Solid rock! Hard and inhospitable! One would think so - about islands formed entirely of lava, welling up and flowing in layers to the ocean, across the ages. But, surprise:
There's lots of water, spurting from high cliffs and flowing from hidden springs - some along the shore and beneath the ocean's surface. In old Hawai'i, those who needed water could swim out to feel a cold fresh current and uncork an empty gourd to carry fresh water back to shore. It's like that in spots like Kahalu'u (means "Place for diving) and Punalu'u (means "Spring for diving") on the Big Island. Where did that water come from and how did it flow hidden beneath the lava?
The trade winds bring the moisture. They have the wide Pacific to draw it from. And when they reach Hawai'i mountains and rise to pass over them, the winds cool; the moisture condenses, and it rains abundantly. No, not along the coast, but in the higher elevations - where the tourists are fewer. So, on the side facing the trade winds, there's lots of water. On the downwind side, the reverse happens - and the landscape can be desertlike. Remarkable that it should be so on an island like Kaua'i which is only about 35 miles in diameter. But it happens especially on the Kaua'i volcano, Wai'ale'ale, which gets 40 ft. of rain annually.. That time worn volcano is only 5000 feet above the ocean - which is where there's still lots of moist air. (Haleakala, and Moana Kea are 10,000 ft or more and lie above the moist zone. They are desertlike).
Now the water is on the ground. So why doesn't it flow on the surface, like the rivers in the continental US? Well, the lava is porous and contains hollow lava tubes, like the Thurston Lava Tube in Volcanoes National Park. Consider how a lava tube forms:
Molten lava, flowing in a river down the mountainside, loses heat to the air above. Then, it skins over and the insulated hot lava below continues to flow. If lava flow is interrupted at its source, the tube drains - leaving a cave which may run for miles. Besides that: some lava (a'a) is clinker-like, allowing water to percolate through it.
I'll try to find and post some pictures of waterfalls and lava tubes later. But not all the photos are accepted by the RCI website, for reasons I don't understand yet.