Isla Sorna has multiple caves on its windward side, two of by which entrance to the jungle river is possible. These caves naturally formed from the erosion of the side cliffs of the island, the water eating away at the soft volcanic rock to cut caves into the cliff. After Lewis Dodgeson pushed Sarah Harding off of the boat, she nearly drowned but managed to swim to one of the caves while still weak from the current. As Ian Malcolm‘s group successfully escaped the island, they exited via boat through one of the two entrance caves which contained a waterfall.
The Lost World by Michael Crichton page 197-198,206-208, 413(First Edition, Thirty Fourth printing)