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Lake Suwa ASI-31

Riparian Nation(s) Japan
Lat. 36°03' N Lng. 138°05' E Alt. 759 m
Surface Area 13.3 km2 Mean Depth 4.7 m Volume 0.06 km3
Shoreline 15.9 km Catchment Area 515.3 km2 Residence Time 0.11 yr
Frozen Period Jan-Feb   Mixing Type Dimictic   Morphogenesis/Dam techtonic 
Related Info/Site  


Lake Suwa is a tectonic lake located on the central highland of Honshu at an altitude of about 760 m above sea level. A single stream, Tenryu River, drains the lake water into the Pacific Ocean. The lake surface remains frozen for 2-3 months during winter. Long straight rows of ice ridge locally called 'Omiwatari' sometimes traverse the frozen lake surface in the winter morning after a cold night.

The lake has been filled with sediments transported by rivers from its relatively wide drainage basin to become a typical eutrophic lake with an average depth of about 5 m. Nearly 30% of the drainage basin are covered by forest vegetation, while the greater part of the population is concentrated in a few cities near the lake shore. The spectacular growth of industrial activity around the lake since the 1960's caused a very rapid hypertrophication of the lake as indicated by heavy blooms of Microcystis (blue-green algae) that take place every summer.

In October 1986, Lake Suwa was appointed to the sixth "designated lake" based on the Law Concerning Special Measures for Preserving Lake Water Quality following the first five lakes registered in December 1985. According to the law, Nagano Prefectural Government is responsible for re-evaluating the Water Quality Conservation Plan for the lake and preparing a new one once in every 5 years. Now the Phase Seven Plan is being prepared in 2017.

Photo of Lake Suwa
Photo: Nagano Prefectual Government