Flow regime is recognized as a prominent factor which determines stream biota, and thus the alteration of flow regime by dams or land use may cause serious degradation of river ecosystems. Natural and altered flow regimes have been evaluated using a variety of flow hydrologic metrics. Such analyses can contribute to elucidate the habitat template of stream-dwelling organisms and to support environmental flow management. Yet, flow regimes of Japanese rivers have not been analyzed to the present using hydrologic metrics in a nation-wide scale. We calculated 171 hydrologic metrics from 20 years of flow records of 452 gauging stations in Japanese rivers. These metrics were used to characterize flow regimes by principal component analysis (PCA). PCA showed that the magnitude of flow variability is the most important component of flow regime, which generates the spatial variability in flow regime across Japanese rivers. This result was consistent with those of previous large scale studies, but the importance of the frequency and the timing of flow events were found to be lower in the present study. Flow regimes of Japanese rivers were suggested to have relatively high temporal synchronism owing to small land area. Next, Japanese rivers were classified into eight hydrologic groups by K-means clustering based on PC factors. The groups were spatially overlapped, whereas some of them showed clear geographical differences. Finally, we used generalized linear models (GLM) to assess influence of catchment factors on the obtained PC factors. Climatic condition was a predominant factor in determining the flow regime of Japanese rivers. Flood control by dams and land use seemed to alter flow regime but the influences of those anthropogenic factors were weaker than that of climate, probably because of small sizes of dams, flashy flow, and narrow plains in Japan.

We examined precision and accuracy of stream habitat measurements in relation to transect spacing, by using seven study reaches, where wetted width, water depth, and current velocity were measured on the basis of 25-cm （＜0.1 width） interval transects. Means and SDs of wetted width, water depth and current velocity based on the 25-cm interval transects were considered the true values, and these six variables were calculated on the basis of different transect spacing from 0.25 to 3.0 wetted width. We assessed （1） decline of the precision, expressed by coefficient of variation （100SD / mean） of repeated measurements, （2） decline of correlation coefficient of the true values with the values based on each spacing, and （3） increase of differences in frequency distribution between the true values and values based on each spacing, due to increasing transect spacing. We also conducted a literature survey to examine （4） transect spacing used by previous studies. Mean wetted width, mean depth and mean velocity based on 0.5-width interval spacing were sufficiently precise and accurate （precision ＜10％; correlation coefficient with the true value: r ≈1.0）. Overall results suggested that reasonable precision and accuracy would be expected up to 1.5-width interval spacing （precision＜15％; r with the true value ＞0.95）, when mean wetted width, mean depth and mean velocity are used as habitat variables for among-reach analysis. However, the precision and accuracy for SDs of wetted width, depth and velocity were lower than those for means of the three variables, suggesting that, when SDs （or other measures of variance） of these variables are used to represent within-reach habitat variations, finer spacing would be needed. Further, when a detailed analysis is conducted on within-reach habitat variation （e.g., microhabitat preference by fishes）, ＜0.5-width interval spacing was suggested to be ideal and 1.0-width interval would be necessary.

In the mouth of the Naka River in Tokushima Prefecture, approximately 24％ of the estuarine environment has been altered because of heightening of the embankment as a countermeasure for the earthquake and tsunami in 2012. Artificial tidal flats, aimed at conserving the habitat of endangered benthic species, were created for mitigation of negative impact due to the construction. To assess the effectiveness of the mitigation, we set nine survey sites in the area, and conducted monitoring the sites 10 times between 2013 and 2017. Thirty three species of crabs and 27 species of gobies were found in the surveys. All threatened and near threatened crabs and gobies found during the first survey were collected during the subsequent surveys as well, suggesting that the habitat of these species has not disappeared due to the construction. We confirmed a decrease in the mean mud content in most of the survey sites in winter 2015, due to a large flush in summer 2014. Contrarily, the mean mud content increased at the mitigation sites due to the introduction of silt dredged from a neighboring river after the large flush. Nine survey sites were analyzed separately in the high （four sites） and low compartments （five sites） according to the range of elevation. The results of the high compartment sites, including two mitigation sites, suggested that the sites had fauna similar to that of the reference sites, after about two and a half years from the construction. On the contrary, the results of the low compartment sites suggested that the sediment condition and fauna changed in most of the sites due to the large flush. However, the dredged silt was supplied around the mitigation sites. Therefore, the fauna similar to that of before the embankment construction was formed around the mitigation sites after its construction.

To develop a generalized method for the primary assessment of inter-levee floodplain pond environments in large Japanese lowland rivers, a generalized prediction model for freshwater mussel occurrence was built, and an application method for river management was suggested. Mussels were present in all of nine study rivers. Mussels occurred in 73 of the 363 surveyed ponds, at 1 to 21 ponds in each river. The probability of mussel occurrence was predicted using the relative height and area of each pond, with hump-shaped and positive relationships with mussel occurrence, respectively. As an example to demonstrate its application to river management, a floodplain assessment figure with a form similar to that of the discharge capacity figure, was created for the Kiso River.

