In estuaries, seawater mixing creates unique water environment. Observations on distribution of seawater mixing layer and on water quality were conducted in this study in order to understand the mechanism of seawater mixing and the vertical distribution of water quality. At the point which has deep water depth, the mixing layer changes easily. Suspended material as well as chlorophyll-a have maximum concentration at the upper edge of the mixing layer. In ebb tide phase, phosphorus has its maximum value at the bottom layer.
Laboratory experiment was also carried out in this study to investigate the flocculation process using river bottom mud. The experiment result shows that flocs diameter, which is used for flocculation, are differed by suspended matter concentration.

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The large-scale experimental channels were constructed in 1994 in the course of the Takaya-gawa river which is a branch of the Ashida river, in Fukuyama-city. In these experimental channels, two kinds contact oxidation materials such as coral stone and deoiled coal paved into one channel and another channel which called a control channel were paved by nothing. The conduction period of water to these channels is about 80-55 days for each season during Aug. 1999 to Nov. 2000.
The purification effects for the strong polluted river-water were estimated by several items of water quality and sludge. In this paper, items of BOD, nitrogen and phosphorus were especially considered from the angles such as particle component and dissolved component.

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Dynamics of colloidal phosphorus (colloidal-P) in river water after the Tokai Heavy Rainfall was investigated in the Yahagi River. The highest concentration of colloidal-P was observed one week after the Tokai Heavy Rainfall in which colloidal-P accounted for almost 80% of dissolved total phosphorus (DTP). Only at this period, the colloidal-P concentration exceeded that of molybdate reactive phosphorus (MRP), and it means a significant amount of phosphorus in colloidal fraction was eroded from the soil. Colloidal-P concentration was 12.4% of DTP in average during the observation period (from Jun. 2000 to Jul. 2002). The yearly load of colloidal-P from Jun. 2000 to Jun. 2001 (including Tokai Heavy Rainfall) was 2.65 times greater than the next year. High concentration of colloidal-P was observed over many months after the Tokai Heavy Rainfall, due to the effects of its accompanied events such as landslides, restoration works and soil runoff.

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Field observations were conducted to investigate the transport process of fine particulate organic matter (FPOM) during a small flood in a lower reach of the Nanakita River. Sediment samples on riverbeds were collected at four stations before and after the flood. The flowing water was collected at three stations during the flood. The concentrations of suspended solid (SS) and particulate organic carbon (POC) were measured in both sediment and water samples. The total deposition and erosion rates of SS and POC were estimated from SS and POC in sediment samples. The time variation of the SS and POC concentrations, and the total transport rates were calculated from SS and POC concentrations in flowing water during the flood. The basic characteristics of the transport processes of SS and POC during a small flood in the Nanakita River were elucidated.

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In this study, water temperature variations at the Nagaoka gauging station of the Shinano River are analyzed within different time framework. It is found that the daily maximum, daily mean and daily minimum air temperatures are good indicators of the daily maximum, daily mean and daily minimum water temperatures at yearly scale. But the relationships between air and water temperatures depart from linearity on monthly or shorter time scales. It is also shown that the 7-day moving averages of daily maximum water temperatures are linearly correlated with the 7-day moving averages of daily mean water depths over seasonal base. Besides, the issue of temperature sampling frequency is explored. It is revealed that one sampling per day in the morning may be able to capture the overall variation pattern of water temperature. Finally, the influence of upstream control upon downstream water temperature is discussed in light of diurnal change of water temperature.

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Water temperature affects nearly every physical property of concern in water quality management and, unfortunately, the change of water temperature is often associated with undesirable. Control of water temperature therefore becomes a prerequisite for maximum utilization of water resources. Reservoirs often provide a means for exercising such controls. In this study, to evaluate artificial effect to the downstream environment, changes in river water temperature by dam construction were estimated. Then mitigation effect by the adoption of surface intake on the dam site was examined.

