I found a great Tama River spot in Mitake and a dog-friendly pizza restaurant called YOSHIZO cafe. If you are looking for something to do out of the city with your family and friends, this is a great Sunday Fun-day getaway.
To the parking lot nearby the spot was about a 40 min drive from the base. There was a group tour heading out on a rafting trip nearby. There is a walking bridge crossing overhead that will take you to the other side, likely into the town of Mitake (I was on the other side of the river from the actual town). There was a family with a tent set up (camping might be allowed). Parking was Y100/hour and there were more than enough spaces in the lot (I was there in the morning). There are public bathrooms adjacent to the parking lot.
I highly recommend aqua-socks, keens or some protective footwear you can wear in the water because the rocks hurt!
There is a pizza restaurant called YOSHIZO cafe up the hill advertised dogs OK.
They spoke limited English. I tried to order lunch on the patio but they told me it was reserved, so I ended up ordering pizza to go and sat on the grass overlooking the river instead.
YOSHIZO cafe website: http://yoshizo-cafe.tokyo/index.html
Hours: Fri-Wed Lunch time 11:30am-4:00pm (Last Order 3:00pm)
Dinner time 5:00pm-9:00pm (L.O. 8:00pm)
Mitake Parking spot near Tama River