In tidal section of river, erosion control works associated with improvement of water quality are necessary, because riparian scour has been caused by daily and biweekly fluctuation of tidal level and water pollutants such as nutrient and several organic chemicals tend to accumulate. Therefore, the purpose of this paper is to establish the basic direction of conservation of the tidal river environment by utilizing ecosystem based on sabo works. It is shown by photointerpretation and ground survey that using a series of groins suits with the characteristics of river basins for flood control on tidal section, the diverse area consisting of several microtopographies are designed on the riparian zone. These area such as tidal flats, channel bars, marsh, and lagoon play a significant role in removal of nutrient loading, the self-purificative function of natural area must be considered and utilized for wastewater treatment. Therefore, it is important to conserve microtopographic environments in the tidal section of the river for improvement water quality by erosion control works and planting works.

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The river flows with a series of spur dikes are investigated by a numerical model based on the depth-averaged plane 2D shallow water equations. The numerical method proposed by Nagata (1999) is used for the computations. The basic equations in the model are composed with contravariant components of velocity vectors in generalized curvilinear coordinate system. The effects of spur dikes are incorporated into the model by tuning the local value of manning roughness coefficients. The numerical method is applied to Kiso River with effects of tidal flows and the model performance is examined. Fundamental features of the river flows with spur dikes are simulated qualitatively by the present model.

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　流路が短く，河川勾配が急な日本の河川では，流下藻類，特に真の河川棲浮遊藻類は，陸水学者にはほとんど注目されてこなかった。しかし，堰やダムの建設により，緩流域や止水域が人為的に作られている現状では，浮遊藻類生産の知識抜きでは，河川環境を理解することが難しくなりつつある。日本での河川流下藻類の研究史を概観し，流下藻類研究が関わった社会的課題として，阿賀野川水銀中毒事件（新潟水俣病事件）と長良川河口堰問題を事例として紹介する。前者は，水銀を取り込んだ付着藻類の流下と下流での堆積が，後者は緩流化した河川での河川棲浮遊藻類の発生が法廷での争点となった。河川浮遊藻類についての既存の知識の少なさと，陸水学研究者だけではなく河川事業者や行政，法律家にも共有されていた誤った先入観により議論は深まらず，判決は下りたものの，科学的な真相解明は不十分なままである。

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