The experimental flood from Glen Canyon Dam in spring 1996 and Columbia basin program to bring back salmon in the 1980s initiate a new era of dam management in the United States. Conservation and improvement of river environment was included in the purpose statement of the new River Law in Japan, which was revised in 1997. Reservoir dams, erosion control dams and other dam structures are thought to have considerable effects on the structure and function of river ecosystem, but very few studies in Japan have demonstratively elucidated such effects. In order to provide present knowledge on influences of dam structure, studies in Japan and other countries were reviewed from the viewpoints of hydraulics, geomorphology, riparian forest dynamics, water quality, benthic animals, and fish life history. We believe that the idea of Adaptive Management will be useful as a future management strategy within a limited knowledge of dam influences.

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Dams and reservoirs are constructed in order to regulate river discharge from the view points of flood control and water resource development. Reservoirs store water but bring discontinuity of sediment, other materials and even organisms. Reservoir sedimentation is one of globally serious problems. Ecosystems not only around reservoirs but also in the downstream reaches must be degenerated. The latter is focussed on in the present paper particularly from the aspects of river engineering and hydraulics. The effects of dams on the downstream reaches are brought by changes in the temporal pattern of water discharge and decrease in sediment supply. Fluvial processes in such rivers are characterized by river-bed degradation, armoring, drying up of bars and flood plains, invasion of vegetation, deposition of fine material and so on. Understanding of such processes must be important in environmental assessment of new dam projects and following-up management of previously constructed dams. Evaluation of habitat suitability is also required there as one of rivereneineering subiects.

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Most bedrock channels in Japan are characterized by very rough channel walls and beds with protruding bedrock and with residual large blocks left on the bedrock floor. Few studies have been made on longitudinal gradient of bedrock channels because they have been believed to be far from equilibrium. However, river terraces indicate that these channels are close to equilibrium conditions, although they continue to downcut at a high rate. As the boundaries of a bedrock reach grow rougher, the channel gradient grows steeper, because the flow energy of bedrock channels is expended on the rough boundary. Flume experiment shows clearly that an increase in number of residual blocks on the river bed results in steepening the gradient of alluvial beds. Depositional beds above dams, free from obstacles, have gentler slopes comparing to the bedrock reaches, because the depositional bed is smooth. The reduction rate of the channel slope in such cases is variable depending on the roughness of the depositional bed-surface relative to the original bed roughness. Based on the recognition of the effect of residual blocks on the beds, we can propose a new engineering technique for preventing downcutting in mountain streams instead of sabo-dams. It may be possible to cause aggradation by placing large blocks to increase the bed roughness, because the blocks give the channel a steeper gradient. In order to elucidate the effect of bed undulations in bedrock rivers quantitatively, field measurements of controlled floods from reservoirs will give us suitable chances. We should refer to the Glen Canyon Environmental Studies in the Colorado River on this subject.

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Riparian forests play an important role in a river ecosystem. Dams for water regulation and erosion control substantially alter the regeneration habitats of riparian species, thereby affect their recruitment, composition and growth rate. This paper reviewed the effects of dams on riparian forest dynamics. The effects of dams can be seen in the regulation of water discharge, sediment transport and frequency of flood disturbances. For example, spring floods in snowy regions mobilize riverbed sediment and create the moist, alluvial substrate ideal for seed germination. These are essential habitats for the recruitment of Populus and Salix spp. In contrast, dams attenuate spring flooding and/or stabilize summer flows. This flow regime limits the development of sand bars and may induce drought stress for seedlings and old trees. The recent degradation of riverbeds, widely observed in Japan, is due to gravel mining, and sediment entrapment by reservoirs and erosion control dams. This situation provides stable habitats for riparian forests, resulting in expansion of woodlands. However, decreased flood frequency and magnitude along the downstream reach of dams may reduce densities of pioneer species and shift species distribution from pioneer to late successional trees. Seed dispersal by water which is especially important for maintaining species diversity in wetland forests may be affected by dam construction. The methodologies applied to previous studies on dam effects are based upon a paired-reach comparison (up- and downstream) and/or comparison among rivers with and without dams. Some studies only examine differences in species composition, but others analyze species distribution and population densities with hydrologic, geomorphic parameters. Pre- and post-regulation effects were examined by dendrochronological analysis of core samples taken from matured trees and interpretation of a series of air-photos. Simulations were also conducted based upon niche differentiation and forest succession models. In order to mitigate the effects of dams, I suggest that an adaptive management strategy, such as initiation of artificial floods, is required.

