, located in

Kitakyushu City is a Cities of Japan, city located in Fukuoka Prefecture, Japan. As of June 1, 2019, Kitakyushu has an estimated population of 940,978, making it the second-largest city in both Fukuoka Prefecture and the island of Kyushu after the city of Fuk ...

Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...

, is a long, narrow

inland waterway A waterway is any navigable body of water. Broad distinctions are useful to avoid ambiguity, and disambiguation will be of varying importance depending on the nuance of the equivalent word in other ways. A first distinction is necessary bet ...

-like bay measuring five to six hundred meters wide and 13 kilometers long. In modern times, it faces heavy industrial areas and once suffered from severe

pollution Pollution is the introduction of contaminants into the natural environment that cause harm. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the component ...

and

water contamination Water pollution (or aquatic pollution) is the contamination of water bodies, with a negative impact on their uses. It is usually a result of human activities. Water bodies include lakes, rivers, oceans, aquifers, reservoirs and groundwater. W ...

, but after a long period of successful purification efforts, the water quality has improved and the ecosystem has recovered.

Physical geography

Dokai Bay from Tembodai Station of Sarakurayama Slope Car 2

Dōkai Bay, located in the northern part of

Kyushu island is the third-largest island of Japan's four main islands and the most southerly of the four largest islands (i.e. excluding Okinawa and the other Ryukyu (''Nansei'') Islands). In the past, it has been known as , and . The historical regional ...

, streches from the sea () on the east side of the island to the west into the land. It is long and narrow, shaped like waterway, and measures five to six hundred meters wide and 13 kilometers long. The average depth is shallow, about 7 meters. The formation of Dōkai Bay is closely related to the

Quaternary The Quaternary ( ) is the current and most recent of the three periods of the Cenozoic Era in the geologic time scale of the International Commission on Stratigraphy (ICS), as well as the current and most recent of the twelve periods of the ...

cooling. During the last glacial period, rivers formed a valley here. There are rivers that flow into the bay, including , but they are all small and they all flow into the bay at the innermost point of the bay. The flow pattern of water masses in Dōkai Bay is a typical estuarine circulation. Throughout the year, the predominant current in the surface layer is toward the mouth of the bay, while the predominant current in the bottom layer is toward the interior of the bay. The surface water in the inner bay does not mix vertically, but flows along the surface and flushes out of the bay within two or two and a half days.

Flora and fauna

Dōkai Bay was a tidal flat several thousand years ago, and was home to many shellfish and other creatures suited to the tidal flat environment. By historic times the bay had become deeper and was known for its abundant shrimp. In modern times the bay became a coal shipping port. The bay was lined with factories, and wastewater from these factories caused the bay to become severely polluted. In the 1960s, the bay was so polluted that it was called the "Dead Sea," because no fish could live there. After that, the government strengthened its anti-pollution measures, and Japan's industrial structure changed. As a result, the bay has recovered to the point where it is home to more than 100 species of fish and shellfish as of the 2020s.

References

{{coord, 33, 53, 17, N, 130, 47, 44, E, type:waterbody_region:JP, display=title Kitakyushu Landforms of Fukuoka Prefecture Bays of Kyushu