Headwaters, the streams that make up the beginnings of rivers, are unique components of catchments as they usually support a taxonomically and ecologically unique fauna. However, since headwaters are generally narrow and have a fluctuating course, they have received little attention from ecologists, and their benthic fauna, including insects, is poorly studied. This paper focuses on an endemic Japanese mayfly Dipteromimus tipuliformis, which has possibly very limited dispersal ability, and is restricted to headwaters. Its ecological relationships (its dispersal ability) and genetic variation (genetic distance within and between populations in the mitochondrial 16S rRNA gene sequences) were discussed, and compared with some other mayfly species (ephemerid mayflies, and other related mayflies) living in other water systems. Dipteromimus tipuliformis showed greater inter-population genetic distances than those of any other of the mayfly species examined.

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We examined the habitat characteristics of larval Cyprinidae (larvae: < 13 mm in total length [TL], juveniles: 13-30 mm TL) at a base flow (0.1 m³s^-1) and under experimental flooding conditions (0.5 and 1.0 m³s^-1 discharge) in two zones (BRZ: Backwater Research Zone, IFRZ: Inundation Frequency Research Zone) of experimental stream B in the Aqua Restoration Research Center in Gifu, Japan. Although the two zones appear as a straight channel, the environmental characteristics of the BRZ, which had three backwaters, were more diverse than those of the IFRZ. In the base flow study, most larvae and juveniles were captured near the shore of the two zones (low abundances) and in the backwater areas of the BRZ (high abundances), where water velocity was slower (2 cm/sec) and there were more areas with high cover (larvae: 56%, juvenile: 44%). However, fish were absent in areas of high cover (> 78%), with a high water velocity (6.6 cm/sec). Sites in the BRZ where fish were present were separated into two different habitat types: 1) shallow, slow moving water and fine substrate on the backwater borders; 2) deep water in the open areas of backwaters. This result suggests habitat segregation of cyprinid species at early growth stages. Larvae congregated in the backwater areas during the experimental floods, whereas juveniles did not differ significantly in the periods before and after flooding. These results suggest that diversity of floodplain morphology should be preserved to maintain larval habitats not only at the base flow level but also under various flow regimes.

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The Shibetsu River winds through Eastern Hokkaido before emptying into the Okhotsk Sea. The river had many meanders before 1940’s, the mountains at the upper reaches used to be covered with natural forest, and a large wetland extended at the lower reaches. After 1950’s flood control projects by straightening channel significantly contributed to reduce flood hazard, and the whole floodplain has developed into a dairy farming area. The circumstances of the river and its basin have changed over time, and locals today are calling for restoration of the river to its original state and creation of a river environment where agriculture and fishery, the basin’s key industries, can prosper together. In response, the Kushiro Development and Construction Department of the Hokkaido Development Bureau, Ministry of Land, Infrastructure and Transport, has launched nature restoration projects. For these projects, the Advisory Committee for the Shibetsu River Basin Management and the Shibetsu River Technical Committee were founded. These committees are conducting onsite surveys, planning-related prediction and assessment.

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The Shibetsu River originally had a number of meanders, and a wetland extended at its lower reaches. Cut-off works to straighten the meandering river course began in 1953. As a result, safety against flooding was improved, the wetland disappeared and farmland development in the river basin progressed. In recent years, however, there have been increasing demands from local residents for the creation of a better river environment. Restoration of the natural river environment has therefore been promoted since 2000. Since it was considered to be difficult to ensure safety against flooding if only the old meandering river course was restored, it was ultimately decided that the old river would be restored while maintaining the present straightened river course. Water was reflowed into the restored meandering river course on March 18, 2002. The meandering river experienced six floods during which annual maximum daily discharge exceeded the average annual maximum daily discharge rate. Eight field studies were conducted to visually observe changes in the channel and flow conditions. And to quantitatively understand changes in landform, five cross-section surveys were performed. These investigations showed that bank erosion, other than the expected bank erosion at the bifurcations, occurred in many places because of meander bends and flow deviation due to sandbars. The erosion resistance of banks differed markedly between the pre-improved channel and the excavated channel. Therefore, it is essential to take this behavior into consideration when predicting channel migration. It is also important to gain a thorough understanding of the areas to be affected by the plan through the examination of relevant data, including the centerline of the channel, bed configuration and flow conditions.

