Hitomi-numa Pond is situated in the lowland areas in the central region of Sarobetsu Mire. This small pond (area, 0.7 ha) contains a relatively large floating islet which is composed of peat. This paper reports objective data that describe the current state of the pond and considers the processes underlying the formation of the pond and the floating islet. The islet was formed within only a few decades of the 20th century, under the influence of agricultural development in the surrounding mire. An excavated drain was constructed in this region for agricultural purposes. Through this drain, sand and mud flowed into the lowland area, eventually forming a natural levee. This levee altered the flow of ground- and surface water, and the water level in the lowland area gradually rose. Owing to the buoyancy of peat, the peat deposits in the lowland area surrounding the pond floated to the top of the pond as the water level elevated. A fragment of the floating peat presumably separated from the peat matrix and drifted on the surface of the pond. The floating islet of Hitomi-numa Pond is peculiar since it was formed because of an artificial act and since the time required for its formation can be accurately determined. Moreover, the process of its formation was complex, involving multiple factors.