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It is important to grasp dynamic process of growing attached algae because it provides food resources to the other aquatic organisms, from the viewpoint of river ecosystem conservation.
In this paper, we discuss on the application of logistic equation to evaluate the change of algal biomass. Firstly, the vertical distribution of saltating sand particles around cobbles is considered to estimate the effective detachment rate of algae in a river of which bed is composed of sands and cobbles, adequately. And then, by making use of the algal physiological data in the previous studies and the limate data at the research site, the growth-detachment dynamics model of algae is properly adjusted. The simulation is applied to the prediction of the algal biomass fluctuation in the middle reach of Yahagi River that is a typical gravel river with less disturbance.

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The relation between firefly habitat and the environment factors in the Natori River basin is investigated. We prepared digital information maps based on GIS and hydrological simulation results. Seven habitats were detected by some interviews and field surveys in the basin and sample data were collected from the maps. The habitat has the specific conditions for firefly habitat. We made the distribution map of a number of habitat suitability in the whole Natori River basin and found eight regions scoring high habitat suitability. The result of field survey in the regions proved the affectivity of this methodology.

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Hydraulic structures such as a dam, weir and water gate affect on the flow depth, velocity, bed shape, water quality and so on, so that the hydraulic structures may affect on the water environment for plants and aquatic lives in an around rivers. Therefore, before construction of such hydraulic structures, it is necessary to conduct the environmental assessment. However, the accuracy of the environmental assessment is not so high at present, because it is not clear that what is the most suitable parameter which affects on the water environment. In this study, the most suitable parameter for spawning of Ayu were selected from the several values such as water temperature, velocity, flow depth, incline, BOD, COD, pH, SS and so on, on the basis of a principal component regression analysis. It was found that the most suitable parameters are temperature, velocity, flow depth and SS.

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In this study, physical quantities (velocity, depth, light quantum, etc.) in riffle-pool structure were measured. Further, since riffle and pool is habitats for benthic animal, those areas were surveyed. The object of this study is to analyze and appreciate about river environment including ecosystem by means of these survey of physical environment and benthic community.
The field survey was performed at research point being compatible with riffle, fast rapid and pool. As a result of, the reached lights on river-bed surface was intense in riffle comparing with pool. Because of dispersions, the standard deviation of light quantum in riffle was larger than that in pool.
The results of benthic community survey were performed in April and August showed a diversity of specie in riffle. And the process of succession to the climax was observed by means of classification of benthic life style.

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In this study, quantitative analysis on the relation between the physical diversity of river channel and the diversity of kinds of fishes has been carried out.
In this paper, the authors tried to find the physical factor of river channel which had an influence effectively to preserve the diversity of kinds of fishes by increasing the rivers and areas to analyze it, in addition to the results until now. The diversity of kinds of fishes was assessed by using the Simpson's diversity index. On the other hand, the diversity of river channel was expressed by standard deviations of the channel hydraulic variables, such as water depth and flow velocity.
The results of this examination showed that the diversity of kinds of fishes was secured in the area which the diversity of kinds of the young fishes was rich. And, as for the rate of the area of the shoreline, it found that it was important as well as “riffle and pool” which was important in the existent research.

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PHAB SIM is one of the most famous methods to evaluate fish habitat. The structure of this method is quite simple and it is easy to use this model for river management. But because of its simple structure, this model cannot delineate suitable condition for fishes sufficiently. Especially suitability criteria that assign suitable range of depth and velocity for fishes are too simple to be used for habitat evaluation.
In this paper, we propose new suitability criteria. At first, we focused on the interaction between depth and velocity, and proposed new suitability criteria on depth-velocity plane. Next, we focused on the hierarchical structure of physical habitat, and we introduced several suitability criteria on unit scale and subunit scale. Finally, we validated our model with new criteria, and we showed effectiveness of our evaluation method.

