今年も球磨川流域や筑後川流域等で水害が発生し，多くの人がなくなり，多くの家屋が被害を受けた。水害のない社会，洪水で人の死なない社会をつくることは人々の共通の願いであり，そのための対策を取り続けることは社会の最優先事項である。その際，ダムが水害を防ぐ有力な手段であることは間違いない。特に今回，球磨川流域で発生した水害は，ダムなき治水を目指していた地域で発生しただけに，「川辺川ダムを造っておけば今回の水害を防ぐことができた」とか，「川辺川ダム計画を止めた民主党，熊本県知事が今回の水害の原因である」といった過激な言葉も現れている。では，果たして川辺川ダムができていたら，今回の水害を防ぐことはできたのか。今後の球磨川流域の治水対策において川辺川ダムは不可欠なのか。そして，私たちはダムによる治水とダム以外の手段による治水をどのように考えていったらよいのか，について考えていく。

　川辺川ダムがあったら水害を「防ぐことができた」，「防ぐことができなかった」と現時点で断定することはできない。様々なデータが揃い，科学的検証をした後に判断すべきだ。ただ，手元にある状況証拠で語れる部分があるとすれば，「川辺川ダムがあったとしても水害を防ぐことはできなかった」と思われる。また，同時に「川辺川ダムがあれば，その能力に応じた水害の軽減はできた」であろう。川辺川ダムは建設中止宣言にもかかわらず，計画は消えていない。ダム治水容量は 9,200 万 m³である。今回の水害時にこのダムが貯水量を空にして存在すれば，9,200 万 m³の流量を貯めることができた。一方，球磨川本流には既に市房ダムがあったが，人吉市内での堤防溢流時，洪水流量をカットする機能を失っていた。そしてこの球磨川本流からの残流量が 7 月 4 日 7 時頃に人吉市内での堤防溢流をもたらしたと大熊（2020）は述べている。

　河川整備基本方針によれば，人吉地点での球磨川の最大流下能力は毎秒 4,000m³である。それに対して堤防溢流時，球磨川には毎秒 5,000ｍ³を超える水が流れたと推測されている。従って「今回の水害において川辺川ダムができていた場合，球磨川を流れる河川流量の一定程度をカットすることができた。しかし，それでも河川に流れる流量は毎秒 4,000m³ を大きく超えるため，少なくとも溢流に伴う水害を防ぐことはできなかった」。「ただ，川辺川ダムの流量カット機能は大きく，水害被害の緩和には役立ったであろう」。

　では，川辺川ダムがあった場合，どれくらい被害を緩和させることができたのか。これについてはよくわからない。球磨川本流，川辺川以外の球磨川支流の洪水によって亡くなった人は，川辺川ダムがあっても助けることはできない。球磨川本流の溢流で市街地に水が流れ込んだ結果亡くなった人も，川辺川ダムでしか助けることができなかったかは不明である。一方，川辺川ダムが河川流量を大きくカットしていれば，それによって逃げる時間がより確保できたかもしれない。さらに，市街地への洪水流入による建物への被害は，川辺川ダムによる流入流量の減少分に応じて緩和されたであろう。

　ダムは環境を破壊する。ダム下流部での河床低下，海岸浸食，ダム地点での水生生物の移動遮断，ダム上流部での堆砂，水害誘発等，これだけ自然環境を痛めつける構造物も他にない。従って，ダムは，自然環境に及ぼす影響が大きいことなどのため，原則として建設しない，というのが 2003 年淀川水系流域委員会の答申である。また，予算制約の中でダムを無限に造り続けることはできない。こうした環境制約，経済制約をどのように考えるかが今後の議論の焦点となろう。球磨川流域は球磨川のもたらす恩恵によって地域経済の基盤を作りあげてきた。その中にはダムがないことによって得られた恩恵も大きい。そうしたダムのメリットとデメリット，ダムなし治水対策のメリットとデメリットを冷静に議論すべきである。

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We report recent occurrences of Fragilaria longifusiformis ssp. eurofusiformis Lange-Bertalot & S. Ulrich from Japanese lakes. This diatom was firstly found from Uokiri Dam Reservoir on the Yahata River, Hiroshima in 2011, followed by Lake Kasumigaura, Ibaraki in 2014 and Lake Biwa, Shiga in 2021. It is considered to be introduced into Japan around 2010. We displayed morphological characters of Japanese specimens and distinguishing points from other confusing species.

