Per the Goshogawara city tourist bureau, the lagoon is called Lake Jusan because thirteen (十三, jūsan) different rivers flow into it.

The only outflow of the lagoon is to the Sea of Japan at the northwest corner of the lake, opposite to the mouth of the Iwaki River.

[6] The Iwaki River region, located in the vast northern Mutsu Province, did not come under the control of the central government until the late Heian period (794 – 1185) [7] The Northern Fujiwara clan, based in Hiraizumi in present-day Iwate Prefecture, controlled the trade with the Asian continent and Hokkaido via the Port of Tosa, located on the western shore of the lake.

Despite the loss of control of much of Japan later in the Heian period, the court maintained some level of military presence in Mutsu.

By the mid-Kamakura period the Andō clan ruled the area, and Fukushima Castle was built on the north shore of the lake.