I'm a wine professional, living in Bangkok and travelling frequently throughout South East Asia. I'm a huge fan of Laos and Cambodia and off the beaten track travel. Feel free to contact me at: www.facebook.com/mil.elephants
The mighty Mekong enters Laos at the triangular border shared with China and Myanmar. It's one of the main arteries for this region and transport ships ply their way down stream from Xishuangbanna in South Western Yunnan. Although travel from China into Laos by river is possible, it's challenging and difficult to arrange. The easy option is to make your way to Huay Xai, in Bokeo province which has become a well-trodden path for tourists making their way to Luang Prabang, the former capital and traditional seat of power in Laos.
Many people make their way to Huay Xai via Bangkok, which most often involves a long bus ride from Bangkok. My preferred option is the fly/ride option offered by Nok Air, which will pick you up from Chiang Rai airport and then drop you off at the bridge which now serves as the border crossing. In the recent past, you would cross the border by tiny boat, walking up the steps to immigration which was quite romantic.
Boats leave daily from Hauy Xai for Luang Prabang, however a stop half way is necessary and the only place you can stop is Pakbeng, which has sprung up as a busy little town (by Lao standards) full of guesthouses and restaurants catering to tourists. Most of the boats are fitted with old car seats which are relatively comfortable, if cramped. Some of the older boats still have wooden benches. Smaller boats plying less popular routes and rivers are much smaller and very cramped. Imagine sitting with your knees up around your chin on a wooden bench inches from the floor for 6 or seven hours. Beautiful but cramped.
Boats usually leave Huay Xai around midday, but at quiet times, and especially during wet season there may not be a boat every day, as boats won't leave without sufficient passengers.
Earplugs should be considered as the huge outboard diesel engines provide a constant and load hum throughout the journey.
Limited food and drinks are available on board but there are several small shops near the port, where food for the trip can be bought.
You'll likely arrive at Pakbeng and easily find accommodation as guesthouse owners meet the arriving boat. Most accommodation is basic and cheap, but clean. The river thrusts through massive rock formations at Pakbeng.
The further you travel into Laos, the more specular the scenery becomes. The culmination of your two-day trip will be arriving just north of the beautiful city of Luang Prabang, at the port, where you'll find transport and agents for accommodation in the ancient city.