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Limnological characteristics of a shallow oxbow lake (maximum depth about 2 meters) were investigated in a lower reach of the Shibetsu River in Hokkaido Island, Japan. Water temperatures, under-water light attenuation coefficients, and concentrations of dissolved oxygen, nitrogen, phosphorus and chlorophyll-a were measured on July 21 and November 17 in 2001 and July 30 in 2002. Water temperature varied among sampling stations and depths, ranging from 10°C to 24°C in July, while all measurements were constant at about 5°C in November. Dissolved oxygen concentrations at all stations on all sampling dates were 10 mg L^-1 or more. Its maximum concentration reached 25 mg L^-1 with 250% saturation at the lake bottom on July 21 in 2001. The depths of sampling stations that obtained high concentrations of dissolved oxygen were between 50 cm and 60 cm. Water temperatures at these depths ranged from 10°C to 15°C, which were lower than surface water by 5°C to 10°C. A large filamentous green alga, Spirogyra sp., propagated in these depths. Concentrations of dissolved nitrogen in lake water varied from 11 μg L^-1 to 250 μg L^-1, which declined in July at all stations and depths. Concentrations of PO₄^3--P showed a range from 7 μg L^-1 to 14 μg L^-1, not so different among stations, depths and sampling dates. Particulate phosphorus and chlorophyll-a concentrations were approximately constant between 33 μg L^-1 and 35 μg L^-1 and 10 μg L^-1 and 13 μg L^-1, respectively. The concentrations of total phosphorus and chlorophyll-a on 21 July 2001 indicate that trophic status of the oxbow lake is in meso-eutrophic conditions. Under-water light attenuation coefficients (k) ranged from 1 m^-1 to 2 m^-1 at all stations and sampling dates. These values are comparable to the maximum value for eutrophic lakes with waterbloom of phytoplankton. However, because chlorophyll-a concentrations in this oxbow lake were lower than those in the eutrophic lakes, under-water light seemed to be attenuated by non-living particles and dissolved organic matter. Therefore, there is a possibility that the oxbow lake is in dystrophic conditions. These results suggested that the limnological characteristics of the oxbow lake were greatly different from those of the main channel of the Shibetsu River. It is believed that the oxbow lake maintains the characteristics of a lowl-and marsh, which was former landscape of the lower region of the Shibetsu River watershed.

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In March 2003, a re-meandering experiment, connecting main channel with an oxbow lake, was implemented in Shibetsu River, located in eastern Hokkaido, northern Japan. The objective of this study is to examine the changes in distribution and structure of macroinvertebrate community before and after the re-meandering experiment in the alluvial lowland river. We investigated composition of macroinvertebrate community in straight reach, oxbow lake (pre-experiment), and re-meandering reach (post-experiment). After the oxbow lake was connected with the main channel, we set five transect lines for quantitative evaluation of macroinvertebrate at three study reaches (natural meandering, re-meandering and straight channel). The hypothesis we tested was that the re-meandering reach containing more diverse habitat environments has more diverse macroinvertebrate community than do straight channel. The study results showed that most lentic species were replaced with lotic species after the connection. Most taxa identified on transects, however, were found at shallow shorelines of point bar, which we did not expect before the experiment. Since taxa richness and abundance were negatively correlated with shear velocity, a low shear stress and resultant bed stability at the shorelines of point bar are responsible for a high abundance and richness of macroinvertebrate community. Those results indicated a role of point bar for keeping abundance and diversity of macroinvertebrate in alluvial lowland river. Taxa richness and abundance of macroinvertebrate community in re-meandering channel had once recovered to the level of natural meandering channel in June, but this recovery was not maintained until November. We believe that this is due to the continuous movement and changes of bed material as well as channel morphology. The re-meandering reach is still dynamic and we have to monitor the physical process as well as ecological response until it reaches to the stable phase.

