Okama is a spectacular crater lake located in the central part of the Zao Mountain Range, on the border between Miyagi and Yamagata prefectures. Also known as Goshiki-numa, or the “Five-Color Lake,” Okama is one of the most famous natural landmarks in the Zao Mountains.
Surrounded by a circular outer volcanic ridge and a central volcanic cone, the landscape resembles a traditional Japanese cooking pot filled with water. This distinctive shape is believed to be one possible reason for the name “Okama,” which means “pot” or “kettle” in Japanese. Changes in the lake’s water temperature caused by volcanic activity may also be associated with the name.
Situated in one of the highest areas of the Zao Mountain Range, Okama is widely regarded as a symbol of Zao. Its striking scenery, combined with the mysterious changes in the color of the water, makes it one of Miyagi’s most impressive mountain attractions.
Volcanic activity in the Zao area began approximately one million years ago, with activity confirmed by at least 700,000 years ago. Okama lies within a volcanic depression created by a major collapse of the mountain approximately 30,000 years ago.
The lake itself formed when water accumulated in an explosion crater located on the western mid-slope of the central volcanic cone.
The Zao volcano is still considered an active volcano, and volcanic gases continue to emerge from the bottom of the lake. This ongoing volcanic activity is closely connected to one of Okama’s most fascinating characteristics: its changing water color.
During periods of relatively low volcanic activity, the lake can appear as a clear, deep lapis-lazuli blue. When volcanic activity increases, sediment on the lake floor can be stirred up, causing the water to become cloudy and whitish.
Changes in acidity can also affect minerals in the water. Iron may oxidize and give the lake a reddish appearance, while the formation of iron sulfide can make the water appear darker.
Because of these natural processes, the color of Okama is not fixed. It can subtly change depending on volcanic conditions, making each visit potentially different.
The ridge surrounding the crater forms a shape resembling the Latin letter “C” or a horseshoe. It extends from Robanomimi-iwa, known as “Donkey’s Ear Rock,” on the northern side, through Umanose, or “Horse’s Back,” on the west, to Mount Katta on the south.
Inside this volcanic basin stands the central volcanic cone known as Mount Goshikidake, which rises to approximately 1,674 meters. An explosion crater about 400 meters in diameter lies on its western mid-slope, and the water-filled crater at its bottom is the lake known as Okama.
According to a survey conducted in 1968, Okama had a shoreline of approximately 1,080 meters, a diameter of about 325 meters, a maximum depth of 27.6 meters, and an average depth of approximately 17.8 meters.
The maximum depth of the lake can vary over time and is influenced by volcanic activity. The lake water is strongly acidic, with a pH of approximately 3.5, and as a result, aquatic organisms do not live in the lake.
Melting snow flows into the lake until around July, while outside the snowmelt season, rainfall is the main source of incoming water.
The lake water is believed to flow underground into the Nigorikawa River. The river flows through Miyagi Prefecture before eventually reaching the Pacific Ocean via the Abukuma River.
Water levels in Okama naturally fluctuate depending on the season and the amount of rainfall.
Zao has experienced volcanic activity throughout its history. An eruption occurred in 1918, after which volcanic fumes were observed at Okama.
By around 1939, an increase in the lake’s water temperature had been observed. Volcanic gases continue to be released from the lake floor today, providing a visible reminder that Okama is part of an active volcanic environment.
This ongoing geological activity is one of the reasons the lake remains such an unusual natural attraction. The beautiful scenery is not simply a static landscape; it is part of a volcanic system that continues to change.
Okama can be accessed from both Miyagi Prefecture and Yamagata Prefecture. The Zao Echo Line crosses the Zao Mountain Range from east to west, providing a scenic route through the mountains.
From the Echo Line, visitors can take the Zao High Line toward its terminal parking area. From there, a walking trail leads to viewpoints overlooking Okama.
Another option is to use the Katta Lift or one of the mountain trails to reach viewpoints from which the crater lake can be seen.
For visitors approaching from the Miyagi side, the nearest expressway interchanges include Murata Interchange and Miyagi-Kawasaki Interchange.
From the Yamagata side, the nearest interchanges are Yamagata-Zao Interchange and Yamagata-Kaminoyama Interchange.
Buses operate from Shiroishi-Zao Station on the Miyagi side and from Yamagata Station on the Yamagata side. However, these services do not operate during the winter season.
The Zao Echo Line is closed during winter. As a result, visiting Okama during the winter season requires winter mountaineering and appropriate equipment and experience.
Travelers should therefore plan their visit according to the season and check mountain conditions before setting out.
Okama Crater Lake is a remarkable natural landmark where volcanic geology and dramatic mountain scenery come together. Its horseshoe-shaped crater, acidic lake water, and continuing volcanic activity create a landscape unlike an ordinary mountain lake.
Most fascinating of all is the lake’s changing color. From deep blue to cloudy white, reddish tones, or darker shades, Okama can appear different depending on volcanic conditions and the chemistry of its water.
For visitors exploring Miyagi or Yamagata, a trip to Okama offers the chance to see one of the most distinctive landscapes of the Zao Mountain Range and experience the powerful geological forces that have shaped the region over thousands of years.