The lake is situated in an over-deepened valley formed by a glacier[1]: 16  over 20,000 years ago in the Pleistocene era.

[3] Early colonial explorers found the shores of the lake covered in mānuka (or kānuka), kōwhai, cabbage trees, flax and general swamp plants.

[1]: 32  Changing water levels caused by the operation of the Coleridge Power Station killed most of the kānuka in 1914 and rata in 1923.

[1]: 143  In modern times a mixture of native and introduced plants surround the lake, including matagouri, broom, gorse, briar, coprosma and biddi-biddi.

[1]: 62 The lake was named by the chief surveyor of the Canterbury Association, Joseph Thomas, on a sketch map prepared in early 1849.