The Upper Mangde Chhu is a wonderful Class IV run near Trongza. The road to put-in is way above the river making this a wilderness run with many good rapids.
This river is generally pool drop with a few rapids that deserve scouting since wood is common. The three big rapids are named after the three poisons: Ignorance, Attachment, and Aversion.
Mile 0: Like many runs in Bhutan there is a very distinct trail to the normal put-in. The trail starts on a switchback on the road near a few farms.
Mile 0.1: Ignorance (IV) is a long, fun affair that can be scouted from put-in.
Mile 0.2: Attachment (IV) The next rapid should be scouted from the left bank since wood is common here.
Downstream is the largest rapid, Aversion (IV+), that should be scouted on the right. This one would be tough to portage.
Mile 3: Take-out where a small creek enters on the right. You should scout the take-out on your way to put-in since there is a very large rapid just downstream. The next few miles of river are Class V and require an 800+ foot hike up to Trongza Dzong.