Development of the middle creek delta plain, Narrabeen Lagoon, NSW

Abstract

Middle Creek drains 26 per cent of the total Narrabeen Lagoon catchment and has formed a 1.6 km‐long delta plain where the channel debouches into the coastal lake. The upper delta plain is characterised solely by fluvial landforms because reworking by lateral migration, cutoffs, channel widening and floodplain stripping has totally eroded the previously existing delta landforms. A combination of lateral and vertical accretion has formed a floodplain characterised by point bars, cutoffs, levees, backswamps and floodplain ridges. The lower delta plain consists of landward retreating silt jetties, an enlarging straight channel, and a subaqueous delta comprising an arcuate river‐mouth bar, delta front and pro‐delta. Extensive landfill was used to stabilise the silt jetties and prevent a channel avulsion through the southern silt jetty during the 1960s. The whole delta plain has formed over the last 6000–7000 years, since the stabilisation of sea level following the postglacial marine transgression, by the progradation of channel, floodplain and delta sediments over estuarine muds and transgressive sands in a narrow bedrock valley.

Key Words:

• Fluvial delta
• river‐mouth bar
• silt jetty
• natural levee
• floodplain
• avulsion