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We investigated effects of experimental channel re-meandering on river ecosystem in lower reaches of the Shibetsu River, Hokkaido, northern Japan. Field samplings before and after re-meandering were conducted to compare fish species abundance and physical conditions of instream habitat to those in a straightened main channel (control reach). The reach used for re-meandering, an isolated old channel of the river, had been a stagnant lentic zone. Physical conditions in the past channel therefore were extremely different from those in control reach. Whereas a large number of lentic fish and crustaceans were found in past channel, only a few lotic fish were found in control reach. After re-meandering, physical conditions of past channel were dramatically changed and showed similar depth and velocity distributions to those in control reach. Re-meandering also greatly lowered lentic fish abundance. On the other hand, many masu salmon were also found in re-meandering reach. We recorded large-sized salmonids at a concave part of re-meandering reach, which were not found in control reach. The channel re-meandering improved its planar geometries and successfully restored diverse instream habitats complex in longitudinal and cross sectional structure. However, unlike past channel, limited availability of slow-flow habitats in re-meandering reach hindered lentic species to colonize.

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Stream restoration has been conducted in the Shibetsu River in northern Japan, which aims at the recovery of its river-floodplain ecosystem altered dramatically in the last 40 years. Well-designed monitoring before and after restoration should be conducted to evaluate the effects of the stream restoration on its ecosystem. An analysis of stable isotope ratios (δ-values) is one of the most effective methods for monitoring material cycling in river ecosystems. Because product δ-values are roughly dependent upon their source δ-values, we can trace material flows by comparing the δ-values of various compounds. In the first section of this review, we introduced a basic concept of the δ-value analysis by using an analogy from various diffusion patterns of salt water. The second section described how to calculate δ-values and discussed the difference between the analyses of natural stable isotope ratios and the tracer analyses (another powerful tool for material flow analysis). The third section presented the theoretical basis of mechanisms which change the δ-values. A product δ-value becomes different from its source δ-value in two kinds of processes: isotope exchange equilibrium and kinetic isotope fractionation. The magnitude of this difference (Δδ-value: δ_product-δ_source) tells us about the processes and activities of the reactions. The fourth section introduced specific examples from the carbon and nitrogen δ-values (δ¹³C, δ¹⁵N) of aquatic organisms to infer the degree of eutrophication and carbon recycling at ecosystem levels as well as to determine food web structures and photosynthetic activities at biological interaction levels. In the fifth section, we presented the result of our study indicating differences in food web structure between a main-stream channel and oxbow lake in the Shibetsu River watershed when their δ¹³C and δ¹⁵N values of diverse aquatic organisms were compared. The advantages and limitations of the stable isotope analysis in environmental assessment were also discussed.

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Many restoration projects have been carried out and various topics associated with these restoration projects also have been discussed in the United States. In general, these discussions focused on ecological theory and technique; however, discussions focusing on the perspectives of the social sciences and humanities on actual restoration projects have been increasing. This paper introduced some typical assertions of these recent discussions in the U. S., and the lessons from these discussions were interpreted for Japanese restoration efforts which have been just started. There are two crucial lessons: first, it was more important for Japanese restoration projects than U. S. cases to discuss the ‘effective restoration’ from not only the aspect of natural science but also the aspects of social sciences and humanities. Second, these aspects included the careful consideration to local history, culture, and an existence of a target community, since restoration projects in Japan took place closer where people lived than the U. S. cases. The lessons lead to require a discussion table, which various restoration aspects can be explored for each restoration project.

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