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hope to accomplish. At present these steps are rarely taken. Our aim in this article is not to present a technical discussion of flu- vial geomorphology and stream habitats (see the excellent treatment of this subject by Kellerhals and Church 1989) but to stress the im- portance of geomorphological analy- sis in river conservation and man- agement. To illustrate how subtle geomorphological adjustments may profoundly influence the ecological relationships downstream from dams, we present three case studies from our research. We then outline 2,000 E 1,500 1,000 L Ln 500 0 Cougar Dam (1963) Klee River Dam (1969) 0 1920 1940 1960 1980 2000 Year Figure 1. Annual maximum instantaneous peak discharges, McKenzie River at Vida, 1923-1989. Since the construction of two dams in the 1960s, peak dis- charges have been reduced by more than 50%. (No data are available for 1987.) a geomorphically based approach for assessing and possibly mitigat- ing some of the environmental ef- fects of dams by tailoring dam de- sign and operation to minimize geomorphic changes. The premise of this approach is that by minimiz- ing the alteration of the physical dynamics and morphology of rivers, many complex species interactions and physical requirements can be maintained without scientists un- derstanding or even acknowledging their importance. If the physical foundation of the stream ecosystem is pulled out from under the biota, even the most insightful biological research program will fail to pre- serve ecosystem integrity. Case I: Channel simplification and salmon decline The McKenzie River in Oregon ap- pears to be a pristine river, little touched by the demands of twenti- eth-century US society. Although two Army Corps of Engineers flood- control dams have been built on tributary streams, most of the wa- tershed (73%) is still unregulated. The annual hydrographs appear natural with numerous discharge fluctuations in response to storms. The main effect of these dams has been the reduction of peak flows. Instead of peak flows reaching 1100-1800 cubic meters per second (cros), they now reach approximately 500-800 cros (Figure 1). A flood of 500-800 cros is approximately bank- full discharge (the discharge that just overtops the stream's banks; Dunne and Leopold 1978). Towns and farmlands just overbank on the floodplain are now protected from flooding (Figure 2). Bank-full dis- charge is often recommended as the high-flow release that is likely to maintain channel integrity in regu- lated rivers (e.g., Hill et al. 1991, Rosgen et al. 1986), so one might expect from a geomorphological perspective that all is fine for this relatively pristine river. Upstream of a point 40 km from its mouth, the McKenzie River flows in a narrow, bedrock-confined can- yon, and boulders are common on the bed. Downstream of this point, the river valley opens up into a broader floodplain bordered by Pleistocene river terraces derived from glacial outwash and perhaps also younger alluvial terraces, both composed of a large range of sedi- ment sizes. In these lower, uncon- fined reaches, the McKenzie is what is known as a wandering river (Kellerhals and Church 1989). For the most part, it has a low sinuosity, single-thread channel, but it has occasional braided reaches with midchannel bars and islands (Figure 3). The flood-control dams are ar- resting the development of mid- channel bars and islands, and the river is becoming exclusively a single- 184 BioScience Vol. 45 No. 3 Figure 2. Developments, such as this house built on the floodplain of the McKenzie River, are now protected from flooding during major storm events.