Lake Toya is a caldera lake in the western part of Shikotsu-Toya National Park in southern Hokkaido. There are Toya Hot Springs and Usu Volcano Group on the southern shore. The lake is more or less circular in shape and has a group of islands at its centre. The bottom is covered by pyroclastic sediments.

There are some 30 inflowing streams, but their discharge rates are very small except that of R. Horobetsu. R. Sibetsu is the sole outflowing river. Since 1937, however, R. Oru became a new outlet through the diversion of the lake water for hydroelectric power generation and flood control.

The lake water became acidic since 1937 owing to the inflow of mining wastewater until pH 5 is reached around 1970, but the acidity has recently been improved to pH 6.8-7.0 after the neutralizing treatment in 1972.