The lake's catchment area is large, and includes the eastern slopes of the
Remutaka and
Tararua Ranges. The lake receives inflows from several rivers and was originally the main point of outflow for the
Ruamāhanga. Lake Wairarapa drained slowly into
Palliser Bay and
Cook Strait ten kilometres south near the settlement of
Lake Ferry, via what is now
Lake Ōnoke, but was prone to flooding. The Ruamāhanga River was diverted away from Lake Wairarapa in the 1960s to assist its flow, reducing the risk of flooding for farmers. The lake today therefore covers an area significantly smaller than its historical size of around . The area around the lake is low-lying and swampy. Agriculture and river development, flood control, and waterway diversions have resulted in the drainage of large areas of the surrounding wetlands, reducing their ecological values. Recent efforts have been made to preserve its important wetland features and restore wildlife habitat that has been degraded through human use. Lake Wairarapa is classed as supertrophic on the
trophic level index, meaning that it is polluted with a combination of nitrates which leach from intensified agriculture in the catchment area of the lake, and the outflows from sewage treatment plants for Masterton, Carterton, Greytown and Martinborough. ==History==