The Tone River which has the largest river basin in Japan rises in Mt. Oominakamiyama in the northern part of Gunma Prefecture. It runs down south to Maebashi City, and changes course to the east, then discharges into the Kashimanada Sea at Choshi City in Chiba Prefecture. It, however, used to flow south near Kurihashi Town in Saitama Prefecture, which lies mid-way between Maebashi and Chosi, and flowed in to Tokyo Bay. This rerouting was achieved by the work of the Tokugawa Bakufu in 1654. Before rerouting, the estuary had been in the northern part of the present Sumida Ward, the eastern part of Tokyo. In the estuary area, which corresponds to the Kohtoh region, consisting of Sumida and Koto Wards and the eastern part of Edogawa Ward at present, the sea was shallow and a lot of sand bars scattered over. The Kohtoh region naturally had favorable conditions for reclamation.
When Ieyasu Tokugawa entered Edo Castle in 1590, the environs of the castle were limited, because the east side of the castle faced to an inlet called Hibiya Irie, and the other sides were surrounded by rough plateaus. Topographically the site was good for a fortress but too small to make a town and farming estate. Soon after his settlement, Hibiya Irie was reclaimed to build a town for warriors and citizens, and the Onagi cannel was excavated in the shallow sea which spread on the east of the Edo City for transporting food and salt. The soil dredged from the Onagi channel was used for filling the northern part of the channel. It was the first reclamation work in the Koto sea region. Since then reclaiming works have continued in the Koto sea region, which used to be the estuary of the Tone River. A lot of land has been reclaimed due to garbage disposal in the city since 1655.
In the Tokyo Bay area, about 2, 700 ha was had reclaimed during the Edo era period over 270 years, and about 6, 000 ha from Meiji Era to the present over about 140 years. As a result of those reclamation works, the sea area of the Koto region has been replaced by man-made lands, with the exception of some ship routes.
