This review paper provides useful information for understanding studies on the ‘riverbed environment’ of the upper and middle reaches of rivers. Firstly, as the basis for evaluating the characteristics of a riverbed, the physical properties of bed material (classification and distribution of grain size, porosity, specific gravity etc.) are discussed so as to find ways to apply these physical properties to ecological studies. It is proposed that sediment population analysis provides a useful framework for discussing the habitat functions of riverbed. Secondly, we summarize the results of studies on flow resistance, flow structure in the roughness layer, riverbed forms, and sediment transport in a mixed bed. We point out the importance of focusing on bed forms and sediment sorting, as a cause of the heterogeneity of the riverbed environment. Thirdly, index parameters and classification of the physical condition of a riverbed are discussed including properties that an appropriate index should possess. Finally, future issues and expected actions are proposed based on the review of sediment hydraulics and fluvial geomorphology.

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Six stream habitat (morphology) classification systems were reviewed from the aspect of spatial scale and location within watershed. Classification systems at a segment scale (10³-10⁴ channel width) covered whole river segments within a watershed, but they did not adequately classify mountain streams. Segment subdivision using a reach-scale classification system can solve this issue. Classification systems at a reach scale (10¹-10²⁾ and a channel unit scale (10⁰-10¹⁾ were constructed for ≤3^rd-order mountain streams. These classifications can be applied to ≥4^th-order rivers in canyons, valley plains, and coastal alluvial plains. Subunit types were commonly classified according to various purposes and targets. However, general subunit types can be based on river morphology and planar flow conditions that are important spatial units, particularly in large alluvial rivers. Our review found that existing classification systems and general subunit types can be applied to better understand the overall physical and ecological features of streams and rivers, and can promote effective river management.

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Geology is one of the crucial components for defining riverbed materials and textures especially in mountainous streams, which might effects on their aquatic communities. In this study, the Kushida River basin, central Japan, composed of different geologic units such as plutonic, metamorphic, and sedimentary terrains, was selected to clarify the relationships between riverbed textures and benthic macroinvertebrates. We conducted field surveys at riffle and pool units in each of the 18 stations (three sites each for six tributaries) by taking into account their watershed geologies and landuses. Sediment samplings, topographic measurements and water quality monitoring (temperature, pH and electric conductivity) were undertaken at these sites. The individuals of macroinvertebrate were picked out from each of the sediment samples and identified at the species level with six “life types”, whereas their residual samples were used to analyze the riverbed textures. The topographic, geologic and ecological differences among each of locations in the tributaries have been discussed with the one / two way ANOVA tests. According to the results, it could be understood that there are strong relationships among the watershed geologies, the physical characteristics of riverbed textures and the features of species and life type compositions of benthic macroinvertebrates.

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Recently, large numbers of young ayu (Plecoglossus altivelis altivelis), one of the most commercially important freshwater fish in Japan, have been found at the lowest reaches in the Kamo River, migrated upstream from the Yodo River. However, the migration of ayu was thought to be hindered by channel fragmentation through a series of drop works. Therefore, we investigated their distribution and habitat use by field observations in 20-km reaches of the Kamo River. Although the water temperature condition was higher than the physiological limit of the fish in the lower reaches of the river, a considerable number of fish survived in the reaches. In addition, we found that they foraged at riffles in the main channel during the early morning, whereas they moved to spring-fed pools behind the sandy bar during the daytime. Furthermore, their stomach contents and nutritional conditions during the afternoon were lower than those in the morning and worse than those in the middle reaches, respectively. These results indicate that the ayu population in the lower reaches of the Kamo River can survive because of the spring-fed pools as a refuge from high water temperature. However, considering that the growth of the fish was less efficient than that of those migrated upstream, the fragmentation of river channel seemed to undermine the production of ayu in the Kamo River. Therefore, improvement of the drop works by lowering the height and/or constructing fish ways as well as conservation of spring-fed pools and groundwater recharge is required for increasing the wild ayu population in the Kamo River.

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