Biwako - Lake and Rivers

(Biwako = Biwa-ko)

1. Biwako: basic facts

Located in Shiga Prefecture, at just about the center of the Japanese Archipelago, Biwako is the largest lake in Japan. It was formed about 4 million years ago and is the third oldest - after Lake Baikal and Lake Tanganyika - of the world′s ancient lakes. It gets its names because on the map it resembles the shape the biwa, a Japanese lute.

During its long history, many species found only in the lake have evolved. These include the Biwa trout and Seta shijimi (Corbicula sandai). The lake is listed in the Ramsar Convention.

Water streams into Biwako from 118 Class A rivers and flows out to Osaka Bay through a watercourse that is called the Seta River, the Uji River, and the Yodo River. This water has always supported the lives of people.

So, now let′s look at the some more detailed facts about Biwako.

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1.1 Geographical features

map

1.2 Data

Biwako area, volume, and other data
Water surface elevation	T.P.+84.371m / O.P.B.85.614m std. water level (B.S.L.±0.00)
Lake surface area	about 674km²
Length of shoreline	about 235km
Lake capacity	about 27,500,000,000m³
Max. depth	103.58m
Length	63.49km
Max. Width	22.80km
Min. Width	1.35km

Biwako water level is standardized to the datum for the low water mark in Tokyo Bay (T.P.) so B.S.L.±0 is set at T.P. +84.371 m (O.P.B. +85.614 m). The surface level is gauged at Katayama, Hikone, Omizo, Katata, and Miho-ga-saki and is expressed at the average of the five readings.

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1.3 Comparison with other lakes - in Japan and worldwide

Surface area, depth, and elevation of Biwako compared with world lakes and wetlands

Surface area, depth, and elevation of Biwako compared with other Japanese lakes and wetlands

Source; Environment at Shiga

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1.4 Topography and climate

1.4.1 Topography

Along with an alluvial plain called the Omi Basin, Biwako is surrounded on all four sides by the mountainous areas of Hira, Nosaka(mountains between Shiga prefecture and Wakasa, Fukui prefecture), Ibuki, Mt. Hiei and the Suzuka Mountain Range. To the south and east of Biwako, the plains of the Omi Basin, famed since ancient times for grain, have been formed by the Yasu River, Hino River, and other rivers flowing from the south, and by rivers such as the Echi River, Inukami River flowing from the east. By contrast, the northern and western plains are small-scale features, comprising not much more than alluvial fans at mouths of the Ane River, Takatoki River, and other northern streams, and at the mouths of the western streams such as the Ishida River and Ado River.

map

The surface area of Biwako is 674 km². At its deepest point, it is 104 m deep, and the average depth is 41 m. It extends 63.5 km north to south and 22.8 km east to west. The lake has two parts. North of the Biwako Bridge that links Katada and Moriyama, the main basin is filled by the northern lake, which has surface area of 616 km². Meanwhile, the southern lake in the secondary basin has a surface area of 58 km². Underwater, topography of the lakebed is extremely complex. While inclines in the south and east are gentle, the north and west has steep slopes. Biwako reaches its greatest depth of over 100 m at about 2.3 km offshore from the Ado River, and the deepest continuous contour is at the west side of the northern lake. By contrast the southern lake, where depth varies between four and seven meters, is relatively shallow.

1.4.2 Climate

1.4.2A Precipitation

The Japanese Archipelago is located in a place where warm maritime air masses meet cool continental air masses. This means there is much precipitation and the weather is liable to sudden changes and extremely heavy local rain or snowfall.

The major precipitation in the Biwako catchment area occurs in the highlands north of a line running though Kitakomatsu on the western shore and Hikone opposite. Here, because the mountains receive heavy snowfall brought by winter winds, annual precipitation is between 2,000 and 3,000 mm. Most of the snow fall occurs in the period from late January to early February.

The climate in the area from the upper reaches of the Kizu River in the Takami Mountains to the eastern catchment of Biwako in the Suzuka Mountain Range is under the dominant influence of the Pacific Ocean. Much of the high annual precipitation of more than 2,000 mm in this area is brought by summer typhoons.

In the Biwako - Yodo River catchment area, the central lowland area from the southern tip of Biwako, including the Kyoto Basin and Osaka Plain, receives the least amount of precipitation, 1,400 mm or less.

map

1.4.2B Temperature

In the Biwako - Yodo River catchment area, the mountains around Biwako, upstream in the Suzuka Mountain Range to the south and the Tamba Mountains in the east, the monthly average temperature tends to be cooler than in the lower-lying flatlands.

The northern part of the Biwako catchment is cool throughout the year and is particularly cold in the winter. Owing to the moderating effect of Biwako, the Omi Basin is not subject to great extremes of temperature.

The climate of the lowland from Biwako to the main stream of the Yodo River is influenced by the Seto Inland Sea and so stays relatively warm during the winter. For the Kyoto Basin the annual mean temperature is about 16°C, and for the Osaka Plain about 18°C. In recent years global warming has become a concern. During the past 30 years this warming tendency has been observed in the Biwako - Yodo River catchment area, where the mean annual temperature has risen by 1°C-2°C.

graph

Typical monthly precipitation and temperature
(at Hikone)
	Precipitation		Temperature
Year/Month	1976 - 2006	2007	1976 - 2006	2007
Jan.	104.8	53.0	3.6	5.1
Feb.	103.2	104.0	3.8	6.1
Mar.	121.6	110.0	6.8	7.3
Apr.	120.0	34.0	12.2	11.9
May.	152.7	125.0	17.1	17.0
Jun.	197.9	234.0	21.5	21.8
Jul.	201.0	328.0	25.4	23.9
Aug.	118.3	101.5	26.9	27.9
Sep.	174.5	83.5	23.1	25.1
Oct.	110.1	81.0	17.0	18.1
Nov.	89.3	101.5	11.4	11.4
Dec.	89.2	118.0	6.2	7.2
Total Precipitation	1582.5	1473.5	-	-
Mean temperature	-	-	14.6	15.2

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