MASHU-KO (LAKE MASHU)

Overlooking the lake from the west

Photo.
Photo: A. Kurata


A. LOCATION

  • Hokkaido, Japan.

  • 43:35N, 144:32E; 355 m above sea level.


B. DESCRIPTION

    Lake Mashu is located in the eastern part of Akan National Park and is known as a mysterious scenic spot. The "mystery" is attributed to the fact that the lake can be rarely seen in the summer tourist season due to frequent heavy fog. The Environment Agency prohibits entry to the lakeshore area and allows only viewing from observation towers.
    It is a beautiful crater lake with its steep encircling wall in complete shape. There is a 300 meter high overhanging cliff in the western part of the crater wall. In the center of the lake there is an oval-shaped small island, Bentenjima (Kamuisshu in Ainu language), 70 m x 50 m in size. The central portion of the island stands 25 m above the water surface. The maximum depth of the lake is 211.5 m and the lake bottom is almost flat and covered by pumice deposits.
    There are two inflowing streams, but no observable outlet from the lake exists. It is believed that the lake water seeps out through porous bottom sediments, since the water level remains fairly constant. The lake is oligotrophic and the lake water appears indigo-blue. The transparency of 41.6 m measured in August 31,1931 was said to be the highest in the world, surpassing that of Lake Baikal at that time (40.5 m). The transparency has, however, decreased substantially in recent years (Q).


C. PHYSICAL DIMENSIONS (Q)

    Surface area [km2] 19
    Volume [km3] 2.7
    Maximum depth [m] 211.5
    Mean depth [m] 141.3
    Length of shoreline [km] 19.8
    Catchment area [km2] 33

D. PHYSIOGRAPHIC FEATURES (Q)

D1 GEOGRAPHICAL
  • Bathymetric map: Fig. ASI-22-01.

  • Names of main islands: Bentenjima (70 m x 50 m).

  • Number of outflowing rivers and channels (name): None.

D2 CLIMATlC
  • Climatic data at Teshikaga*, 1982-1983

    Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Ann.
    Mean temp. [deg C] -7.6 -9.1 -3.95 3.55 8.65 9.5 14.5 17.95 14 7.9 2.05 -4.05 4.45
    Precipitation [mm] 41.5 13 52.5 74 63.5 101.5 96 200 92 87.5 86 31 938
    * Altitude 250 m.

    Fig. ASI-22-01
    Bathymetric map (1).

  • Water temperature [deg C]

    Station 1, 1984-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 - - - - - 5.5 - 19.1 16.0 12.7 - -
    30 - - - - - 4.2 - 7.1 8.0 10.5 - -
  • Freezing period: December-April.


E. LAKE WATER QUALITY (Q)

E1 TRANSPARENCY [m]
    Station 1, 1984-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    - - - - - 25.4 - 23.3 22.0 23.3 - -
E2 pH
    Station 1, 1984-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 - - - - - 7.2 - 7.4 8.0 7.4 - -
    30 - - - - - 7.2 - 7.3 7.6 7.2 - -
E3 SS [mg l-1]
    Station 1, 1984-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 - - - - - <1 - <1 <1 <1 - -
    30 - - - - - <1 - <1 <1 <1 - -
E4 DO [mg l-1]
    Station 1, 1984-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 - - - - - 12.0 - 9.3 9.2 10.1 - -
    30 - - - - - 12.2 - 12.8 10.8 12.5 - -
E5 COD [mg l-1]
    Station 1, 1984-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 - - - - - <0.5 - <0.5 <0.5 0.8 - -
    30 - - - - - <0.5 - <0.5 <0.5 0.8 - -
E6 CHLOROPHYLL CONCENTRATION [micro l-1]
    Station 1, 1984-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 - - - - - 0.26 - 0.35 0.32 0.22 - -
E7 NITROGEN CONCENTRATION
  • Total-N [mg l-1]

    Station 1, 1984-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 - - - - - <0.05 - <0.05 <0.05 <0.05 - -
    30 - - - - - <0.05 - <0.05 <0.05 <0.05 - -
E8 PHOSPHORUS CONCENTRATION
  • Total-P [mg l-1]

    Station 1, 1984-1985
    Depth [m] Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
    0 - - - - - <.003 - .003 <.003 <.003 - -
    30 - - - - - <.003 - <.003 <.003 <.003 - -

F. BIOLOGICAL FEATURES

F1 FLORA
  • Phytoplankton: Melosira spp., Synedra sp.

F2 FAUNA
  • Zooplankton: Daphnia longispina, Bosmina coregoni.

  • Fish: Salmo gairdneri, Oncorhynchus nerka.


G. SOCIO-ECONOMIC CONDITIONS

G1 LAND USE IN THE CATCHMENT AREA
    Natural landscape 100%.
  • Main types of woody vegetation
    Evergreen conifer forest (Picea jezoensis, Abies sachalinensis); birch forest (Betula ermanii).
G3 POPULATION IN THE CATCHMENT AREA
    None.

H. LAKE UTILIZATION

H1 LAKE UTILIZATION
    No direct uses; sightseeing.

J. WASTEWATER TREATMENTS

J1 GENERATION OF POLLUTANTS IN THE CATCHMENT AREA
    a) Pristine lake environments.

M. INSTITUTIONAL MEASURES

M3 RESEARCH INSTITUTES ENGAGED IN THE LAKE ENVIRONMENT STUDIES
  1. Hokkaido Research Institute for Environmental Pollution, Sapporo
  2. National Institute for Environmental Studies, Tsukuba

N. SOURCES OF DATA

  1. Questionnaire filled by Hokkaido Research Institute for Environmental Pollution.
  2. Geogr. Survey. Inst., Ministry of Construction (1986) 1:10,000 Map of Lake Mashu (1:10,000 Kosho zu, Mashu-ko).* * Printed in Japanese. The title is tentatively translated into English with the original title in romanized Japanese in parentheses.