Manly Lagoon

Our water quality monitoring program has shown Manly Lagoon to have poor water quality. Find out more about the estuary and its unique features.

Manly Lagoon is situated in the Sydney Metropolitan region of New South Wales. It is classed as a creek and has an intermittently closed entrance that meets the sea at the north end of the popular Manly–Queenscliff Beach.

Manly Dam, which was constructed in 1892, regulates the major creek that feeds into Manly Lagoon.

Estuary health and features

Water quality

As part of our water quality monitoring program we assess the water quality and ecosystem health of an estuary using a range of relevant indicators. We have supported Northern Beaches Council develop and carry out a monitoring program to assess the water quality of estuaries in their local government area. Sampling is carried out fortnightly between October and April each year. Three sites are sampled in Manly Lagoon.

The monitoring program has been ongoing since 2012. The most recent sampling was completed over the 2021–22 summer.

Manly Lagoon water quality report card for algae and water clarity showing colour-coded ratings (red, orange, yellow, light green and dark green, which represent very poor, poor, fair, good and excellent, respectively). Algae is rated 'very poor' and water clarity is rated 'fair' giving an overall rating of 'poor' or 'D'.

This report card represents 2 water quality indicators that we routinely measure: the amount of algae present and water clarity. Low levels of these 2 indicators equate with good water quality.

The report card shows the condition of the estuary was poor with:

  • algae abundance graded very poor (E)
  • water clarity graded fair (C)
  • overall estuary health graded poor (D).

The past results of the monitoring program, including detailed water quality report cards, are available on the Northern Beaches Council website.

Find out more about our estuary report cards and what each grade means. Read our sampling, data analysis and reporting protocols and find out how we calculate these grades.

We have monitored water quality in Manly Lagoon since 2007. This table shows the water quality grades for this estuary over that time.

Manly Lagoon historic water quality grades from 2007-08 for algae and water clarity. Colour-coded ratings (red, orange, yellow, light green and dark green represent very poor (E), poor (D), fair (C), good (B) and excellent (A), respectively).

Grades for algae, water clarity and overall are represented as:

  • A – excellent
  • B – good
  • C – fair
  • D – poor
  • E – very poor.

Physical characteristics

Estuary type: Creek

Entrance
location 		Latitude (ºS) –33.79
Longitude (ºE) 151.29
Catchment area (km2) 17.2
Estuary area (km2) 0.1
Estuary volume (ML) 35.6
Average depth (m) 0.4
Notes: km2 = square kilometres; m = metres; ML = megalitres.

Water depth and survey data

Bathymetric and coastal topography data for this estuary are available in our data portal.

Land use

The catchment of Manly Lagoon is highly disturbed. About one quarter of the land within the catchment remains forested, the remainder is used for urban development, including two golf courses.

National and marine parks

  • Part of Garigal National Park falls within this catchment.
  • This estuary does not flow into a marine park.

Community involvement

  • The Manly Environment Centre, run by Northern Beaches Council, educates the community and provides opportunities for locals to get involved in coastal environmental management.
  • Northern Beaches Council also facilitate a Bushcare Program for local residents to get involved in conservation and restoration of the natural environment around Manly Lagoon estuary.
Manly Lagoon

Local government management

Local councils manage estuaries within their area unless the estuary is attached to a marine park. Northern Beaches Council manage this estuary.