Publisher：Limnology and Oceanography  

River estuaries are habitats for a variety of organisms, including many temperature-sensitive species; water temperatures in estuaries are affected by several factors as they are influenced by both terrestrial and marine environments. Therefore, understanding the factors that influence river estuaries are essential for environmental management. However, no information exists on temperature change at high temporal resolution and over a wide area; moreover, the relationship with environmental factors has not yet been clarified. Here, we have described the actual status of water temperature change in riverine estuaries and its relationship with environmental factors for a wide area of the Japanese archipelago. Our results indicated that 217 of 294 rivers showed a significant monthly increase in water temperature. The average annual change in rates of water temperature increase was the highest in October at 0.090°C yr⁻¹ and lowest in February at 0.068°C yr⁻¹. Furthermore, snowfall, air temperature increase, sea water temperature, and land use were identified as factors increasing the rate of water temperature increase. Factors influencing the increase in river estuarine water temperature varied from month to month—with meteorological factors being the strongest influencers from spring to fall, and anthropogenic factors in winter. Our findings emphasize the importance of considering not only meteorological and oceanographic phenomena, but also anthropogenic influences and topographic features to understand the pattern of water temperature changes in river estuaries.

DOI： 10.1002/lno.12507

Scopus

Publisher：Water Switzerland  

Japan’s rivers are shaped by distinctive topography and abundant rainfall, and they face flooding and sediment supply escalation concerns under climate change. Small- and medium-sized rivers tend to catch unprecedented forces that exceed planned levels, leading to substantial widening and excavation. Thus, there is a demand for a method that is capable of managing significant flood flows over an extended period. The spur dike can maintain channel clearance by promoting erosion as well as providing bank protection. However, the effectiveness of this spur dike function has not been well studied in small- and medium-sized rivers and curved reaches. In this study, we evaluate the function of spur dikes in improving channel sustainability based on examples of small- and medium-sized rivers that have maintained their channel for more than ten years after spur dike installation. First, the applicability of the empirical rule was evaluated by comparing it with actual cases of erosion depths in curved sections in Japan. Next, one-dimensional simulations were performed to evaluate the sustainability of the section over a long period. Finally, a depth-averaged morphodynamic simulation, including the secondary flow effect, was applied to evaluate the location of the flow core and elevation changes due to the spur dike. The results showed that a slight difference in the ratio of river curvature radius to river width (r/B) caused the river channel to be erosive and sedimentary. The reasons for the difference were the cross-sectional expansion caused by the excavation of the bend and the difference in the plane flow regime caused by the shift of the flow core to the inside of the bend. Although it is structurally challenging to reproduce localized scour around a spur dike in a depth-averaged simulation, it is essential for designing to apply the simulation model and combine empirical knowledge.

DOI： 10.3390/w16040575

Scopus

Publisher：Estuaries and Coasts  

This study was conducted to clarify the relationship between fish species composition and environmental factors at various scales in river estuaries in the Japanese archipelago. The estuaries harbor diverse fish species composition due to their complex topography, varied climatic zones, and ocean currents. Non-hierarchical cluster analysis classified the fish species composition of 100 river estuaries (58 families, 152 species, and 486,356 individuals) into five groups. The highest number of species was confirmed in the group of river estuaries located in southwestern Japan and flowing into the inner bay (55.1 ± 9.6 species), characterized by an abundance of the number of Gobiidae species. In contrast, the species number was the lowest in the groups consisting of river estuaries that primarily belong to the northern Japanese archipelago (15.3 ± 3.3 species) that were characterized by an abundance of the number of cold-water fishes such as Salmonidae, Pleuronectidae, and Osmeridae. The highest fish species diversity was confirmed in the groups consisting of river estuaries flowing into the inner bays of southwestern Japan, which had a large tidal range (344 ± 132 cm), suggesting that the large tidal differences create a diverse tidal flat environment that supports fish species diversity. These results indicate that regional species diversity and geographical factors influence the classification of fish fauna. In northeastern Japan, the classification was influenced by geographic area and associated water temperature changes, whereas in southwestern Japan, tidal differences and wave effects could be more important. Understanding the factors that influence the fish species composition of a target river is fundamental to estuarine conservation, and the findings of this study provide basic knowledge for conservation.

DOI： 10.1007/s12237-023-01248-0

Scopus

Publisher：Regional Environmental Change  

Intense anthropogenic impacts in tidal rivers can cause habitat loss and ecosystem degradation. In addition, changes in water temperature associated with climate change are significantly impacting the distribution area of fish fauna within tidal rivers. In the present study, we used long-term fish fauna data to determine the relationship between climate change-induced increases in water temperature and changes in the distribution of fish species in tidal rivers in the Japanese archipelago. The distribution ranges of many subtropical and tropical fish species were found to move northward in areas affected by warm currents, suggesting further possible distributional dispersal in future. This study is the first to examine the nationwide distributional changes and future projections of fish fauna in tidal rivers. The results suggest that many subtropical and tropical fishes are expanding their distribution areas in tidal rivers and in coastal and estuarine areas.