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The author firstly introduces three fundamental concepts regarding river ecosystems, i.e. river continuum, serial discontinuity and nutrient spiralling. Secondly, the author reviews the states of discontinuity caused by impoundment of rivers in five categories of water quality, i.e. water temperature, particulate matter, chlorophyll-a, nutrients and anaerobic water environment, taking into consideration with the effects of water quality changes on the downstream ecosystems. Finally, the author discusses measures to mitigate discontinuities using natural water movements and natural substances, and recommends the throughflow regulation method and the application of barley straw, especially against eutrophication.

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Influences of dams on benthic animal communities in tailwater rivers were reviewed focused on changes in their species richness, population densities and biomass. It was commonly observed that the benthos communities below dams had more population densities of fewer numbers of taxa in comparison with control rivers without dam effects. However, not a few works reported different results such as a case of less species richness, less population densities and less biomass or a case of more species richness in benthos communities below reservoirs. We considered functional processes of dam effects on benthos communities in the following respects: i.e., 1) impacts of flow regime regulation including stabilization of water flow, frequent flow fluctuation by electric power station, and drought in an extreme case, 2) alteration of habitat structure such as stabilization of river bed, water courses and armoring, 3) alteration of thermal regimes derived from surface or deep waters of the reservoirs, 4) effects of siltation, 5) effects of increasing plankton supply from the reservoirs, and 6) effects of dams and reservoirs themselves as an obstacle for migration by adult aquatic insects. The importance of Serial Discontinuity Concept and its future perspectives were also discussed. In order to assess the influences of dam construction and its management properly, the SDC model should be modified to reflect individual characteristics of each river and to include influences on diadromous animals and estuarine ecosystems.

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Damming causes many changes to the river. A dam and the resultant man -made lake change the river environment through a complex web of biological and physical impacts. The change from the river ecosystem to the lake ecosystem at the dam site will inevitably result in alteration of species composition, food habits and population dynamics of fishes. River impoundment affects endemic fish fauna and their life-histories through the following mechanisms : i. e., transverse blocking of the river channel, alteration of flow regimes and thermal regimes, and hindering upstream and downstream migration. Changes in fish communities have been recorded within considerable distances both upstream and downstream of dams. Hydropower reservoirs suffer high mortality if fish pass through the turbines. Since reservoir becomes a sink for sediments, seston transport is dramatically affected. Turbidity will increase during the construction phase and often over long distances. These constraints may lead to catastrophic falls in the abundance or extinction of some fish populations. The extinction might be associated with the introduction of exotic species. Flow regulation by dam can prevent or delay the inundation of floodplains and riparian zones, reducing or eliminating areas for breeding and reprodution of fish. These rapid and often extensive changes in the discharge regime cause displacement of fish, particular-ly juveniles, and can lead to loss of the fisheries. The modification of seasonal and diurnal thermal regulation causing the alteration of fish assemblages, usually results in reduction of species diversity in the river, when the dam releases the hypolimnial and/or epilimnial waters. The numerous unfavorable effects of impoundments and other forms of river regulation must be well documented for the planning and execution of future projects for the rehabilitation of water courses. Ecological evaluation is essential part of the programme on the base of "long-term environmental sustainability". It is hoped that the principles and guidelines of habitat improvement including the idea of removing dams will be made based on long-term ecological assessment.