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In order to evaluate the habitat of the fish inhabiting the river, it is necessary to know the relation between the amount of habitations of a fish, and the physical characteristic of a habitat “National Censes on River Environment” contains information about plants and animals as well as the physical features of rivers in a certain format. In the present study, the dominant physical attributes forming a fish habitat in various rivers of the large area in Japan are discussed by using the above database.
The population of fishes (for example Carassius gibelio langsdolfi) and physical characteristic in each river are approximated by a regression function which is derived from a multiple regression analysis. Contribution of the physical characteristic to the quantity of fishes in a different district is compared, and the feature for every area is described. And district feature is described by result from a principal component analysis. CPUE (Catch per Unit Effort) is proposed to standardize the amount of fishes capture and is used in analysis shown above.

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A slope with 45 degree against a horizontal line was attached on the first weir in a pool-and-weir fishway, which is located beside of a large weir in the Onga River, to allow fishes migration. The observation of migrating fishes at the fishway with a digital video camera and velocity measurements with a 3-D electromagnetic current meter were carried out after the improvement. It was found that the migration rate near the side wall is larger than that at the central region of the fishway, because instantaneous velocity near the side wall is smaller than that at the central region of the fishway, irrespective of that the time averaged velocity of both regions are almost same and also the eddy scale around the fish is larger than the body length of the migrating fishes.

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Settlement and upstream migration in the Japanese mitten crab (Eriocheir japonica) was investigated in the Seseragi-fishway in the Nagara estuary barrage. Megalopa larvae settled and metamorphosed to crab stage in the upper tidal zone in the Seseragi-fishway. The youngest crabs were nearly two millimeters in carapace width. After attaining a minimum of four millimeters, the crabs began to migrate upstream and reach the freshwater area. Settlement of megalopa larvae occurred mainly in early summer (June) and early winter (December). Their upstream migration to the freshwater area began in July and August. The majority of larvae that settled in winter did not survive and metamorphose to the crab stage due to low water temperatures. Thus, the growth and upstream migration of the Japanese mitten crab seemed to be strongly influenced by water temperature in the estuary. This mode of settlement with two peaks in summer and early winter reflected the temporal pattern of reproduction observed in the sea. In this study, it is clarified that the Seseragi-fishway have a function of settlement and upstream migration for supplement of decreased estuary zone. With respect to engineering works in estuaries such as the Seseragi-fishway, it is recommended that we take into account the processes of settlement and migration of young crabs.

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Aquatic insects were surveyed to evaluate a biotope of Aono dam fishway ecologically. The Aono dam is located on the Aono river, a tributary of the Muko river which flows through Hyogo Prefecture, Japan. The fishway consists of pool-type fishway, riffle and pool sequences and etc. Aquatic insects were sampled by picking up surface bed materials in about 0.09m₂ quadrat and washing adhered materials out. Sampled aquatic insects were identified to genera and individual numbers were counted for each genus. The genera and their individual numbers were examined in relation to hydraulic and bed conditions. EPT, Shannon Diversity Index and etc. were calculated for each sample. Most sampled insects belongs to genera which have tolerance to organic pollutions and usually live in lower reaches. Algae and mud which are supposed to come from the dam reservoir strongly influence spatial distribution of the genera of insects.

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From the engineering and biological view point, it is important to understand the movement of fish in the flow with artificial structures (e. g. fishway) or natural bed topography (e. g. bar and pool). There have been a large number of works on river flow with structures or fishway including numerical computational studies, however, the interaction between real river flow and fish movement have not been clarified. In this paper, an attempt is made to understand the fish movement in the flow by numerical experiment and flow visualization technique. According to these calculations, the movement of fish in the flow is indicated and visualization of result is shown.

