Glossary
Unmodified lakes
An unmodified lake is one with no seawalls or other artificial structures on its shore and with the land around it left entirely natural. The national lake survey examined how far each lakeshore had been altered by people, and called lakes whose shores were entirely natural unmodified lakes.
How lakeshores are classified
In the survey, the lakeshore is the zone extending 100 m inland from the waterline (where the water meets the land at its highest level). The shoreline was divided into three categories:
- Natural shore: the waterline and the 20 m inland of it are natural, with no artificial structures
- Semi-natural shore: the waterline is natural, but there are artificial structures within 20 m inland
- Artificial shore: the waterline itself is made of artificial structures
An unmodified lake has 100% natural shore and 100% natural land use (such as woodland) along its shore, with sections facing open water counted in both.
Most unmodified lakes are small
In the 1991 survey, 210 of Japan's 478 lakes (43.9%) were unmodified. However, 195 of them had shorelines under 5 km, and 101 under 1 km. The report suggests that many small lakes were simply never used for power generation, tourism or farming.
Only 15 relatively large unmodified lakes, with shorelines of 5 km or more, existed nationwide, and 14 of them are in Hokkaido; the other is in Fukui Prefecture.
| Rank | Lake | Prefecture | Origin | Shoreline (km) |
|---|---|---|---|---|
| 1 | Lake Mashu | Hokkaido | Caldera | 19.8 |
| 2 | Yudonuma Pond | Hokkaido | Lagoon-origin | 17.8 |
| 3 | Onneto Pond (Nemuro) | Hokkaido | Lagoon-origin | 13.9 |
| 4 | Lake Panketo | Hokkaido | Dammed | 12.4 |
| 5 | Busshugaike | Fukui | Dammed | 9.7 |
| 6 | Lake Chobushi | Hokkaido | Lagoon-origin | 8.6 |
| 7 | Panketo Pond | Hokkaido | Lagoon-origin | 7.5 |
| 8 | Lake Chimikeppu | Hokkaido | Dammed | 7.4 |
| 9 | Junsainuma Pond (Toyotomi) | Hokkaido | Other | 7.0 |
| 10 | Naganuma Pond (No. 2, Toyotomi) | Hokkaido | Other | 6.4 |
| 11 | Poronuma Pond | Hokkaido | Lagoon-origin | 6.0 |
| 12 | Lake Takkobu | Hokkaido | Lagoon-origin | 5.8 |
| 12 | Penketo Pond | Hokkaido | Lagoon-origin | 5.8 |
| 14 | Lake Horokayanto | Hokkaido | Lagoon-origin | 5.7 |
| 15 | Bentennuma Pond | Hokkaido | Lagoon-origin | 5.2 |
Table III-1-16 of the report. Lake names follow this catalog.
Unmodified lakes in Hokkaido
68 of the 127 lakes in this catalog are unmodified lakes. The survey dates from 1991, so some shores may have changed since then.