Lake Utonai is an inland sea-lake situated on the tip of the vast Yufutsu Plain, with a circumference of 17km, an area of 230ha., and a depth of 0.6m. As described by the name, "Utonai" -the place where small streams gather, the lake is fed by clear streams,including the Bibi River. Surrounded by remaining nature with mires and marshes,it is a paradise for waterfowls and a treasury of wetland flora.
In May, 1981, the lake and surrounding wetlands, together covering approximately 510ha., was designated as Japan's first "sanctuary" ,or sacred ground for wild birds by the Wild Bird Society of Japan, with the aim of nature preservation, its promotion and enlightenment. The "sanctuary" which allows people to deepen their contact with wild birds while giving thought to protecting the regional nature is especially meaningful to be located in the developing city of Tomakomai.
On December 12,1991,the area was also designated as a registered
wetland under the Convention on Wetlands Of International Importance especially
as a Waterfowl Habitat, known as the Ramsar Convention, as the 543rd site
in the world, and the 4th in Japan.