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1172 Issessin", llydrokwic.Illeallion hand, is much shorter in the post-dam period. This is a byproduct of hydropower generation, wherein water is stored in the reservoir until sufficient head is attained to generate power efficiently, at which time it is rapidly re- leased through the dam turbines. The effect on the hy- drologic regime is to create a greater frequency of high and low pulses of lesser duration (Group 4: frequency and duration of high and low pulses) and also to in- crease the number of hydrograph rises and falls (Group 5: rate and frequency of change in water conditions). The magnitude and timing of the annual minima have changed, with a shift from higher fall season to lower mid-winter annual lows (Fig. 6). This probably results from attempts to capture winter flows for later spring and summer use in hydropower generation. Surprisingly, the average hydrograph rise rate (Group 5: rate and frequency of change, Fig. 7) for the Roanoke is reduced from the pre-dam period. Typically, areas downstream of hydropower dams experience steeper hydrograph rises because of the rapid release of water from the reservoir during peaking power generation. The apparent reduction in rise rates on the Roanoke is probably due to the fact that flow releases seldom ex- ceed 566 cros (20,000 efs), which corresponds to tur- bine capacity limits. In the pre-dam period, flows com- monly rose more than 1132 cros (40,000 cfs) in a single day during rainstorms. Changes in the variability of the 32 IHA parameters are also evident (Table 2; Figs. 4-7). In general, variability has been reduced in summer and winter monthly means, in extremely low water conditions, in timing of the an- nual highs and lows, in high and low pulse durations, and in frequency and rate of hydrograph rises and falls. On the other hand, coefficients of variation increased for springtime monthly means and long duration (e.g., 30- and 90-day), high flow magnitudes. Dam-related alterations to the Roanoke flow regime have been blamed for the ,drastic reduction of striped bass populations,(Zincone & Rulifson 1991). Higher av- erage streamflows in the spring months (May June) have been associated with less successful rates of juvenile bass recruitment. Aquatic invertebrates inhabiting the littoral zone along the river's edge may be severely af- fected by the greater frequency of hydrograph pulses, rises, and falls. Rapidly reversing cycles of wetting and drying have been shown to decimate littoral-zone benthic fauna unable to migrate with the shifting river edge (Armitage 1984; Walker et al. 1992; Moog 1993). Such losses of benthic fauna may be substantially reduc- ing the availability of prey for the Roanoke's fishes. Altered flood patterns may lead to significant alter- ations in the composition and structure of the Roanoke's bottomland hardwood forest by changing the.magnitude and duration of floods (Lea 1991; Richter 1993). This forest has been heralded as being the "highest quality and most extensive" bottomland hardwood forest on the Ricblcr el III southeastern coastal plain (Lynch 1991). The different plant species and floodplain forest communities along the Roanoke are thought to be distributed along a gradi- ent of inundation duration (or anoxic stress). With the elimination of high-magnitude flooding, higher flood- plain surfaces are now seldom if ever inundated, en- abling less flood tolerant species to become established on lower sites, and thus lowering overall vegetation di- versity. Changes in the forest could also have serious im- plications for Neotropical migratory birds using this area (Zeller 1993)• Using the IRA Method During development of the IRA method, a longer list of statistical parameters was consolidated to minimize the number of computations and to reduce redundancy; at the same time we retained as much sensitivity to differ- ent forms of hydrologic alteration as possible. The 32 IRA parameters appear to be robust in-their ability to quantitatively describe alterations peculiar to specific human influences such as flood control. We also consid- ered aggregating the results across each of the five groups of hydrologic characteristics. Users must bear in mind, however, the risk of losing information when relative differences are averaged across parameters within IHA groups (Suter 1993). We strongly recommend that IHA results be presented in the full scorecard format as shown in Table 2 to retain information about the specific hydro- logic alterations associated with the perturbation under investigation. Reporting the full suite of hydrologic parame- ters also enables investigators to explore relationships be- tween individual parameters and biotic responses. This caution about lumping hydrologic parameters into IRA groups and averaging results within groups should not inhibit exploration of the relationship be- tween overall group averages and specific types of hu- man influences such as reservoir operations, groundwa- ter pumping, timber harvest, or urbanization. Such integrative analysis is urgently needed to enable ecosys- tem managers to better assess or anticipate the effects of certain land and water uses. The sensitivity and robust- ness of individual IRA parameters and IHA groups to a wide range of human influences in different ecoregional settings remains to be tested. The U.S. Clean Water Act Amendments of 1972 (PL 92-500) called for the restoration and maintenance of the "chemical, physical, and biological integrity of the nation's waters." Increased use of analytical methods such as the IRA will demonstrate how far we have to go toward restoring the physical integrity of U.S. rivers, lakes, and aquifers. We anticipate that the IHA method will be used in conjunction with other ecosystem met- rics that evaluate more directly the biological conditions and ecological degradation within an ecosystem, such as Conservation Biology Volume 10, No. 4. August 19%