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RESPONSE OF BEDROCK-GORGE ALLUVIAL SAND BARS TO OPERATIONS OF LARGE HYDROELECTRIC DAMS--COLORADO RIVER IN GRAND CANYON, ARIZONA, AND SNAKE RIVER IN HELLS CANYON, IDAHO AND OREGON. J.C. Schmidt, Geography, Utah State University, Logan, Utah 84322 and P.E. Grams, Geology, Middlebury College, Middlebury, Vermont, 05753• The area of eddy-sand bars and fine-grained alluvial banks exposed at low discharge along the Snake River in Hells Canyon has decreased by about 75X since dam closure. Changes downstream from Glen Canyon Dam on the Colorado River in Grand Canyon. over a similar period, have been much less. Although daily peak power operations of the two dams are similar, the rivers differ in that (1) high discharges are much more frequent on the Snake River, and (2) there are significant contributions of sediment from unregulated tributaries downstream from Glen Canyon Dam. These differences indicate that eddy sand bars, eroded by daily peak-power fluctuations, can only be reconstructed by regulated clear-water floods if sediment has previously been delivered to the affected channel from downstream tributaries. Dams with little reservoir storage (Snake River) more frequently release clear-water "floods" in excess of power plant capacity and provide insufficient opportunity for the accumulation of bed sediment delivered by downstream tributaries. This accumulated sediment can become available for entrainment by the infrequent floods of streams with large reservoir storage (Colorado River). THE EFFECTS OF A HYDROELECTRIC DAM ON EPIIITHIC COMMUNITY METABOLISM IN THE CLEARWATER RTi~ER, IDAHO, USA. Mazk D. Munnt and Merlyn A. Brusven2. 1) E.V.S. Consultants Inc., Seattle, WA 98102 and 2) Department of Plant, Soil and Entomological Sciences, University of Idaho, Moscow, ID 83843. The majority of research on regulated rivers has focussed on benthic community structure, with little emphasis on benthic community function. The Clearwater River system provides a unique opportunity in that the fully regulated reach flows for 2.5 km before merging with a larger, nonregulated branch thereby fomung a semiregulated river. Epilithic community metabolism was determined on a seasonal basis in riffle reaches of nonreguhited, regulated and semiregulated waters of the Clearwater River in northern Idaho. Metabolism was estimated using thee, 12-liter recirculating chambers and the dissolved oxygen method, with parameters expressed as g OZ m-2 d-i. In the nonregulated reach above the reservoir, gross community productivity (GCP) ranged from 0.8 - 3.2, net community productivity (NCP) from 0.5 - 2.6, community respiration (CR) from 0.3 - 1.2, and production/respiration ratios (P/R) from 1.2 - 3.3. Epilithic metabolism in the regulated reach immediately below the dam increased sharply; GCP ranged from 42 - 25.5, NCP from 3.1 - 22.2, CR from 1.9 - 9.7, and P/R ratios from 1.4 - 5.7. Increased primary production and respiration in the regulated reach was due to the dense growth of an aquatic moss (Fontanalis neo-mexicanus), which completed dominated the riffle portions of the regulated reach. The regulated reach also undergoes frequent flow changes and possesses a dampened thermal regime. The effects of Dworshak Dam on epilithic community metabolism was mitigated 2.5 km downstream of the dam due to the merger of a lazger nonregulated river. The semiregulated reach has a more normal dischazge pattern, thermal regime, and epilithic community which is dominated by algae. Epilithic metabolism in this reach was similar to that found above Dworshak Dam (GCP = 1.2 - 2.6, NCP = 0.8 - 1.8, CR = 0.6 - 1.3, and P/R = 1.4 - 2.2). This study demonstrates that while Dworshak Dam has altered both primary production and respiration directly below the dam, the placement of the dam only 2.5 lan upstream from a nonregulated reach greatly mitigates the longitudinal effects. CHANGE AND EXTINCTION OF BAZZANIA TRICRENATA COMMUNITIES AFTER RIVER REGULATION IN AURLAND, W NORWAY. Arvid Odland, Botanical Institute, University of Bergen, Allegt. 41, N-500'7 Bergen, Norway. The hygrophilous moss Bazzania trfcrenata, which is quite rare in the driest, eastern parts of Norway, was locally common in the spray-zones of waterfalls within the Aurland river before it was regulated for hydroelectrical development. After the regulation the species is still growing in some areas, but in several localities it has gone extinct. Before the regulation, 16 stands dominated by B. trfcrenata were described (species abundance given according to the Hult-Sernander-Du Rietz scale), and 20 years after the regulation 23 stands were described. The data have been analyzed by multivariate methods (TWINSPAN, DCA). The analysis indicate that there have been a change in the floristic composition of B. trfcrenata-dominated communities, and this change is assumed to be due to the reduction in spray precipitation. Several hygrophilous species such as Cerastfum cerastotdes and Saxffraga rivularfs have disappeared, and more dry-tolerating species such as Festuca ovina, Linnaea borealis, Hylocomoim splendens and PtiZidium crfsta-castrensis have increased in the communities. In sites which have become too dry, there occur large tufts of dead B. trfcrenata, and on these Festuca viuapara, HyZocomium splendens, Cladina spp. and other species tolerating dry conditions, are slowly expanding. 4