Lake Waiau is a high-elevation lake located at above sea level on Mauna Kea, on the island of Hawaiʻi. It is arguably one of the highest lakes in the United States, after the evaporation of Ka Wai o Pele in 2018 the only lake on Hawaiʻi Island, and one of very few lakes at all in the state of Hawaiʻi. It is relatively small, only about 100 m across, and varies in size as the water level rises and falls. At high water levels a small outlet stream appears at the northwest end, but it is absorbed into the ground after a short distance. The name means swirling water in Hawaiian, though it is usually rather placid. It usually freezes in winter, but aquatic insects such as midges and beetles can be found breeding in the water.