Lake Mashu

摩周湖ましゅうこMashū-ko

A sealed caldera lake of exceptional clarity, often veiled in fog

Lake Mashu (Mashu-ko) is a caldera lake in eastern Hokkaido with no inflowing or outflowing rivers, once measured as the clearest lake in the world. Its sheer walls keep visitors on the rim, where observatories look down on water that is often hidden by fog.

Elevation: 351 mArea: 19.2 km²Maximum depth: 211 mActive volcanoNational park · 1934

National park: Akan-Mashu National Park

Overview

Lake Mashu lies in the town of Teshikaga in eastern Hokkaido, in a caldera formed about 7,000 years ago. The crater walls rise 200 to 400 metres above the water, and there is no public access to the shore: the lake is seen from observatories on the rim.

No river flows in or out; the water level is held steady by rain, snowmelt and seepage. In 1931 a transparency of 41.6 metres was recorded, then the highest ever measured for a lake. Clarity has declined since, but the lake remains among the clearest in Japan and its colour is described as 'Mashu blue'.

Its Ainu name is Kamuy-to, 'lake of the gods'. Sea fog drifting inland in summer frequently fills the caldera, which gave rise to the popular song 'Kiri no Mashū-ko' (Mashu Lake in the Fog) in 1966.

Landscape and formation

The Mashu volcano built a stratovolcano that collapsed in a series of large eruptions around 7,000 years ago, leaving the caldera. Later activity created the lava dome of Kamuinupuri (857 m) on the south-east rim and the tiny island of Kamuishu, the tip of a volcanic cone rising from the lake bed. Mashu is monitored as an active volcano by the Japan Meteorological Agency.

Highlights

Observatory No. 1
摩周第一展望台Mashū Daiichi Tenbōdai
Main viewpoint on the south-west rim, reached by road from Teshikaga.
Observatory No. 3
摩周第三展望台Mashū Daisan Tenbōdai
Viewpoint further north with views over the lake to Kamuishu and to Lake Kussharo on the other side.
Ura-Mashu Observatory
裏摩周展望台Ura-Mashū Tenbōdai
Viewpoint on the eastern rim, reached from Kiyosato.
Kamuinupuri
カムイヌプリKamuinupuri
The 857 m peak on the rim, also called Mashu-dake, with a trail from Observatory No. 1.
Kamuishu Island
カムイッシュ島Kamuisshu-tō
A small rocky islet in the lake, the summit of a submerged volcanic cone.

Ecology

The lake was originally without fish. Introduced kokanee salmon and rainbow trout now live in it, and the lake is used as a background monitoring site for global air and water pollution. Forests of fir, spruce and birch cover the outer slopes.

Flora

Subarctic mixed forest
Sakhalin fir, Ezo spruce and birch on the caldera slopes.

Fauna

Ezo sika deer
Common along the roads to the observatories.
Kokanee salmon
ヒメマスhimemasu
Introduced in the 20th century to the originally fishless lake.

Culture

For the Ainu the lake was a sacred place. A legend tells of an old woman who, separated from her grandchild, waited on the island until she turned to stone – the islet of Kamuishu. The fog and the song 'Kiri no Mashū-ko' made the lake a fixture of Japanese travel imagery in the 1960s.

Kamuy-to
カムイトーKamui-tō
The Ainu name of the lake, 'lake of the gods'.

History

c. 5000 BCE
Climactic eruptions of the Mashu volcano form the caldera.
1931
A transparency of 41.6 m is recorded, a world record for lakes at the time.
1934
The area becomes part of Akan National Park (Akan-Mashu National Park since 2017).
1966
The song 'Kiri no Mashū-ko' by Fuse Akira popularises the lake.

Through the seasons

Spring

Snow persists into May; the road to Ura-Mashu opens after the thaw.

Summer

Fog is most frequent from June to August; mornings are often clearer.

Autumn

Clear air and colouring birch and maple make September and October good for views.

Winter

The lake usually freezes in cold winters; Observatory No. 1 remains accessible, while the road to No. 3 is closed.

Routes and trails

Kamuinupuri trail
Rim walk of about 7 km from Observatory No. 1 to the 857 m summit, with continuous views into the caldera.
Observatory circuit
Road link between Observatories No. 1 and No. 3, often combined with Lake Kussharo and Iō-zan.

Practical notes

Mashu Station on the JR Senmō Line lies in Teshikaga; the observatories are reached by bus in season, taxi or car.
Descending to the shore is prohibited.
Fog is common, especially in summer; checking the rim before driving up can save a wasted trip.

Frequently asked questions

Why is Lake Mashu so often foggy?

Cool, moist air from the Pacific drifts inland in summer and settles in the enclosed caldera, filling it with fog.

Can you go down to Lake Mashu?

No. The shore is off limits; the lake is viewed from the observatories on the rim.

How clear is Lake Mashu?

A transparency of 41.6 m was measured in 1931. Clarity is lower today but still among the highest in Japan.

Where is Lake Mashu?

In the town of Teshikaga in eastern Hokkaido, within Akan-Mashu National Park, near Lake Kussharo.

Sources

1.統計FAQ 日本の湖沼の面積 (opens in a new tab) — 総務省統計局
2.阿寒摩周国立公園 (opens in a new tab) — 環境省
3.弟子屈町 (opens in a new tab) — 弟子屈町