Freshwater and brackish lakes

Lakes are divided into freshwater and brackish lakes depending on whether seawater mixes into them. The national lake survey recorded whether each lake was brackish or freshwater along with its lake type.

Freshwater lakes

Lakes with no seawater mixing in. Mountain lakes are freshwater, and so are lakes near the coast that are not connected to the sea. In the 1991 survey, the great majority of Japan's lakes were freshwater.

Brackish lakes

Lakes connected to the sea, where seawater and river water mix. In the 1991 survey, 58 of the 478 lakes nationwide (12.1%) were brackish, found throughout Japan.

By origin, 81% of brackish lakes are lagoon-origin lakes, and those classed as dammed or other also lie right by the sea. The report attributes them to the action of the sea, such as coastal currents building sandbars across the mouths of bays and inlets. By nutrient level, 62.1% are eutrophic and 17.2% are dystrophic, fed by water from coastal lowland wetlands.

Telling them apart by electrical conductivity

The more salt dissolved in water, the more easily it conducts electricity. Electrical conductivity (EC) is a few tens of µS/cm for rainwater, hundreds to thousands of µS/cm for freshwater lakes and rivers, and about 40,000 to 50,000 µS/cm for seawater. In the 1991 survey, freshwater lakes generally measured 1,000 µS/cm or less, while brackish lakes ranged from 1,000 to tens of thousands of µS/cm.

Brackish lakes in Hokkaido

This catalog has 23 brackish lakes, 23 of them lagoon-origin lakes. Many line the Sea of Okhotsk and the Pacific coast of Nemuro and Kushiro.

Lake Saroma, Lake Furen, Lake Notoro, Lake Akkeshi, Lake Abashiri, Lake Tofutsu, Onneto Pond, Lake Komuke, Yudonuma Pond, Hichirippunuma Pond, Panketo Pond, Lake Shibunotsunai, Poronuma Pond, Oikamanainuma Pond, Lake Chobushi, Lake Mokoto, Mosappunuma Pond, Lake Horokayanto, Pashukuruto Pond, Tosamuporonuma Pond, Sarukotsunuma Pond, Onishito Pond, Yasoshinuma Pond