The putin is located at the Route 177 bridge in New Boston. On the downstream side of the bridge, on river left, is a drainage ditch that leads down to the river bank.
For a decent run, the water level must be high at the New Boston bridge. If the water level just looks "runnable" at the bridge, you will not have a good time. Further downstream, the river braids into wide channels than can be very shallow (unrunnable).