19-Jun-2016 -- This is my first trip to Japan so I naturally had to visit at least one confluence, and this one is both close to Tokyo where I have a business meeting tomorrow, and easy to reach via a train journey:
Shinagawa to Akihabara via the Yamanote Line (about 15 min)
Akihabara to Moriya via the Tsukuba Express (30+ min)
Moriya to Mitsukaido on the Kantetsu-Joso Line (about 10 min)
Including the time to buy separate tickets for the first and the two last trains, the significant walking distance between the lines and the fact that it was impossible to buy the second train ticket with a credit card (I had to find an ATM machine first), the trip took about 2 hours for a net distance of less than 60 km!
I had planned to run to & from the confluence but after a few minutes I realized that it was impossible to do so without getting far too sweaty for the return train ride, so I had to take it easy. Just like David Moews noted it took about half an hour to walk along the river, across the small bridge and zig-zag between the rice fields to get to the confluence area.
Here it was obvious that the plants were in the middle of the growing season so I could not just take off my shoes and walk between them in the mud.
The nearest road is 30+ meters from the point but I noticed that each rice field is about 20 m wide, with narrow (10 cm) concrete dividers between them. I therefore balanced out on both sides of the field and got within 11 m on the SE side and within just under 9 m on the NW side, thankfully without losing my balance and dropping into the mud!
Afterwards I reversed the route and went back to my hotel in Shinagawa, now (after a nice shower) it is time for some of the sushi Japan invented!