Here is the full list of all 14 The Treehouse Man achievements.
Defeat the corrupter of the cosmic river
Complete a green river without taking damage
Unlock all companions
Find all messages in a bottle
Defeat the menace of the fertile river
Defeat the arsonist of the barren river
Find all health upgrades
Unlock all weapons
Defeat the unborn raptor
Defeat the arachnid manipulator
Get all achievements
Complete a blue river without taking damage
Complete a red river without taking damage
Complete a moonlit river without taking damage