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Sedimentation is an essential issue on long-term management of dam reservoirs. Cost and benefit of the sediment manage project is mainly analyzed based on the benefit of flood damage reduction by securing or keeping the reservoir capacity for flood control and the cost of construction and maintenance of dam structures. However, other important issues such as the improvement of downstream environment by necessary sediment supply, the effect of recycling of sediment, additional costs for downstream facilities such as electric power stations, CO₂ emission by excavating and transporting of sediment are not considered. In this study, we studied how to integrate these costs for the economic evaluation of comprehensive sediment management projects. The benefit of improving river environment was estimated by the results of CVM (Contingent Valuation Method) and Conjoint Analysis on Yahagi river.

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To establish indicator species for estimating the effects of sediment replenishment on the downstream ecosystems of dams, we investigated the densities of macroinvertebrates and the factors of river bed compositions at the seven dammed rivers in Kinki region of Japan. Case-bearer and burrower families would be nominated as the indicator species, since their densities were strongly correlated to the river bed condition, especially amounts of fine riverbed materials. Generalized linear mixing models showed that many case-bearer and burrower families were able to approve as the indicator for river bed conditions, and that Glossosoma spp. was the most useful indicator. Such indicator density would provide important information to determine the amounts and frequency of sediment replenishment on the dammed rivers for restoration of downstream ecosystems.

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Sedimentation is an essential issue on long-term management of dam reservoirs. Cost and benefit of the comprehensive sediment manage project is now a challenging issue by taking account not only keeping the reservoir capacity but also other important issues such as the improvement of downstream environment by necessary sediment supply, the effect of recycling of sediment, additional costs for downstream facilities, CO₂ emission by excavating and transporting of sediment. In this study, we studied how to integrate these costs and benefits for the economic evaluation of comprehensive sediment management projects. The benefits of improving river and coastal environment were estimated by the results of CVM (Contingent Valuation Method) and Conjoint Analysis on Yahagi River. We have also proposed that preparing a stock yard in the middle river reach is a key factor to manage various objectives.

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Drawdown zones in lakes and marshes are ecotones between terrestrial and aquatic environments and many organisms inhabit the boundary areas. In addition, drawdown zones can serve as temporary refuges for terrestrial and aquatic organisms. Evaluating the role of drawdown zones as refuges is an important topic of study in terms of regional biodiversity conservation. In this study, I focused on the drawdown zone in the dam lake of the Kasegawa Dam. From 2016 to 2018, I surveyed the fruit galls induced by the aucuba fruit gall midge Asphondylia aucubae Yukawa et Ohsaki (Diptera: Cecidomyiidae) on an evergreen shrub Aucuba japonica Thunb. var. japonica (Garryaceae) in an area where terrestrial plants had died due to the initial impoundment between October 2010 and February 2012 (submerged area) and the adjacent non-submerged area. Galled fruit rates were significantly lower in the submerged area (about 60％) than in the non-submerged area (more than 95％). In contrast, there was no significant difference in the parasitism rate of the gall midge by a hymenopteran parasitoid Syntomernus asphondyliae (Hymenoptera: Braconidae) among sites. These results suggest that the drawdown zones can act as refuges for the plant to escape fruit infestation by the gall midge but do not function as refuges for the gall midge to escape from their hymenopteran parasitoids. Such differences may be due to the differences in the dispersal and host-seeking abilities of the insects.

