Arakawa Shizen Park (荒川自然公園) is a public park in Arakawa City, near the Arakawa Nanachome (荒川七丁目) stop of the Toden Arakawa line. It has a couple of ponds (one is crawling with turtles), a few sports facilities and some weird statues. It is also a popular spot during sakura season.
In May, starting at Kameido station, I walked east towards Edogawa City, until I reached the Arakawa River. I then crossed it in order to visit Funabori Tower Hall and its observatory. I went back to the other side of the Arakawa and walked along the Kyu-Nakagawa River (旧中川) then turned west at the Kitajukken River (北十間川), which goes all the way to the Sumida River, passing in front of the Tokyo Skytree. I ended the day with the Sanja Matsuri in Asakusa.
The Old Iwabuchi Sluice Gate (aka Red Sluice Gate or Akasuimon) is a flood gate located in Akabane (Kita City), at the spot where the natural Arakawa River becomes the Sumida River. The construction of the gate in the 1920’s was accompanied by the digging of an artificial canal, starting at the same location: This canal is the Arakawa River that flows through Tokyo. The system was designed so that, in case of a flood, the gate would be closed and overflow water would instead get into the canal and quickly dumped into the sea. The Red Gate still stands but its role has been taken over by the new Iwabuchi Sluice Gate (in blue). This page has more context about the floods of the Sumida River and the gates.
The gates are in the middle of a large park that I visited a few weeks ago.