One arm of the Wainamu sand dune system advances into the Waiiti tributary of Wainamu Valley.
During the colder climate of the Last Ice Age sea level was up to 130 m lower than today. At these times the streams and rivers eroded their valley floors down below present sea level as they flowed out and across the exposed coastal plains. Between 18,000 and 7500 years ago sea level rose rapidly and the sea advanced across these coastal plains and invaded the lower parts of the stream and river valleys creating bays. By 7500 years ago the Te Henga/Bethells area was a deeply embayed coast with the sea extending right up the Waitakere River and its tributaries the Wainamu and Mokoroa Streams. Quite quickly, however, excess sand in the coastal sand system built sand spits across the mouth of these newly flooded valleys near the present location of Bethells Beach. This created tidal lagoons and estuaries in behind, extending up under what are now Bethells Swamp and Lake Wainamu. Over the last 5000-6000 years more sand has been thrown up on the beach and has blown inland forming sand dunes that have advanced into the mouth of the Wainamu Valley and built up the bed of the outlet stream with sand so that the dammed valley has become a deep freshwater lake.
At the same time the Bethells Swamp estuary filled up with sand carried in by incoming tides and it transitioned from an estuary to a freshwater swamp now choked with swamp vegetation. Alluvial clay, sand and gravel transported down the Waitakere River into the upper parts of the swamp have built alluvial flats that will eventually advance and engulf the whole swamp. This is the natural cycle from bay through estuary to lake through swamp to alluvial plain.
Today the Wainamu Valley is still in the freshwater lake stage, dammed by the mobile sand dunes. The stream that flows in at its head is small and has not brought down the same amount of alluvial sediment as the Waitakere River has into the Bethells Swamp.
About 50 years ago, dune vegetation was planted in the active sand dunes behind Bethells Beach and a small subdivision created between there and the Wainamu dunes. This cut off the supply of sand to the Wainamu dunes and so over time these active high dunes are slowly being lowered and will eventually become fixed dune remnants blocking the lake. Much of the active sand on the dunes today is blown over the edge into the outlet stream which returns the sand downstream to the beach and out of the otherwise closed system.