The limited water-level method is used for the reservoir of the Miharu Dam, located in upstream of the Abukuma River, which flows into the Pacific Ocean, and drawdown of the water level is routinely practiced just before the flood season (June to October). At this time of the year, the surface water temperature of the reservoir reaches around 15℃ at which spawning of largemouth bass, an invasive species, is thought to begin. Hence, the drawdown period coincides with the spawning period of largemouth bass. Several patterns of drawdown were performed in order to evaluate the effects of the drawdown on the spawning of largemouth bass. In an ordinary drawdown, the water level is lowered at a constant rate till the onset of the flood season. In the Miharu Dam reservoir, drawdown is temporarily paused for 2 to 4 days in the midst of drawdown to promote the spawning of largemouth bass. Then, drawdown is resumed to dry up the spawning beds established at the maintained water level. As a result, a considerable number of exposed spawning beds were detected by setting a few maintained periods during a drawdown, called “Stepped Water-level Drawdown”, and exposed spawning beds were observed at depths of 0.5 to 2.0 m from the maintained water level. Fries of largemouth bass were captured and the dates of their spawning were estimated from the relationship between length and daily rings (otoliths). The analysis revealed that spawning during the drawdown period scarcely led to hatching. It was also found that 4-stepped drawdown is more effective for exposing spawning beds than 3-stepped drawdown. Population growth simulated by the Leslie matrix given a scenario of 4-stepped drawdown yielded a stable trend over 50 years. These analyses suggest that 4-stepped drawdown before the flood season could be an effective management method to control the recruitment of largemouth bass.

In recent years, invasive alien plant species, Sicyos angulatus has been spreading across the shoreline area especially in the drawdown zone, where the water level fluctuates （8 m） between the flood and non-flood seasons, in the Miharu dam. Particularly, the expansion of the S. angulatus community has been conspicuous since 2009, when the water level rise to the non-flood season has slowed along with the maintenance work for the reservoir shoreline in the end of the flood season. It is hypothesized that decrease in the number of flooding days given to the seeds of S. angulatus during the non-flood season relates to the proliferation of S. angulatus community. Hence, the research was focused on the ecological characteristics such as water tolerance of S. angulatus seeds. It is found that many S. angulatus fruits observed along the shoreline turning their color green to brown in October, leaving seeds to mature. Green and brown fruits had different water tolerant properties. All green fruits sank in the water in 178 days and were decayed, and the germination rates in the next spring were low. Therefore, it was considered that fruit germination could be reduced if the water level was raised to immerse S. angulatus community by the middle of October, when the fruits were ripened. In the case of the Miharu dam, germination of S. angulatus could be inhibited within the range of 2 m above drawn water level in flood season. On the other hand, about 40％ of the brown mature fruits were floating even after the seven months of the flooding period, and more than 20％ germinated in the next spring whether they are floating or sunken. Since the water level during the non-flood season is raised back near the normal high-water level and maintained in the Miharu dam, it was thought that the floating fruits were diffused near the normal high-water level and germinated as seedlings in the next spring. This could be the reason S. angulatus community in the Miharu dam was large near the normal high-water level. In order to suppress S. angulatus, it is crucial to immerse the fruits while immature, cut down S. angulatus before attaching the fruits, and prevent fruits from reaching to the shore.

Effects of multiple fishways made by citizens on the fish community and the redds were examined in the Komaoi River of the Abashiri River system in eastern Hokkaido. Before installing fishways, in the river section above artificial drop structures any salmonid fish nor any redd were found but Cottus nozawae and Lethenteron sp. were present. After having installed seven handmade fishways in 2012, fish of Salvelinus leucomaenis and Oncorhynchus masou masou and their redds were observed at the river section. On a quantitative investigation at the river section in 2013 O. m. masou was the only collected salmonid species. In 2017 O. m. masou and S. leucomaenis were collected. Meanwhile, the number of C. nozawae seems to be decreased after fishway installations. These results clearly show that multiple handmade fishways installed to the Komaoi River enable the upstream migration of salmonid fish to above artificial drop structures. However, due to river bank protection at the middlestream region, it might cause the delay in the population recovery of a sedentary fish.

Space information on the forest floor is limited compared to that on the canopy. Therefore, this study aimed to propose a simple method to obtain ortho mosaic images and digital surface models (DSM) of the forest floor and present the vegetation information (density of tall trees, density of seedlings, density of knee-roots) from interpretation of or tho mosaic images as an example. A mangrove forest of the Okukubi River run in Kin-cho, Okinawa prefecture was investigated. The canopy was captured as still images with un manned aerial vehicles (UAV), and the forest floor was captured as videos with a camera attached to a camera stabilizer. The images and videos were processed with Structure from Motion (SfM) to produce ortho mosaic images and DSM of the canopy and forest floor. The positions of tall trees and seedlings, knee-roots of Bruguiera gymnorhiza could be read out of the ortho mosaic images of the forest floor. The forest floor DSM had problems on abso lute height accuracy, such as the influence of the ʻdomingʼ effect. Nevertheless, they largely reflected the geography. In the canopy height model (CHM), differences of 2 m or more from measured tree heights were observed at some points, suggesting that the ʻdomingʼ ef fect of the forest floor DSM may be a cause of CHM errors. A method of producing ortho mosaic images and DSM of the forest floor proposed in this study is simple, and may pro vide a new investigation method. In mangrove forests, this method is expected to allow us to obtain the forest floor information that we never had before and information useful for mangrove forest management. However, it is necessary to accumulate examples further to ward practical use because of the insufficiency of accuracy verification.