Merimbula Lake is a moderate sized estuary with a waterway area of some 5.6 km2 and a catchment area of approximately 37.9 km2. The lake entrance is permanently open to the ocean through a narrow entrance channel, and the lake is largely divided into two sections split by the road bridge. Due to the extensive tidal flushing and relatively small catchment area, the water quality of the lake is typically dictated by tidal effects, with freshwater inflows only impacting water quality for short periods of time after heavy rainfall. Shallow areas of the lake support extensive seagrass meadows, with over 50% of the lake covered in seagrasses – the fourth largest beds on the NSW South Coast. The lake foreshores and tributary streams also support fringing mangrove, saltmarsh and freshwater wetland areas. The lake fauna is both diverse and abundant, and includes most popular estuarine recreational fish species. Merimbula Lake is also a regionally important tourist and oyster aquaculture centre. Recreational activities are centred around the lake entrance area, the town centre and sheltered estuary beaches.
Back Lake is situated to northeast of the Merimbula township. In contrast to Merimbula Lake, Back Lake has a relatively small surface area of only 0.4 km2 in proportion to its large catchment area of 31.4 km2. Technically the “lake” is actually classified as a semi-mature, saline coastal lagoon. Merimbula Creek is the primary drainage inflow into the lagoon. No National Park estate or State Forest is located in the catchment however, 80% of the catchment remains currently vegetated. Back Lake is classified as an Intermittently Closed and Open Lake or Lagoon (ICOLL), meaning that its entrance to the ocean is sometimes open to tidal exchange, but at other times is closed off by a sand barrier at the beach. The lake entrance is typically open to the ocean for only about 17% of the time, with the entrance actively managed by BVSC under its Entrance Management Policy to reduce the impacts of foreshore flooding when the entrance is closed.