DOI： 10.1007/s10113-023-02098-z

Scopus

Publisher：River Research and Applications  

Dams have contributed to population growth and technological innovation by reducing the risk of flooding and allowing humans to cultivate productive alluvial soils on river floodplains. However, in countries where many dams have already been constructed, many problems have been observed, such as river environment degradation, aging of the embankments, and inadequate dam performance. As the social and natural environment surrounding dams is becoming more complex and diverse, it is necessary to understand the current overall status of dams and the history of their construction such that we may consider how social capital and dams are related to human society and the natural environment. In this study, the relationship of basic information of dams and the social background in which the dams exist was examined for Japan, where dam construction began in earnest in the late 1800s and became a driving force for economic development. The situations surrounding dams are drastically changing, as maintenance and management costs increase due to the aging of dams and water demand varies due to changes in the industrial structure of a region. Simultaneously, there are instances where dams are expected to reduce damage, such as in the presence of more severe and frequent droughts and floods caused by climate change. It is therefore necessary to remove or restructure dams while considering numerous complex and diverse factors, such as the local social conditions, potential future changes, and environmental impact of rivers.

DOI： 10.1002/rra.4129

Scopus

Publisher：Estuaries and Coasts  

This study investigated the fish fauna diversity of the Shimokita and Tsugaru peninsulas in the northernmost part of the main island of Japan. Since ocean currents influence brackish and saltwater fish distribution and dispersion, it is assumed that in the target area where ocean currents are complex, the ichthyofaunal diversity of river estuaries may vary significantly between rivers. In addition, the target area is located on a peninsula with complex topography, and topographical factors are also considered major factors influencing the fish fauna. During the field survey, 6879 individuals of fish from 34 species were collected from 25 river estuarine areas. Furthermore, the cluster analysis results indicated that the Tsugaru Warm Current, which flows from the warm current (Tsushima Current) on the Sea of Japan side to the Pacific Ocean side, strongly influences the presence of fish fauna in the regional river estuarine areas. Moreover, the wave exposure was marginal in rivers flowing into the Tsugaru Strait (7.4 ± 5.8), and saltwater fishes were absent, whereas fishes that depend on the gravel bed were observed. The bed slope of the estuarine river area of this group was large (1.65 ± 0.99%). The riverbed comprised stone and gravel, suggesting that in addition to biogeography, environmental factors strongly influence the presence of fish fauna. Furthermore, compared with previous studies conducted in the Shimokita Peninsula, the distribution area of the southern Gymnogobius species expanded, possibly due to the increase in seawater temperature caused by climate change. In the target area, where currents and topography are complex, it became clear that the area into which the river flows and topographical factors determine the fish fauna of river estuaries, suggesting that elucidating the relationship between complex environmental factors and biota is important for riverine environmental management and conservation ecology.

DOI： 10.1007/s12237-022-01097-3

Web of Science

Scopus

Publisher：Water Switzerland  

An open levee system is a traditional flood mitigation system for reducing the expansion of inland flooding and decreasing the peak flow. However, there have been few quantitative studies on its feasibility. Furthermore, the differences in applicability depending on the topography and the construction of continuous levees have not been fully examined. We studied its feasibility based on simulations in the Kuji River area, where the vast Typhoon Hagibis occurred. Morphological models representing the past (the 1940s) and the present (2019), obtained by modifying the highly accurate digital elevation models (DEM) via the tracing of aerial photos, were applied to a 2D unsteady flow simulation model to reveal the effects of the levee system on river hydrography and overland flood behavior. The results indicated that inundation flow through an open area decreased both inundation duration and depth, while the reduction of peak discharge is relatively insignificant at approximately 10%. The sub levees are not adequate under the current conditions and floodwater volume, and their effectiveness depends on the surrounding conditions, such as the development of continuous levees.

DOI： 10.3390/w14091343

Scopus

Publisher：Limnology  

Headstream areas, which are located in the uppermost reaches of mountain streams, are important sections for investigating the geomorphology of stream channels and their generative mechanisms, because they are close to sediment supply sources and generally suffer little anthropogenic impacts. In this study, the occurrences and characteristics of bed morphologies in a sandstone-dominated headstream area are explored at two scales—reach and unit—which are often overlooked. As a result of field investigations, cascades and step–pools were found to occupy the majority of the stream channel laterally at the reach scale. However, most of the elevation differences were accounted for by fall–pools, cascades, and step–pools, suggesting the importance of bed morphologies with large elevation losses in the headstream area. In addition, the characteristics of the bed morphologies at the reach scale were closely related to the longitudinal profiles of each stream. The mean gradients of the reach-scale bed morphologies were 46.9% for fall–pools, 23.9% for cascades, 16.8% for step–pools, and 5.9% for pool–riffles. At the unit scale, pools occupied the majority of the stream laterally, and > 80% of the elevation differences in all the studied rivers were occupied by falls, bedrock cascades, cascades, and steps. Although there have been no studies on the elevation loss functions and characteristics of falls, this study revealed the importance of fall–pool reaches and fall units in a sandstone-dominated headwater area in the Kantō Range of Japan.

