia in Vancouver, Canada, used sediment traps to track the movement of preplaced magnetized particles, as well as detailed channel maps, to study the effect of salmon redding on four mountain streams in British Columbia. The researchers found that the salmon account for up to 50% of the annual amount of sediment migration in a given stream, visibly deepening channels in the headwaters and filling in pools and channels downstream. "People have known for a long time that salmon dig up the stream bottoms," Hassan says. "But until now, nobody knew how much."
The salmon excavating is actually beneficial to a stream's inhabitants. Just the act of piling sediment on the downstream sides of redds, for example, churns up the current enough to increase its oxygenation, improving the health of the ecosystem. Recognizing how this process works is critical to understanding the dynamics of streams frequented by salmon, says Hassan. And stream-restoration plans need to consider this effect because "off-the-shelf restoration designs" that don't account for the actions of salmon may not hit the mark.
The paper is important because it illustrates that the size and shape of rivers are not controlled entirely by physical variables, says hydrologist Gregory Pasternack of the University of California, Davis. And more and more assessments are revealing that river-restoration projects based only on physical factors are not succeeding in improving the quality of the environment, he says. Research hydrologist Thomas Lisle of the USDA Forest Service in Arcata, California, adds that the paper shows how dramatically an organism can affect its physical environment. With species as large as salmon, he says, the fish could be shaping larger-scale valley features and even influencing landscape evolution.