The lake's signature turquoise color, often called 'Shima Blue,' is a result of light scattering from microscopic mineral particles in the acidic water.
The pH of the reservoir water is highly acidic, typically around 3.5, due to natural geothermal activity in the surrounding mountains.
Construction of the dam was completed in 1999 to mitigate flooding risks in the Agatsuma River watershed.
The reservoir capacity is approximately 13,000,000 cubic meters.
The dam serves as a key component in the local green energy network, powering the nearby Shimagawa Power Station.
Shimagawa Dam is a multi-purpose concrete gravity dam located in the Shima River basin of Gunma Prefecture. It creates Lake Okushima, a reservoir known for its distinct turquoise water caused by colloidal particles suspended in the acidic lake water. The dam functions primarily for flood control, industrial water supply, and hydroelectric power generation. Visitors can access the top of the dam to view both the reservoir and the steep downstream valley. The structure stands approximately 89.5 meters tall. It is situated within the Joshin'etsu Kogen National Park area. The area features a dedicated observation deck and perimeter walking paths. It is part of a larger hydroelectric system that includes several other dams in the Agatsuma District.
The pedestrian viewing area at the center of the dam crest looking toward the reservoir.
Bring polarized sunglasses to better observe the 'Shima Blue' color shift in the water.
Visit during autumn to witness the contrast between the turquoise reservoir and the vibrant fall foliage.
Check the local weather forecast specifically for the mountain region, as conditions can change rapidly compared to the lowlands.
Do not attempt to swim in the reservoir, as the high acidity and water depth make it dangerous and prohibited.
