Biwako - Lake and Rivers

2. Measures at Biwako

2.4 Water supply from the Seta River Weir

2.4.1 Meeting downstream water needs throughout the year

At the Seta River Weir, it is important not to waste valuable water by discharging it unnecessarily. To ensure that discharge can be precisely controlled to enable the supply of even during times of drought, when the Biwako water level is low, facilities such as bypass channels next to the weir have been installed.

Meanwhile, provision has been made to ensure that shipping is unaffected by low water levels. Wharves and moorings have been improved and dredging is undertaken along shipping routes and in places where boats dock.

In addition, so that the local water supply around the lake can be maintained even if the water level drops, water intake facilities have been located offshore and the number of pumping stations has been increased.

It is also necessary to maintain the population of the ayu (sweetfish), an edible fish that is one of the valued aquatic resources of the lake. To ensure that the ayu have places to spawn if the lake water level declines, artificial river beds were created at the mouths of the Ado River and Ane River. After adult fish spawn in these locations, three to five billion ayu fry hatched from eggs annually return to the lake.

Presenting the appearance of Itsukushima, off the Omi shore rises the torii of Shirahige Shrine. So that the gateway is not left high and dry if the level of the lake falls, it has been relocated further offshore in deeper water. To maintain the famous scenery around the Ukimodo of Mangetsu Temple, dredging has been carried out around the ‘floating pavilion’ so that declining water levels do not spoil the scene immortalized in ‘Geese descending at Katata,’ one of Hiroshige′s Eight views of Omi.

2.4.2 Maintaining downstream water supply

Operation

During normal times, to efficiently feed water to downstream users, the Seta River Weir is managed as an element in the overall upstream dam and reservoir system of the Yodo River system.

At the measuring station at Hirakata, which is a reference point for flow in the Yodo River, during the rice irrigation period the target constant flow is 183.97 m³/s, and during other times, 167.370m³/s. Discharge through the Seta River Weir and from other upstream reservoirs is carried out to maintain the flow at Hirakata and make up the differentce when it falls below the target; that is, at times indicated by (1) on the figure. During the normal daily operations of the Seta River Weir, demand for domestic, industrial, and other water drawn from the Yodo River is also monitored.

2.4.3 Conditions in Drought

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