The Iwaki River supported numerous villages as early as the Jōmon period as evidenced by numerous shell mounds. The river was revered as the "mother of the Tsugaru Plain" throughout its early history. The Iwaki River region, located in the vast northern Mutsu Province, came under the control of the central government throughout much of the Heian period. The Northern Fujiwara clan, based in Hiraizumi in present-day Iwate Prefecture, controlled the rich trade of the Port of Tosa in present-day Goshogawara. Despite the loss of control of much of Japan later in the Heian period, the court maintained some level of military presence in Mutsu. This control ended when the Northern Fujiwara were conquered by Minamoto no Yoritomo and clans of the Kantō region, and the Iwaki River region came under the direct control of the Kamakura shogunate. By the mid-Kamakura period the Akita clan ruled the area, and the ranches, agriculture of the Iwaki River region, and trade with the Ainu. The Port of Tosa was destroyed in an enormous tsunami in 1340. The Akita clan was eventually defeated by the Nanbu clan during the Sengoku period. Numerous castles were constructed along the Iwaki River, notably Hirosaki Castle.
The Tokugawa shogunate quickly established stability in the Iwaki River region at the beginning of the Edo period. The course of the Iwaki River was diverted and developed for agricultural as part of an effort by the Tokugawa shogunate for the increase of agricultural output across Japan. The original path of the upper reaches of the Iwaki River remains unclear. There were originally two rivers in the area of the castle town of Hirosaki, and at some point in the late 17th century they were diverted into a single river, the Iwaki. The Tsugaru Domain began large-scale work on the Iwaki River in the same period to open new rice paddies on the Tsugaru Plain. The Iwaki River had a lively riverboat trade, and rice storehouses controlled by the Tokugawa shogunate were built in Sanzeji, Fujisaki, Itayanoki, and Kanagi. The rice was collected at the Port of Jūsan in present-day Goshogawara at the mouth of the river at Lake Jūsan. From there the rice was transported to Ajigasawa on the Sea of Japan to be shipped to the Edo capitol.
Modern period
After the Meiji Restoration in 1868 the construction of railroads in Aomori Prefecture, specifically the Ōu Main Line, ended the comparatively inefficient riverboat trade along the Iwaki River in favor of the movement of agricultural cargo by rail. In the 20th century the central government of Japan took direct control of riverworks to prevent flooding on the Iwaki, a chronic problem recorded as early as the Edo period. The Iwaki River Improvement Plan was officially approved by the Diet of Japan in 1917. Large-scale flood prevention works began shortly afterwards. The mouth of the river at Lake Jūsan, Mitoguchi, suffered frequent blockage between November and April due to strong winds and waves, a feature of the harsh climate of northern Aomori Prefecture. Projects to remove blockages of the river at Mitoguchi began in the Meiji period, but caused a change in water levels on the upper reaches of the river. The controversy over construction work at Mitoguchi became increasingly controversial, resulting in violent clashes between residents of the upper and lower regions of the Iwaki River in 1890. A pier to prevent blockage was excavated and constructed at the mouth of the river between 1930 and 1946. The passageway allows year round access to the river. Numerous bridges were also constructed across the Iwaki River to improve transportation in Aomori Prefecture.
Use
The Iwaki River is a major source of water for irrigation. Numerous irrigation canals have been built across the Tsugaru Plain, and the Iwaki supports the paddies for vast production of rice in Aomori Prefecture. The river is called "the mother of Tsugaru" for this reason. Aomori Prefecture is the largest producer of apples in Japan, and the upper zone of the Iwaki River provides irrigation for an extensive network of orchards, especially in the area of Hirosaki City. The commercial fishery of ayu is no longer held on the Iwaki, but its lower reaches and mouth of the river at Mitoguchi are a source of commercial clam production.
The lower reaches of the Iwaki were once a vast reed bed. Numerous bird species are protected in this area, including the marsh grassbird, also known as the Japanese swamp warbler, and the Japanese reed bunting, both of which are threatened by habitat loss. The mākuosamu and Dytiscidae species of beetles are indigenous to the area. The least weasel, common in other areas of the world, is increasingly rare in the Iwaki River region and may become a protected species. The yamato shijimi, a species of clam used in Japanese cuisine, is common in the brackish area of the mouth of the Iwaki River at Mitoguchi and Lake Jūsan.
