This is a not a trip for beginners, but was an enjoyable trip for me as I had some extra time and don't mind doing a little work. Many in this area don't consider this a worthwhile trip since like many narrow rivers it is prone to blown down trees that can sometimes go from bank to bank. I had to portage a couple of log jams and one small permanent dam. At high water flows some of these might be quite dangerous.
But this is a quiet and scenic area and was well worth my effort. I have driven along the river on US Rt. 6 frequently and have also run and hiked the Hop River Rail Trail from time to time. If you like to scout a river before trying to run it these both provide many good opportunities. There are at least a half dozen bridges that cross this part of the river. I had been meaning to put a kayak in for a long time.
I put in under the bridge at Hendee Rd. And took out under the bridge under Rt. 66 near the Columbia/ Willimantic line (This is actually just after the Hop River has joined the Willimantic River). The water level was average for what I've seen. Right away I was in quick water. The river is easy enough to read for the most part, but just about each time I started to really relax and enjoy the scenery there seemed to be an obstacle that required some concentration. Some easy boulder dodges, branches, etc. The riffles make some sections look like class 2, but I never at anytime thought a skirt was in order, and got very little water in the kayak without one.
There were some shallow rocky stretches when I felt some bumps. There were also some deeper sandy bottom areas where I spotted beavers. Lots of geese and ducks nest along here. You'll probably get a visit from a Blue Heron or two.
The most important permanent obstacle to be aware of is a small dam just before the bridge at Hop River Rd in Columbia. You can easily scout this before you put in by parking in the dirt lot next the bridge. I was able to portage along the east bank (river left). At normal water levels you can put back in under the bridge on the rocks, but at higher water you might prefer to carry over the road and put in on the other side. The tip that you are approaching the dam is that you will have just gone under the footbridge where the Hop river Rail Trail goes over the river. there is a stretch of boulder dodging just afterward. Start to make your way to river left as the Hop River Rd. bridge comes into sight.
It occurs to me that this might have been a more sensible take out instead of continuing downstream from Hop River Rd.
There are a couple sort of nasty log jams that you will need to portage if you do continue down from here. They can be scouted beforehand from the Rail trail if you park on Kings Rd. which is a dead end off of Flanders Rd. in Columbia. The Hop River Rail trail used to cross the river again here, but the old rail bridge has fallen into disrepair and the trail is now diverted onto Kings and Flanders Roads. (scout the downstream jog jam under this broken bridge). There is a need to portage on river left at both of these obstacles. (A short but fun rapids section can be run immediately after the jam.)
To scout the up stream jam you'll have to hike up the Rail trail quite a way in the upstream direction, going under the Rt 6 divided highway bridges. You'll see a steep bank down to the river where the jam can be viewed.
The take out under the Rt 66 bridge where I ended was river right and was a little muddy but convenient.
From Hartford take I-84 east, to I-384, onto Rt 6 toward Providence. Hendee Rd. is on the left near the Bolton Andover line.
Perhaps Google maps?
I also found this link to a Topo Map online: