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11146 ? i on , ?f a(, 913 ?, Downstream Ecological Effects of Dams A geomorphic perspective Franklin K. Ligon, William E. Dietrich, and William J. Trush The damming of a river has been called a cataclysmic event in the life of a riverine ecosystem (Gup 1994). By changing the flow of water, sediment, nutri- ents, energy, and biota, dams inter- rupt and alter most of a river's im- portant ecological processes. The goal of this article is to illustrate a geomorphically based approach to understanding and reducing some of the cataclysmic effects of dams on downstream riverine ecosystems. Additionally, we hope to provoke debate on the merits of different approaches to river conservation. Most resources devoted to identi- fying the downstream effects of dams on riverme ecosystems are commit- ted to biological studies. We suggest that geomorphic studies directed at ecologically significant features of river morphology and hydraulics may sometimes be more valuable in short-term evaluations associated with the environmental assessment of new dams or the relicensing of existing dams. This is not to say that biological research on such issues as the ecological effects of dams on temperature, fish migrations, and Franklin K. Ligon is an aquatic ecolo- gist at EA Engineering, Science, and Technology, 3468 Mt. Diablo Blvd., Suite B-100, Lafayette, CA 94549. Wil- liam E. Dietrich is a professor in the Department of Geology and Geophys- ics, University of California, Berkeley, CA 95521. William J. Trush is the di- rector of the Institute for River Ecosys- tems, Humboldt State University, Arcata, CA 95521. © 1995 American Institute of Biological Sciences. If the stream's physical foundation is pulled out from under the biota, even the most insightful biological research program will fail to preserve ecosystem integrity altered flow regimes is not critically important. But we believe that, more often than has been realized, geo- morphological changes are the key to understanding the long-term eco- logical consequences of dams and other stream disturbances. We also have some reservations about the efficacy of biological re- search in short-term studies to lead to successful management plans. Even for a single intensively studied species such as chinook salmon (Oncorhynchus tshawytscha), the biological research necessary to de- velop a management plan is sub- stantial-often a multiyear under- taking-and may not lead to a clear understanding of important features affecting the population dynamics of the salmon. (For example, Pacific salmon species in the Pacific North- west continue to decline, and de- spite possibly 10,000 to 15,000 re- search articles on salmonid biology and ecology, there is little consensus on the relative importance and in- teractions of dams, commercial har- vest, hatchery-reduced fitness, oce- anic conditions, predation, and habitat degradation resulting from timber harvest-to list only the most commonly implicated mechanisms.) The challenge for biological research to develop effective, affordable man- agement plans is much greater when (as in many river systems through- out the world) there is concern not for just one species but for the biodi- versity of the entire river ecosystem; there is not a large body of literature (and perhaps none at all) on the life history, ecology, and management of the component species; and one does not have the time and money for a long-term study. It is in these situations that short- term geomorphic studies can greatly enhance understanding of how dams may fundamentally alter the stream ecosystem. The physical habitat is critically important to stream eco- systems and can change more easily and more quickly than in most other ecosystems (Powers et al. 1988). When a dam is built, altering the flow of sediment and water, the habi- tat will change-sometimes dramati- cally, but often subtly-and the ecol- ogy of the river can be significantly, and sometimes disastrously, altered. As there is typically insufficient time in the few years between the proposal and construction of a dam to investigate and understand the biological system that will be af- fected, taking steps to minimize changes in downstream habitats may often be all that one can realistically March 1995 183