Here is the full list of all 13 Riverhill Trials achievements.
Land in Riverhill.
Avoid falling in the water.
Explore the cave.
Activate the platforms that lead to the hill.
Enter the city.
Explore the forest.
Find the secret in the cave.
Explore the entire map.
Say goodbye to Riverhill.
Collect 5 UFO's.
Collect 15 UFO's.
Collect 20 UFO's.
Collect 30 UFO's.