Lake Suwa ASI-31

Lat.36.05 Lng.138.084 Alt.759
Riparian Nation(s) Japan
Surface Area 13 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

Description

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. (1987, revised in 2017)

The measures based on the plan such as extensions of sewerage system, regulations of drainage from plants and factories, and reducing contaminant load from agricultural land, are improving the water quality which is indicated by the facts such as the concentration of total prosphorous below the environmental reference point in recent years and the improvement of transparency at the center of the lake. However COD and T-N are still exceeding the environmental reference point and show the similar level since the year 2000 when blue-green algae showed sharp decrease.

Other issues includes the rampant growth of water chestnuts (instead of blue-green algae) since several decades ago and ecosystem incidents such as the effect of extended poor oxygen water to benthic organisms and the massive death of smelt in July 2016. Although the utilization of the lake shore for sports and health promotion by the maintenance of the shore is advancing, further utilization and tourism promotion are expected.

To resolve these issues The Vision of Lake Suwa Creation is being prepared, which will consolidate the Phase Seven Lake Suwa Water Quality Conservation Plan and the basic plan of the cycling road around the lake, to indicate an ideal state of the lake regarding the various aspects like conservation of the ecosystem and lake shore utilization and it will show the overall basic plan and concrete measures related to the future state of the lake. (Februray 2018)

Photo of Lake Suwa
Photo: Nagano Prefectural Government