It is a big, long and wide river through top Thailand into Laos on the boat trip I took from Chiang Mai. It was fairly cheap and comfortable enough for two days or so via Huay Xai the night before for an early start on to Luang Prabang. Visas required for Laos too.

Staying over in a motel – more than hostel, less then hotel, and sharing with people rather than individually as requested by us all. The boat was full but OK, comfortable enough and such an interesting journey through mountains.

We saw elephants the next morning early coming to the river to drink which was impressive. Staying in a small village called Pakbeng overnight and a wander down the road for food and if you wanted, souvenirs.

https://southeastasiabackpacker.com/slow-boat-laos/ gives information for now as I travelled in 2015-6 with a cost then of around $35 I think. Pictures too on here to see.