is a lake in the Kiso Mountains, in the central region of Nagano Prefecture, Japan.
Geography
The lake is the source of the Tenryū River. It ranks 24th in lake water surface area in Japan. The cities of Suwa and Okaya and the town of Shimosuwa are located on the shores of Lake Suwa.
Miwatari
Lake Suwa is the site of a natural phenomenon known as the miwatari, in recent years with an "o" added to the beginning, meaning , large cracks that form in the winter across the surface of the frozen lake. A vertical temperature gradient results in ice pressure ridges forming in the surface ice, reaching heights of or more, although much greater heights were recorded in the past. The record of this crossing is the oldest-known human-observed climate record. This record has been used by scientists to study the impact of sunspots, human development and anthropogenic CO<sub>2</sub> emissions on lake ice formation.
The lake has a natural hot spring under its surface. Recreational development in the 1950s resulted in increased output from this underwater geyser. Since 1945, complete ice cover is considered to be achieved when the entire surface is frozen except for the opening in the ice for the geyser.
As a shallow lake, Lake Suwa is highly sensitive to climatic variation. Recent studies have shown that Suwa's ice-free years correlate to increasing atmospheric CO<sub>2</sub> levels. Research on ice breakup at Suwa and at the mouth of the Torne River in Finland suggest that climate change is driving the changes in seasonal ice cover. As a result, Lake Suwa may remain ice-free for almost the entirety of the 21st century's winters.
As of 2025, the last time Lake Suwa had developed the miwatari was 2018, and the ridge that developed was only a few inches high.
Epson, a global technology company, was founded and is headquartered in Suwa.
Water sports
In 2020, in advance of the scheduled Tokyo 2020 Summer Olympics, a new rowing boathouse and training facility was established in Shimosuwa.
See also
- List of lakes in Japan
- Onbashira