Situated within the De Mond State Forest in the Western Cape, the estuary of the Heuningnes River extends approximately 12 km across the flat coastal plain of the Zoetendals Vallei farm area before breaking out to sea through a double dune ridge.

The wetland was designated a Ramsar site primarily for its importance as one of the few confirmed South African breeding sites of the Damara tern Sterna balaenarum. This species is endemic to southern Africa, is listed as rare and vulnerable and is possibly the rarest resident sea bird in South Africa. It is estimated that about 15% of the national population of this species is found in the De Mond area. The dune areas and pebble slacks of the beaches around the estuary are important for nesting sites while the estuary is a vital foraging ground for the birds and is used as a meeting ground during pair formation.

Other breeding birds include African black oystercatcher Haematopus moquini, blue crane Anthropoides paradisea, Kittlitz's plover Charadrius pecuarius and Egyptian goose Plectropterus gambensis. As the most southerly estuary in Africa, the site is scientifically important for species distribution extremities, including the southernmost records of tropical species like the ginger prawn Penaeus japonicus, giant mud crab Scylla serrata and a gastropod Nerita albicilla. The seahorse Hippocampus sp. also occurs here.