I didn't go as far as the dam, but rode from Nong Khiaw to Muang Ngoy in May last year and then caught a boat back down. The road up was relatively easy.
Here's the Nam Ou at Nong Khiaw.
There were two scheduled boats heading up river, I think at 11am and 2pm... but the locals will know. There's a Kiwi guy running the guest house in the main street. Little yellow scooter out front. The beer's cold here
By road IIRC it was 27km east on 1C, turn left and follow your nose to Muang Ngoy
Fuel is available at the turnoff on 1C
Nice ridgetop track. Easy riding
I stopped at one village where the road forked and asked. Locals were good with the advice.
Near Muong Ngoy, its worth going into this little village, which grows tobacco. I'd have stayed there if not already checked into a place on the river. Rooms $1 a night rather than the $6 I paid. I suspect the main difference was in the bathroom... and the warm beer in the village
The road down to the river has seen a few landslides, but they keep it open
Muang Ngoy had a dozen or more backpackers there when I was there. Spewing over the rails at the end of happy hour.
I counted 52 boats (the joys of dining alone)... from small paddling boats to the big mothers here
I chartered a boat for a special 7:30am run back down the river to Nong Khiaw. It cost me $40 - instead of the $4 in fuel I'd have used... but was worth it.
A couple of locals took advantage of a free ride
Would have been hard to get too many more bikes in this one, with the roof
They let me out a kilometre or two below town. If I did it again, I'd get out at the concrete steps and ride up them. The area they unloaded me was river gravel and it was a pain to get clear of.... but I was having bike issues too... blew up later that day, 4km from the China border.
So - long story short. I doubt you'd have trouble getting a boat to take you towards the dam... assuming river levels are OK