Nagase Reservoir ASI-06

Riparian Nation(s) Japan
Surface Area 2 km2 Mean Depth 28.2 m Volume 0.05 km3
Shoreline Catchment Area 295 km2 Residence Time 0.056 yr
Frozen Period None Mixing Type Morphogenesis/Dam Nagase Dam (1957)
Related Info/Site


Nagase-damu-ko is a multiple-purpose reservoir located in the upper reaches of Monobe River, which is 71 km long and flows through the granary area of Kochi Prefecture on Shikoku Island. The river's watershed, 508 km2 wide, is one of the areas in Japan where frequent visits of typhoon bring the heaviest rainfall. However, the rainfall is so variable that paddy fields in the watershed often suffered from severe drought, while flood disasters were also frequent.

The Japanese Ministry of Construction initiated in 1949 a comprehensive development project for the river system including the construction of this reservoir. The Nagase Dam was completed in 1957 with the total cost of 3,939 billion yen, and now supplies irrigation water at an average rate of 30 m3 sec-1 together with the hydroelectric power of about 111E+8 kWh. The Prefectural Government of Kochi is now responsible for the management of this reservoir.

The catchment area of the reservoir (295 km2) is largely covered by dense forest, containing only a small village situated near the dam site. The water has remained oligotrophic, the annual mean of total phosphorus concentration being 0.006-0.008 mg l-1, though the bloom of flagellate algae, Peridinium spp., has occurred in May-September annually in recent years. The density of algal population usually ranges from 1,000 to 10,000 cells per litre. Otherwise the lake water quality has been free from the effects of siltation, acid rain and chemical pollution and suitable for drinking (Q).

Photo of Nagase Reservoir
Photo: T.Nishijima