奈良県大迫ダム貯水池の水質特性および下流河川に及ぼす影響

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  • ナラケン オオサコ ダム チョスイチ ノ スイシツ トクセイ オヨビ カリュウ カセン ニ オヨボス エイキョウ
  • Characteristic of Water quality in Lake Osako and Its effects on Down the River

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Generally, it is known that the reservoir has peculiar hydraulic characteristic and the water quality differ from natural lakes because of the strong enclosed water and deep depth. The cold and long-term turbid water phenomena and the eutrophication are caused by these characteristics in the reservoir. Recently, remarkable water environmental problem in a reservoir is the long-term high turbid water and the coloring phenomena such as a red and black water of discharge water. Therefore, it is the most important problem to clarify the water quality formation and the mechanism of generation and也e disappearance of the turbid waters in the reservoir. We carried out the monthly field observation to clarify characteristics of the hydraulic and the water quality in the Lake Osako located in the south-eastern part of Nara Prefecture. As a result, Lake Osako is stably stratified with a thermocline and formed an anoxic water mass in the lower layer below the thermocline from summer to autumn. This layer can easily become anoxic due to weak mixing between the upper and lower layer as well as to large oxygen consumption by suspended and dissolved substance in the lower layer and bottom sediment. The formation of an anoxic water mass greatly affects the water quality and biological environment of Lake Osako and around the river. It is clear that the high turbidity water in the resermir and down the river is generated for a long period, when water level decreases.

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