Why does Easter Island have few streams and little surface water?

Easter Island lacks significant surface water and permanent streams primarily because of its geological composition. The island is formed by volcanoes composed largely of tuff—a porous rock made of compacted volcanic fragments—and eroded lava fields.1

Because the soil and underlying rock are highly porous, rainwater does not collect in surface streams; instead, it drains rapidly into the ground. While there are crater lakes in the volcanoes Rano Kao, Rano Raraku, and Rano Aroi that collect rainfall, the only intermittent stream on the island flows down the slopes of Mount Terevaka before disappearing into the porous soil.1