Photo of a waterfall in green landscape
Kauai lush vegetation and ridges

If Kaua'i mountains are so green it's because it rains a lot. Mont Waialeale, Kauai's highest summit is 5148 m high and is considered the rainiest spot on earth.
It rains approximatively 350 days a year and during the last 32 years, an average of 460 inches per year of rain has been measured.
Therefore, no surprise to see so many waterfalls.