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Experimental removal of coarse woody debris (CWD) was conducted in a low-gradient, second-order stream covered by deciduous riparian forest in northern Hokkaido, Japan. We examined channel morphology, habitat variables and fish densities in 2 study reaches (cleaned and control, 100 m length) during autumn (Sept.-Oct.) of 1993, spring (May - Jun.) and autumn (Sept. - Oct.) of 1994. Immediately after the first survey, CWD and other small organic materials within the cleaned reach were removed by hand or hand-winch. Aggraded areas in the cross-sections of the cleaned reach were larger than those of the control reach during the autumn of 1993 - spring 1994. Cross-sectional changes tended to correlate with channel variables, such as curvature and width, in the cleaned reach, and with CWD amounts in the control reach. After CWD removal, small pools associated with CWD disappeared and pool number decreased, but pool volume did not vary. Cover area decreased, but depth, velocity, and substrate did not change in the cleaned reach. Density of juvenile masu salmon (Oncorhynchus masou) declined rapidly, but that of stone loach (Noemacheilus barbatulus toni), a benthic fish, did not show any significant changes.
Deposition of sediment after CWD removal was associated with development of gravel bars at channel bends or wider sections. Pools were recreated by lateral scouring at the opposite sides of banks where gravel bars developed. We concluded that CWD pieces play an important role in forming pools and controlling pool number, although they are not effective in increasing pool volume in low-gradient meandering streams. Thus, a decline in masu salmon population could be attributed to a decrease in cover and pool number in this stream.

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This study aims at clarifying the relations of habitat conditions required for Pungitius pungitius to the environmental factors of springs in non-irrigation period and at considering with the way of spring management desirable for conservation of the fish populations. The study was conducted in five springs in Hataya, Senhata Town, Akita Prefecture, from December 1997 to April 1998. In those springs, the population density of P. pungitius decreased from December to March, when the amount of eruption water decreased. Particularly distinct decrease in the population density of fry was observed in the springs where the average water temperature in December showed a larger temperature balance between at the eruption point and at other measuring points in the spring. Even in the spring with a smaller value of the temperature balance throughout the investigation period, the lower population density was also observed where the vegetation coverage of aquatic plants was small. In the springs with small fluctuations in the water temperature and high coverage of aquatic plants, occurred the higher population density of fry and distinct rise of the total population density during March through April. In order to avoid the affects of farm land consolidation on the fish population, it is desirable to make connect, via ditches, springs being capable of drying up or having large water temperature fluctuation with those having much amount of eruption and less water temperature fluctuation.

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Data continuously recorded for one hour on chlorophyll a at lower reaches of the Nagara and Kiso Rivers made it clear that two planktonic communities with different origin have developed at their rivermouths. One was a community dominated by Cryptophyceae or Dinophyceae originating in a brackish or marine environment. Changes in their biomass synchronized with that in salinity. The other was a potamoplanktonic diatom community drifting down from upstream, and the maximal algal biomass was recorded at low tide.
A rivermouth barrage in the Nagara enhanced potamoplanktonic algal production in the newly created freshwater region by prolongation of the river water retention time. Moreover a large algal biomass downstream of a construction site in the Kiso suggested that brackish or marine planktonic algae accumulation resulted from obstruction of dilution with freshwater and inhibition of exchange of the rivermouth water. The mechanism by which the algal biomass of each community was controlled after construction was different. For assessment and improvement of river eutrophication after barrage construction, a clear distinction between the two planktonic communities should be considered.

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Methods to estimate the season of adult emergence of Genji-firefly, Luciola cruciata, were presented with analysis on the relations of landing date of mature larvae and emergence date of adults to the air temperature at Kiyotaki and Ginkaku-ji in Kyoto City and Nagaoka in Santo Town, Shiga Prefecture. The peak (or initial) emergence date of adult fireflies (Y) could be estimated from the peak (or initial) landing date of mature larvae (X) as; Y=14.23+1.34×X, where X and Y were presented by days from 1st January. The developmental zero and the effective cumulative temperature during the pupal stage (after the landing of mature larva until adult emergence) were estimated to be 13.7°C and 162.6 degree-days, respectively. Without observation on the landing mature larvae, the peak flight season of adult fireflies (Y, presented by days from 1st January) was possible to be roughly estimated with the cumulative temperature above the developmental zero (13.7°C) for 20, 30 or 40 days after average air temperature exceeds the developmental zero (_SUB20_/SUB or X₃₀ or X₄₀, respectively) : Y=174.8-0.248×X₂₀, Y=180.2-0.199×X₃₀ or Y=187.1-0.175×X₄₀. To generalize these estimations for localities other than Kyoto and Shiga prefectures, we need to compare the developmental zero and cumulative temperature of other local populations.

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