The Uchikawa 内川（うちかわ） in Ota-ku is quite short, just 1.55 km, and considered a “secondary river” 二級河川. The source of the river is Magome / Kita-Magome 馬込 / 北馬込, which reaches the Uchikawa by a series of underground culverts and tunnels.
The river surfaces when it intersects with the Tokaido Main Line tracks (map):
The Uchikawa drains into Tokyo Bay 東京湾 near Omori Furusato-no-hamabe koen (beach park) 大森ふるさとの浜辺公園 (map).
River sign:
Links:
the tokyo files: rivers & green roads 東京の川や緑道 