The lakes normally are dry, but fill during periods of heavy rainfall. Some of the small lakes form behind the coastal sand dunes. There are no perennial streams. Vegetation over most of the island consists of Kiawe and lowland shrubs. A small estuary exists at the mouth of the intermittent Keanauhi Stream on the west shoreline of the island.
Sandy beaches are present along much of the northwest coast.
Reefs are found mainly along the western coastline. Two small uninhabited islands lie off Niihau. Lehua is located 1 km north of Niihau. Kaula is located 34 km to the south-west. This island is not in the wave shadow of any other large island and thus is exposed to full impact of large ocean swell from all directions.