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Off-stream reservoir is a unique water management system that has diversion weir(s) only in the mainstream, which may reduce adverse impacts of storage dams on river environment such as water quality change, reservoir sedimentation, downstream degradation and river discontinuity. Off-stream dams were defined and extracted from a dam database and were classified into several groups. To clarify environmental effects of off-stream reservoir, satellite images analysis and field survey were conducted on selected rivers with off-stream dams. There was almost no difference in bar characteristics between the upstream and downstream reaches in the selected four rivers with off-stream dams. Grain size distribution, dimensionless shear stress and taxonomic composition of benthos were also similar between the upstream and downstream reaches in the river with Dodairagawa dam (off-stream dam). Our study suggests that sediment continuity and river environment are maintained in rivers with off-stream reserving system.

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北陸地方4ダム（手取川ダム・真名川ダム・大石ダム・大津呂ダム）の共通点はすべて洪水調節を利水目的の1つとしているため，洪水期および融雪期の前後に貯水量が減少した。総貯水容量が比較的大きい手取川ダム，真名川ダムは，小さい大石ダム，大津呂ダムと比較して，放流量が流入量を上回る月が多かった。総貯水容量が大きいダムは渇水に対する危険性が小さいため，放流量が増加した。年平均回転率は大きいものから順に大石ダム14.5回，手取川ダム5.2回，大津呂ダム4.7回および真名川ダム3.1回であった。真名川ダムおよび大津呂ダムは総貯水容量に対する年平均流入量が小さいため，年平均回転率が小さくなった。大石ダム下流河川生態系を改善するためには，①現行の表層取水から選択取水に変更すること，②麦わらや落葉落枝を利用すること，③4月に放流量を増加させて自然出水再現放流すること，④流入量を一時的に蓄えることによって規模の大きいフラッシュ放流を実施することを提案した。大石ダムは建設後約40年間が経過する。治水や利水施設としてだけでなく，下流河川生態系を保全する施設として適切に維持管理されることが求められる。

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There is a phenomenon called armoring of the river bed as an impact of dams to its downstream, and has been researched previously by several researchers. However, previous researches often focused on one particular dam, therefore comparison of the phenomenon among several rivers which have different characteristics has yet to be conducted. In this research, we focused on several dams in rivers which have different geological features in its basin. As surveys, we took data of bed materials upstream and downstream of dams, and compared them. Based on these data, we found that the armoring of the river bed seldom occurs when the geology of the river basin is sedimentary rocks, and it frequently occurs when the geology is volcanic rocks. More in detail, the location of the landslide affects the size distribution of the river bed. The size will be smaller when the landslide exists in downstream of dams, and larger when it does not exist.

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　ダム湖で問題となっている富栄養化現象は，集水域から流出した栄養塩が, 河川を通じて流れこむことが原因の一つである. そのため，流入河川の水質と集水域環境の関係について，検討していく必要がある．本研究では，全国的な多数のダム湖を対象に，GISを用いて集水域の人口や土地形態を解析した．さらに，これら流域背景と流入河川の総リン濃度を統計的に分析し，年平均の流入河川の総リン濃度を推定する式を作成した．この式を用いて，人口変化に伴う将来の総リン濃度の変化を推定したところ，現在流入リン濃度の高いダムが多い西日本において，特に水質の改善が示された．

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Reservoir sedimentation is one of the most important problems for securing long term achievement of hydropower operation in the future. In order to solve the problem, we have to note that there are two major types of hydropower stations, which are storage type and regulating type. We have already discussed that we have to focus on sediment management in regulating type reservoirs more than storage type ones from the view point of flood disaster prevention in reservoir areas. In this paper, we studied on technical and economical feasibility of hybrid method of draw-down operation and sediment bypass by numerical analysis, and we found that the hybrid method was very advantageous in both ways. We also analyzed the impact of sediment supply to dam downstream river by sediment bypass and found that it was possible to keep the safety of the river channel by some effective excavation and that the improvement of the river environment would be expected.

