Manly Lagoon is situated in the Sydney Metropolitan region of New South Wales. It is classed as a creek with an entrance that is intermittently open and closed to the ocean
Manly Dam, which was constructed in 1892, regulates the major creek that feeds into Manly Lagoon.
Water quality report card
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 2007. The most recent sampling was completed over the 2023–24 summer.
This report card represents 2 water quality indicators that we routinely measure: the amount of algae present and water clarity. The numerical scores for these 2 indicators are averaged to give the overall grade.
Algae
Water clarity
Overall grade
The report card shows the condition of the estuary was poor with:
- algae abundance graded very poor (E)
- water clarity graded good (B)
- overall estuary health graded poor (D).
Grades for algae, water clarity and overall are represented as:
- A – excellent
- B – good
- C – fair
- D – poor
- E – very poor.
Go to estuary report cards to find out what each grade means, read our sampling, data analysis and reporting protocols, and find out how we calculate these grades.
The past results of the monitoring program, including detailed water quality report cards, are available on the Northern Beaches Council website.
We have monitored water quality in Manly Lagoon since 2007. This table shows the water quality grades for this estuary over the past 7 years.
| Year | Algae | Water clarity | Overall grade |
|---|---|---|---|
| 2023–24 | F | B | D |
| 2022–23 | D | B | C |
| 2021–22 | E | C | D |
| 2020–21 | C | C | C |
| 2019–20 | D | C | D |
| 2018–19 | E | B | C |
| 2017–18 | D | B | C |
Only the most recent 7 sampling seasons are shown here, previous water quality data is available here.
Physical characteristics
| Estuary type | Creek |
|---|---|
| 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 2 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 facilitates a Bushcare Program for local residents to get involved in conservation and restoration of the natural environment around Manly Lagoon estuary.
Aerial view of Manly Lagoon estuary
Local government management
Local councils manage estuaries within their area. Where the estuary is attached to a marine park, marine park management teams are responsible for ensuring compliance with park zoning. Northern Beaches Council manages this estuary.
Threatened species
Estuaries provide important protected environments where native plants like seagrass and mangroves grow and create habitat for fish and other animals.
Find out more about the biodiversity in our estuaries.