九十九里浜
The northern limit borders on the Abukuma Highlands, Yamizo Mountain Range, Ashio Mountain Range, and a volcanic field associated with the Nasu Volcanic Belt. The western coincides with the Kantō Mountain Range and the southern edge is defined by the Bōsō Peninsula, the Miura Hills, Tokyo Bay, and Sagami Bay. The Kashima Sea and Kujūkuri Beach define the eastern end of the plain. Most of the rivers originate in the northern or western mountain ranges and flow east or southeast into the Pacific Ocean, Tokyo Bay, or Sagami Bay. In the central part of the plain is the Tone River; in the northern part the Watarase River, Kinu River, Kokai River, Naka River, and Kuji River; and in the southern part the Arakawa River, Tama River, and Sagami River. Of these rivers, the Tone River encompasses a large area of floodplain, for it has the largest drainage area in Japan of . The drainage areas covered by these rivers account for the alluvial lowland of the Kantō Plain.