DOI： 10.1007/s10201-021-00690-y

Scopus

Publisher：Soil Use and Management  

To mitigate urban flooding, conserving pervious areas and securing the infiltration capacity are important. The infiltration capacity of an area can be significantly reduced by compaction; thus, we attempted to build a runoff reduction technology that can ensure the infiltration capacity even after compaction to mitigate urban flooding. Herein, we revealed the relationship between the degree of compaction and infiltration capacity of amended soils using humus and bamboo chips. We performed a watering infiltration experiment, which initially demonstrated the nonoccurrence of surface runoff in all experimental materials for rainfall intensities of 60 and 120 mm hr⁻¹, without compaction. However, as the degree of compaction increased, humus mixed with soil exhibited behaviour similar to that of unimproved soil and the surface runoff exceeded seepage drainage. In contrast, in the case of bamboo chips mixed with soil, the surface runoff remained small even at high degrees of compaction. Consequently, when the rainfall intensity was 120 mm hr⁻¹ and compaction was 3 kg cm⁻², the surface runoff per unit time for bamboo chips mixed with soil was 2.4 times less than that for humus mixed with soil and 2.2 times less than that for unimproved soil. Moreover, such differences were observed in the infiltration capacity with respect to compaction because of the void structure owing to soil improvement. Thus, we conclude that soil improvement using bamboo chips can be considered an effective technology to mitigate urban flooding.

DOI： 10.1111/sum.12705

Web of Science

Scopus

Publisher：Catena  

Pools play an important role among stream bed morphologies in mountain streams. They provide habitat and contribute to energy dissipation. However, they have received only marginal attention compared with step structures, and their forms and driving factors, such as stream-channel characteristics and structural characteristics of upstream morphologies, remain unclear. Therefore, the purpose of this study was to determine the structural characteristics of pools, including morphology, size, and deposit conditions, and the factors controlling them. Based on the results of a field survey for a channel length of 9524 m, with 428 reaches and 1302 pools, the correlations between the pool spacing, morphology, size, and deposit conditions of the pool, and controlling factors were investigated for three rock types (granite, rhyolite, and sandstone). The average pool spacing calculated by dividing the interval length of the pool by the channel width was 8.80, 12.87, 4.56, and 16.14 for fall-pool, cascade, step-pool and pool-riffle respectively. This is due to the fact that the target streams are first-order streams with small widths as well as their regional characteristics, that is, few log jams and forced pools. In addition, the depositional depth of the pools varies depending on the geology. The maximum percentage of sediment deposition in the pool was 78%, 33%, and 34% for granite, rhyolite, and sandstone, respectively. Thus, the sedimentation in granite is significantly greater than in the rhyolite and sandstone headstream areas. This is probably due to the discontinuous weathering of granite during which it is broken down into large-sized core stones and fine grains. The difference in the depositional environment of the pools depending on the geology indicates that the biota that use those pools as a habitat may also differ. However, research on the microhabitats of pools is lacked, and research regarding the future perspectives of ecosystems is required.

DOI： 10.1016/j.catena.2021.105760

Scopus

Publisher：Biodiversity Data Journal  

Background Urbanisation has facilitated changes in the hydrological cycle, sediment dynamics and habitat loss and it has had a tremendous impact on river ecosystems. Moreover, the invasion of non-native species reduces the number of native species with the progress of urbanisation, thereby resulting in the homogenisation of fish fauna and significant decrease in diversity. However, the effects of urbanisation on fish fauna vary depending on the region, degree of urbanisation and number of years since the construction of the city. Hence, it is necessary to collect information on how fish fauna changes due to urbanisation in different regions. The target site of the present study is the Tsurumi River, of which approximately 85% of the basin is urbanised and many of the river channels have been affected by straightening and concrete channelling. Monitoring of biota has been conducted mainly in downstream areas; however, data on fish fauna in upstream areas and tributaries of the river, which have been substantially affected by urbanisation, are lacking. New information Surveys were conducted at 71 sites in the Tsurumi River Basin during summers and winters, focusing on river channels that have been straightened or converted to concrete channels due to urbanisation. As a result of this investigation, 10 families, 28 species and 9,335 individuals were collected. Some of the fish collected included rare species, such as Tanakia lanceolata, Lefua echigonia and Pseudobagrus tokiensis, indicating that, even in rivers that have been severely impacted by human activities, there are still rare indigenous species living there, albeit in limited numbers. In addition, Misgurnus sp. (clade B2), Micropterus salmoides, Lepomis macrochirus and Poecilia reticulata were confirmed to be non-native species. Misgurnus sp. (clade B2) was widespread; however, populations of carnivorous non-native alien species were small. The data are all accessible from the document “database_fish_urban_tsurumi” (https://ipt.pensoft.net/manage/resource?r=database_fish_urban_tsurumi).

DOI： 10.3897/BDJ.10.e83527

Scopus