This is a nice secluded river. I put in at Zeeks Bridge on Harkney Hill Road; public boat ramp, decent parking. If you launch at the ramp and go right its a small pond that brings you to Johnsons Pond.
The other side of Hillfarm Rd. you need to portage the water level at the road is usually to high. On Johnsons the boat traffic is high, so I prefer to stay on the Big River side. If you go left under the road at the ramp, it meanders into the woods - very peaceful.
The river goes under rte 95 and rte 3. 95 is a fairly long tunnel lots of echos for the kids, I think the other parking spot mentioned has been closed in the woods at the 95 tunnel, but at rte 3 you can take out on the side of the road if you plan ahead.
I usually paddle both ways it's really not that far; easily done in half a day. At this point the river gets smaller and smaller winding through the woods over beaver dams and depending on the water level eventually you will need to turn back.
If you're persistent you can go pretty far though. Nice daytrip.
rte 95 to rte 3 (I think exit 7a) you can see the motel sign from the highway. Go right on 3; motel is on your left. you will pass gentlemen farmer on left liquor store on right. Go left at first small light - Harkney Hill Rd. If you pass it you will see Cumberland Farms. Stay on Harkney Hill and on the right you will find a dirt lot at the boat ramp.