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Typha angustifolia were cut in June, July and August in 2002, and in May and July in 2003 for i nvestigating their strategy to survive from cutting at different timing. Zizania latifolia was also cut in July in 2003. The cutting effects were evaluated quantitatively by regrowth characteristics of aboveground leaves just after cutting. The belowground production of T. angustifolia was decreased by summer cutting. This was due to the decrement of transportation from original shoot to belowground and from regrowth shoot to belowground organ. July to August is the best season for controlling expansion of the species, because the transportation can be decreased by the cutting. The amount of transportation from belowground to aboveground organ was a little compared with the decrement of belowground biomass and was almost the same with the two species, but heterotrophic season just after cutting were found 15 days, 40days in case of T. angustifolia and Z. latifolia, respectively. The season was longer and secondary shoot was formed in case of Z. latifolia. This implies the different strategy of Z. latifolia with T. angustifolia.

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Vallisneria higoensis which is an aquatic plant peculiar to Kumamoto Prefecture is on the verge of the present extinction. In this study, it aimed at an improvement of the growth environment of a water plant by making the flow velocity increase using groyne. The indoor experiment considered groyne arrangement. Consequently, it turns out that the one-bank groynes increases the flow velocity of a mainstream part by about 5% compared with a case without a groyne, and it is increasing about 10% in the both-banks groynes. It was admitted by the result of a field survey that the flow velocity in main flow region after both-bank groynes installation was increasing about 11 to 23% compared with before installation. It turns out that 5m distribution expansion was seen at the maximum about growth of aquatic plant, and the area of a aquiherbosa is also expanded.

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The field observations were performed to understand the characteristics of growth and nutrient uptake of Phragmites japonica on flood plain in a gravel river, in which the temporal variation of plant biomass, the content of nutrients of vegetation and the concentrations of nutrients in the groundwater were measured. The result of the field observations show that the growth of the vegetation was restricted by the groundwater level in the present observation site. The hydroponics experiment of the riparian vegetation was conducted to understand the nutrition uptake due to photosynthesis. The results of the experiment show that the nitrogen uptake of the vegetation was closely associated with the amount of respiration of the roots of the vegetation.

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Local flow phenomena and scouring around a modeled emergent macrophytes' colony, were investigated by water-flume experiment. The characteristics of the colony of Phragmites australis, Typha angustifolia and Zizania latifolia were decided as a fuction of the colony's porosity. The porosity of the colony was represented by seven cylinders' diameter and their distances. In case that the distances were two times of the diameter (D), horseshoe vortex were formed in front of each cylinder, however in case that the distance was less than the diameter, the horseshoe vortex around each cylinder was not formed, in stead, large horseshoe vortex was formed around the modeled colony (7 cylinders). Local scour phenomena were changed according to the flow phenomena and the scour depth in front of the cylinder became larger with decreasing porosity. With same porosity, the sour depth became larger with increasing velocity under clear-water scour condition. The scour depth can be evaluated by the single cylinder's formula and the porosity effect.

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Laboratory experiments on the flow with very flexible submerged vegetation was performed, in which the flow velocity, velocity fluctuation, fluctuation and deformation of plants were measured by using various method. It was found that the wavy motion of the vegetation termed honami was observed and the deformation of plant is closely associated with the turbulent flow structure, which yields the larger Reynolds stress near the top of vegetation layer. Numerical simulation was performed to predict the flow and the motion of vegetation. Turbulent flow was computed by 2D LES model, and the motion of vegetation was calculated by using the vibration equation taken into account the higher modes of vibration. The model can predict the flow field and the deformation of subemerged vegetation well.

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Typhoon Na.10 attacked the Pacific side of Hokkaido on August 9, especially in Iburi and Hidaka sub prefectures. The flood generated large amounts of driftwood and caused bridge washout in the Saru River basin. Based on an onsite survey conducted immediately after the typhoon, we examined the driftwood in the Saru River. We studied the distribution of locations where driftwood beached in the river channel, the volume of such driftwood, the distribution of driftwood origin in the river channel, reasons for driftwood originating there. The influence of driftwood to the rivers structures (bridge piers etc) is also investigated. It became clear that the driftwood generated from the river channel and the mountain area occupied 20% and 40% of all driftwood, respectively as a result of investigation. The generating places of the driftwood from the river channel were the flood plain at the inner bend and the eroded outer bank shore.

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