This is a dam lake created by damming the northernmost long river, the Teshio River. The Iwaonai Ohashi Bridge, which crosses the lake filled with water, is reflected on its surface, creating a wonderful harmony between nature and man-made formations. There are campsites and bungalows in the surrounding area, and you can enjoy fishing, canoeing, boating, and more on the lake.

Erica Park is adjacent to Lake Iwaonai. Erica is a plant of the Ericaceae family, native mainly to Africa and Europe. In this park, 60,000 Erica plants of 11 varieties that are adapted to this region, including Erica and its relative Calluna, are planted. The dainty flowers, about 1 cm in size, grow densely and can be enjoyed from June to September.