I was there a few years ago and paddled with Siamrivers. Jason was the guide, nice guy, I think he lived in Colorado for a while. I was there in late January (dry season) I did the Mae Teng river I think. Not bad, with more water it would have been pretty nice. He has some boats to use, and will pick you up. He is in Chiang Mai in Northern Thailand 12 hours by train from Bangkok. Nice part of the country too. Not too hot and sticky like the south. I think he has a website. I also just noticed that he is selling his business, hopefully he is still there. I'm a bit envious, I'd love to go back. Have you been there before?