Manly Lagoon is a highly modified shallow coastal lagoon, located at the Northern end of Manly Beach.
It forms the boundary between Manly and Warringah Councils. It is a natural feature of the Manly and Warringah local district, and receives water from three creeks draining a large urban area.
Estuaries, such as Manly Lagoon, are places where mixing of both fresh and salt water occurs, creating a range of habitat conditions. They are an important breeding grounds and habitats for a wide range of fish species, as well as seagrasses, birds, and other animals. The catchment also serves as a habitat for threatened species and endangered ecological communities.
Rainfall in the Lagoon’s surrounding urban areas provides freshwater to its three creeks Brookvale Creek, Manly Creek, and Burnt Bridge Creek, which subsequently flow into Manly Lagoon. Burnt Bridge Creek is Manly’s only creek, whilst the other creeks are located in the Warringah Local Government Area. This upstream area can be considered a catchment, and the boundary can be defined by the highest ridge or hill lines, marked on the map below in red. This surrounding catchment area is approximately 18 square kilometres, and is very large compared to the small size of the actual lagoon, which has a water body area of 0.1 km2. This means the small lagoon receives a large amount of runoff from the surrounding area.
Natural Sandy Entrance
The natural condition of Manly Lagoon is for wave energy to establish and maintain a small berm of marine sand across the lagoon entrance. This is due to high ocean wave energy at Manly Beach (and most NSW beaches), compared to the small out flowing creek energy from the land (which would be able to maintain an open entrance). Manly Lagoon is classified as an Intermittently Closed or Open Lake or Lagoon – or “ICOLL”. Scouring open of the marine sand berm at the entrance, or ‘break out’, naturally occurs only in response to heavy rainfall events in the catchment, leading to large volumes of water flowing off the land and towards the lagoon entrance.
Manly Lagoon Catchment – approximately 18 square kilometres