The flooded forest is an area of the Mekong River just north of Stung Treng. The unique environment is caused by the annual rise and fall of the Mekong. In wet season, the river rises by many metres and strong currents force the trees to fight against surging water. When the water level drops in dry season (around December), it reveals otherworldly root formations and bent-over trees that now stand proudly above water.
You can take a boat from resorts along the river or arrange it in Stung Treng town. It is best to bring a kayak if possible and explore closer, but river currents can be strong even during dry season.
Visit in the dry season from December to April when the water level is low, otherwise the trees will be underwater. If you are also visiting Sopheakmit Waterfall, you may want to time your visit for December to January so the water level is low enough to see the flooded forest, but there is enough water to provide a good sight at the falls – it can dry up a bit later in the season.
Koh Han is a CBET (Community Based Eco Tourism) site and also offers camping and other eco-friendly activities. You can stay on the islands in the river and kayak around the flooded forest.