Lake Elterwater is a shallow, freshwater lake in southern Marlborough about 4km north of Ward, approximately two kilometres long and 300m wide. Lake Elterwater drains a catchment of approximately 1,600 hectares. Usually the lake has no outlet, however under high rainfall conditions, it drains into the Flaxbourne River. The lake is named after a small lake, Lake Elterwater in Cumbria in the Lakes District of England.
The lake has had the designation of a wildlife refuge since 1956. The lake provides an important habitat for a number of waterfowl. Australasian coots, pukeko, mallard and paradise ducks are among the over thirty species of birds that have been observed on and arond the lake. Shortfinned eels, bullies, and freshwater mussels may be found in the lake.
Crack willow had formed an extensive part of the vegetation around the lake for many years, however successful poisoning operations have eliminated many of the willows, providing an opportunity for native vegetation to reestablish.
In dry summers, the lake bed may dry up completely; in 2017 a game of cricket was held on the dry lake bed, and the lake bed has also been used to grow fodder crops. Some years ago, light aircraft landed on the dry lakebed.
A parking area and toilet are provided at the northern end of the lake off State Highway One. Due to the extensive bird life, the lake is a popular birding location.