The old city of Edo today’s Toky occupied alluvial and reclaimed lands along and to the east of the Sumida River (which flows just east of central Tokyo) and hills to the west of the river. The site was chosen for strategic reasons. It commands the southern approaches to the Kantō Plain, the largest in Japan. Saitama is mostly flat, and in Kanagawa hills prevail, though both prefectures give way to mountains along their inland extremities, as also does Tokyo. Much of the mercantile centre of Edo was reclaimed from the Sumida estuary, which reached to the grounds of the premodern castle (now the imperial palace).
Two other rivers of note in the region are the Tama, the lower reaches of which form the eastern boundary between Tokyo and Kanagawa prefectures; and the Tone, the main course of which lies some distance north of Tokyo. The Tone is the second longest river in Japan, and its drainage basin is the largest. Before the 17th century it flowed through what is now Tokyo and into the bay, but for flood control the Tokugawa shogunate diverted it. The main mouth of the Tone is now at the northeastern corner of Chiba prefecture, although a minor branch, the Edo River, continues to flow into the bay and forms the boundary between Tokyo and Chiba prefectures. The Sumida, of different origins, continued to flood the city until the Arakawa Drainage Channel, roughly parallel to the Sumida and a short distance to the east of it, was put through in the years before the 1923 earthquake.
The eastern districts, because they lie on unconsolidated, geologically unstable land and because they have been the more crowded and less affluent parts of the city, have been prone to disaster. They were almost completely destroyed by the earthquake of 1923 and the aerial bombings of 1945. The palace lies at the boundary between the flatlands and the more prosperous and geologically stable hilly regions. The flatlands—the Downtown, or Low City—dominated the mercantile culture of Edo. The hilly Uptown, or High City, has been increasingly dominant in the 20th century. The shift may be taken as a concise summary of what has transpired since Edo became Tokyo.
From its origins along the Sumida estuary, the city has spread in all directions, even into the bay. Reclamation has been continuous and since 1950 has been so extensive that the reclaimed lands are the centre of highly imaginative, perhaps somewhat dreamy, schemes for the future. This is inevitable, since most of the rest of Tokyo metropolitan prefecture is now full of people and since vast tracts of suburbia lie beyond the authority of the prefectural government. The general direction of movement for this constantly moving city has been westward. Until 1991 City Hall, which might more properly be called the Prefectural Office, was near the old centre of the city, just east of the palace and within the outer moat of Edo Castle. In 1991 it moved to a part of Shinjuku, a western “satellite centre” that was not fully within the city limits until 1932. The new site is nearer the population centre of the prefecture than the old.
By 1932 the city limits were no longer realistic. Twenty new wards were added around the old 15, and Tokyo suddenly became the second (or perhaps third) largest city in the world. It does not matter so much now that the 23 wards, to which the 35 were reduced in 1947, no longer contain the city, because the “ward part” has no administrative significance. A popular saying had it that Edo ended at what is now the campus of the University of Tokyo, to the north of the palace. It would not take an hour for a good walker to go the distance from the old mercantile centre, east of the palace and castle, to the university. A walk today to the farthest northern suburbs would take the best of walkers many hours.