The turquoise coloration of the water is scientifically attributed to allophane, a clay mineral originating from volcanic ash, which scatters light uniquely.
The lake is formed by the Shima River Dam, a concrete gravity dam completed in 1999.
The water clarity and color intensity fluctuate throughout the year based on precipitation levels and sediment inflow.
The area is officially part of the Joshin'etsukogen National Park ecosystem.
The lake elevation is approximately 700 meters above sea level.
Lake Okushima is a reservoir created by the Shima River Dam in the upper reaches of the Shima Onsen hot spring area. The lake is widely known for its distinct 'Shima Blue' color, a result of light scattering from microscopic allophane particles in the water. It serves as an artificial reservoir managed by the Gunma Prefectural government. The surrounding landscape features dense temperate forests that provide a contrasting backdrop to the turquoise water. A loop trail circles the lake, providing access to various viewing platforms and vantage points. The site is a primary destination for those exploring the Agatsuma District of Gunma Prefecture.
The viewing platform near the dam crest offers the clearest panoramic view of the lake's turquoise expanse.
Wear sturdy walking shoes if you intend to complete the full loop trail around the reservoir.
Bring an extra layer even in summer, as temperatures are lower than in the surrounding lowlands.
Check local weather reports for recent heavy rain, which can temporarily turn the water from blue to a murky grey.
Do not attempt to swim in the reservoir, as it is strictly prohibited and dangerous due to submerged machinery and steep embankments.
Take all trash with you as there are no public waste bins; refrain from entering the water.
