Changes in river mouth morphology are mainly affected by longshore currents and sediment deposition by the river. River mouth closures are important because they can lead to flooding. Although jetties and river mouth excavation are employed as countermeasures for preventing or limiting river mouth closure, such measures are not long-term solutions to the problem. Stabilization of the river mouth of the Ishikari River on the Ishikari Coast is described in this study.

Ishikari Bay is an inlet on the Sea of Japan on the west coast of Hokkaido, Japan. The Ishikari Plain, which has an area 3,800 m2, is drained by the Ishikari River which originates in Mount Ishikari in the Taisetsu Mountain Range and flows through Asahikawa and Sapporo before entering Ishikari Bay. The Ishikari River is 268 km long and drains a catchment area of 14,330km2. Total discharge from the Ishikari River is 14.8km3/year, which originate from the major tributaries of the Chubetsu, Uryu, Sorachi and Toyohira rivers. The Ishikari coastline has large areas of quicksand that has been deposited by the Ishikari River, and sandy beaches extend more than 24 km to the north and south of each river bank. Approximately 2,000,000m3 of sand is discharged by the Ishikari River every year. The large Ishikari River Basin supports a wide variety of life forms, including humans. However, even though flood control measures have been conducted since 1834, several flooding disasters have occurred. The prevention of spits development has been particularly important in terms of preventing river mouth closure of the Ishikari River.

The Ishikari Bay New Port was constructed 7.5 km south of the river mouth of the Ishikari River as part of the Third Hokkaido Comprehensive Development Plan approved by the Japanese Cabinet in July 1970. Under the plan, the Ishikari Bay area just to the north of Sapporo and the surrounding area in western Hokkaido was considered to be well suited for extending the existing function of the area as a distribution and transportation hub for physical goods. To establish a new manufacturing base and distribution hub in the region, construction of the port and development of the area in the vicinity of the port was initiated. After being designated a major port in April 1973, full-scale construction of the port was initiated under the direct control of the national government. In April 1978, the Hokkaido Prefecture Government, Otaru City, and Ishikari City jointly formed the Ishikari Bay New Port Authority to administrator the port. Under the new administration system, the construction of port facilities has been carried out methodically, beginning with the East District. The total area of the Ishikari Bay New Port Area development project is more than 3,000 ha.

The main causes of river mouth closure are a decrease in river discharge due to increased abstraction (i.e. drinking water, irrigation), and the strength of longshore currents. Many countermeasure works against river mouth closure in Ishikari River have been carried out, but no effective solution of river mouth closure could be. Namely, despite implementing numerous countermeasures to prevent river mouth closure of the Ishikari River, none were entirely successful. However, construction of the Ishikari Bay New Port decreased longshore sediment transport, which stabilized the Ishikari River mouth and reduced the incidence of river mouth closure. In this study, the relationship between Ishikari River mouth closure and the construction of Ishikari Bay New Port is described by the authors who have been engaged in the development plan and port construction projects over many years.