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After dam construction, the flow and sediment regime change downstream from the dam. For example, the armor coat progresses because of sediment trapping by the dam. To solve these problems, ’sediment replenishment’ has been executed recently. Nevertheless, it is difficult to maintain effects of sediment replenishment over the long term because the restored sediments are washed away easily in steep rivers, especially in slippery bedrock sections. Therefore, the authors have proposed techniques that promote gravel sedimentation by improving riverbed forms. The effects of proposed techniques such as the river widening and multistep masonry weirs were investigated using a two-dimensional numerical model. Results showed that the proposed techniques are useful to reduce tractive force in steep rivers. They can create a desirable bed state for river ecosystems with various gravel sizes.

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It is necessary to detect accurate effects of benthic invertebrates’ communities on water temperature. Headwaters in mountains enable to detect accurate effects of benthic invertebrates on only water temperature. In order to evaluate the impacts of benthic invertebrates on water temperature, we investigated the quantitative relationship between water temperature and benthic invertebrates in the headwaters of the Natori River basin located at the middle of Miyagi prefecture. As results, the population of benthic invertebrates has a positive correlation with the number of taxa (R²=0.59, p<0.01). The Sorensen similarity index decreased significantly with the water temperature difference expanding (R²=0.15, p<0.01). The population densities of Plecoptera decreased significantly with the water temperature rising (R²=0.25, p<0.01).

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This topic presents the effect of trial impoundment of dam on secondary productivity of Stenopsyche marmorata (Trichoptera) at immediately downstream of Takizawa dam, in Japan. S marmorata has very high productivity among all invertebrate species and bivoltine life cycle. Production statistics for S.marmorata in Nakatsu-gawa river calculated by the increment-summation method and the instantaneous growth method, respectively. Values for non-wintering generation, wintering generation and annual production during individual years for the entire study period are given separately. Annual productions were 26.09/23.75 g/m² for 2006-2007, and 46.18/58.11 g/m² for 2007-2008 by increment method / instantaneous method. Because of the minimum outflow were maintained during the long period of trial impoundment, the survival rate of the fourth and fifth instars of larvae were significantly higher than the other study. Also, its individual body weight was heavier than the others. The biomass and productivity strongly depend on river gravel bed size distribution and flowing phytoplankton from the dam.

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There are local scouring of which the maximum depth is up to 20 meters and exposure of a layer of clay in the segment2-2 zone of Kiso River. However, the progression mechanism is unknown. In this study, we figured out the amount of riverbed elevation variance disaggregated by Segment scale and carried out field surveys including river bed morphology measurement in order to identify causes for local scouring developing.

The following causes were found out in Kiso River: (1) as the first stage, a decline in sand sediment transport from upstream basin causes the maximum amount of riverbed elevation lowering in the segment2-2 zone of which riverbed is composed of sand, leading to exposure of a layer of clay and riparian tree expansion on sand bars, (2) as the second stage, the concentrated flood flow and the high-speed flow triggered by forestation, locally, cause exfoliating of a layer of clay suppressing riverbed lowering, leading to the erosion of the lower layer of sand at an accelerated pace. The above phenomenon is specific for the alluvial plain with alternating layers of sand and clay vertically.

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知床半島において,ダムの設置との関係で河川環境の状況を調査した.魚類の移動障害となりうるダムや床固等の一覧を作成し,2003年3月までのダム設置の実態を明らかにした.少なくとも331基のダム等が同半島に設置されていた.ダム等が設置されている河川の割合は半島全体で42.9%にのぼる.設置されているダムは,山地の保全や土砂災害を防止するために設けられる治山ダムと砂防ダムがほとんどであり,洪水調節や農業用水の取水を目的としたダムは確認されなかった.ダムの数は流域の地形や土地利用状況によりさまざまであり,ひじょうに集中的に配置されている河川もある.最初にダムが設置されたのは1960年代初頭で,1980年代にもっとも盛んに設置された(年平均約10.5基).現在はダムの設置数は減少しているが,2000年以降も年間7基以上が造られている.これに対して魚道の数は全部で66に過ぎない.ダム建設時に魚道が本格的に併設されるのは2000年以降である.魚道の管理状態は悪く,総体として知床半島における魚類および河川環境の保護は不適切かつ不十分な状態であった.

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