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tality (which is likely density-inde- pendent) may be responsible for the variability in returns seen in Figure 7, but it is not the reason that the population is declining. Without considering the geomorphic effects of the flood-control dams, the bio- logical research program (no matter how insightful) would not lead to an appreciation of how the habitat is becoming increasingly unsuitable for salmon. Case II: Channel incision and reduced floodplain inundation The Oconee River, as it flows through the coastal plain of Geor- gia, is a different type of lotic eco- system-a sand-bedded, low-gradi- ent, low-shear stress, highly sinuous, meandering river with bottomland hardwood swamps on its floodplain (Figure 8). A hydroelectric dam was built on the Oconee in the 1950s. The river appears to support lower densities of fish than do similar riv- ers in the southeastern United States (Evans 1994). The floodplains of many rivers in this region are inundated for signifi- cant portions of the year, commonly three or four months (Wharton et al. 1982). These floodplains are important to the ecology of many fish species that spawn or rear within them (Bain and Boltz 1989, Lambou 1990, Walker 1984). Also, because sand-bedded rivers do not produce large numbers of invertebrate prey for fish, the inundated floodplains serve as highly productive foraging grounds for many species, such as sunfish and black bass (Centrarc- hidae), catfish (Ictaluridae), and suckers (Catostomidae). The food they obtain there may be quite im- portant to their growth and fitness (Welcomme 1989). The inundated floodplains are also important to many fish as a refuge from preda- tion (Welcomme 1989). The reduc- tion in the availability of this refuge may lead to greatly reduced popula- tion levels of some forage species, which can in turn lead to population declines of piscivorous fish (Power et al. page 159 this issue). As the reservoir on the Oconee is small and has not attenuated the natural peak flows, one might ex- pect that the floodplain would con- Table 1. Areas and perimeters of lower McKenzie River islands and wetted sur- face area for the years 1930 and 1990. Year Features 1930 1990 Islands (number) 60 28 Island area (m2) 2,260,000 1,110,000 Island perimeter (m) 44,000 18,000 Wetted area of river (m2) 4,020,000 2,930,000 tinue to be inundated much as it had been in the past. However, as is often the case below dams, the river has lowered its bed and deepened its channel (incised) perhaps as much as 1 m, as a result of the dam captur- ing most of the river's sediment. Incision of 1 m in a river that al- ready had vertical clay banks 5-7 m high is not easy to detect. The river does not look or behave like an entrenched river. For ex- ample, lateral migration of the stream channel continues at a nor- mal rate (2-5% of channel width per year). We have drawn this con- clusion because we observed a coars- ening of the surface layer of in-chan- nel sediment deposits (armoring) and the presence of terraces on tributar- ies near their confluence with the Oconee, indicating that the tribu- taries have cut down in response to the new lower level of the Oconee. Armoring and tributary incision are common responses to river incision (Parker 1980). We are in the process of modeling the sediment transport in the Oconee River to better esti- mate the extent and magnitude of down-cutting that has occurred since the dam was built. The new, deeper channel requires a higher discharge to overtop its banks and spill out onto the flood- plain. Preliminary estimates indicate that the floodplain is inundated for far less time than before, even though the high flows have not been altered by dam operations. A reduction in the areal extent and duration of floodplain inundation can often re- sult in decreased species diversity and standing crop of fish (Finger and Stewart 1987, Pollard et al. 1983, Welcomme and Hagborg 1977, Wharton et al. 1982, Zalumi 1970) and is likely contributing to the apparently low population den- sities and catch rates of fish in the Oconee. As with the McKenzie, there has been considerable biological re- search on the Oconee into such top- ics as the minimal flow requirements of various fishes and guilds of fish species and the effects of flow fluc- tuations on fish displacement. We believe, however, the overriding problem has been the geomorphic adjustment of the river to the new conditions imposed by the dam. Case III: Increased stability of a braided river Our final example is from our work on salmon ecology in New Zealand's South Island, which is famous for its large braided rivers (Figure 9). Chi- nook salmon, which were introduced from North America in the early 1900s, are one of the primary spe- cies of concern in these rivers. De- spite the extensive gravel visible in Figure 9, little salmon-spawning habitat is available, because these rivers are highly unstable, flood fre- quently, and have constantly shift- ing channels. Succesful spawning occurs pri- marily in small spring-fed tributar- ies to these streams. Rearing habitat for recently hatched salmon (known as fry) is limited in these tributaries, and fry that rear in the main channel may be swept downstream by flood- ing flows. As these braided rivers typically have extremely small estu- aries, fry may be displaced directly into the ocean, where they may die as a result of premature exposure to high salinity levels or predation. The Waitaki River is the only large braided river in New Zealand that is dammed. The salmon are restricted to the lower Waitaki River, below the most-downstream reser- voir, Lake Waitaki. Because flood run-off in the Waitaki is stored to produce electricity later in the year, peak flows in the lower Waitaki are reduced. As a result, the riverbed does not shift and move nearly as often as it did prior to construction of the dam. The increased stability of the bars has allowed vegetation (e.g., willows [Salix spp.] and lu- pines [Lupinus spp.]) to become es- tablished on the bars, further stabi- lizing the locations of existing March 1995 187