Narrabeen Lagoon is situated in the Sydney Metropolitan region of New South Wales. It is classed as a lake that has an intermittently closed entrance.
Northern Beaches Council open the entrance artificially when the water level reaches a specific trigger level, when flooding threatens surrounding infrastructure.
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 Narrabeen Lagoon.
The monitoring program has been ongoing since 2012. The most recent sampling was completed over the 2021–22 summer.
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.
Algae
Water clarity
Overall grade
The report card shows the condition of the estuary was good with:
- algae abundance graded good (B)
- water clarity graded excellent (A)
- overall estuary health graded good (B).
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 Narrabeen Lagoon since 2010. This table shows the water quality grades for this estuary over that time.
| Year | Algae | Water clarity | Overall grade |
|---|---|---|---|
| 2010–11 | B | C | B |
| 2013–14 | B | B | B |
| 2014–15 | D | C | C |
| 2015–16 | B | B | B |
| 2016–17 | B | E | C |
| 2017–18 | C | C | C |
| 2018–19 | B | C | B |
| 2019–20 | C | B | B |
| 2020–21 | A | B | B |
As part of our Beachwatch program we assess swimming suitability at 2 swimming sites in Narrabeen Lagoon using microbial indicators. We collect samples at each site weekly between October and April, and monthly from May to September, to calculate the suitability grade. Sampling is ongoing. Each grade reflects the most recent 100 water quality results to April 2022. Find out more about how we monitor beach water quality.
| Local government area | Swim site name | Grade |
|---|---|---|
| Northern Beaches | Narrabeen Lagoon (Birdwood Park) | Poor |
| Northern Beaches | Bilarong Reserve | Poor |
Estuarine, lake and lagoon water can sometimes be unsuitable for swimming, especially after rainfall, when stormwater and wastewater can overflow into swimming areas. Learn more about what the grades mean and how we grade swimming sites on the Beachwatch How we report webpage.
Physical characteristics
| Estuary type | Lake |
|---|---|
| Latitude (ºS) | –33.7 |
| Longitude (ºE) | 151.31 |
| Catchment area (km2) | 52.4 |
| Estuary area (km2) | 2.3 |
| Estuary volume (ML) | 5251.6 |
| Average depth (m) | 2.3 |
Tidal exchange volume
Tidal exchange volume or tidal prism data is available for this estuary. This tidal prism was measured in 1980.
| Tide state | Flow (ML) | Local tidal range (m) | Sydney Harbour tidal range (m) |
|---|---|---|---|
| Ebb flow | 280 | 0.14 | 1.08 |
| Flood flow | 500 | 0.19 | 1.22 |
Notes: km2 = square kilometres; m = metres; m3 = cubic metres; ML = megalitres.
Water depth and survey data
Bathymetric and coastal topography data for this estuary are available in our data portal.
Land use
Narrabeen Lagoon drains a mostly forested catchment, part of which is protected in a conservation area. Urban development is the other main land use within this catchment and occupies about one-quarter of the total catchment area.
National and marine parks
- A large section of Garigal National Park is located within this catchment.
- This estuary does not flow into a marine park.
Community involvement
- The Coastal Environment Centre in Narrabeen, 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 Narrabeen Lagoon estuary.
Aerial view of Narrabeen Lagoon estuary.
Local government management
Local councils manage estuaries within their area unless the estuary is attached to a marine park. Northern Beaches Council manages this estuary.
Threatened species
This estuary provides important habitat for many threatened species such as the red-crowned toadlet and spotted-tail quoll.
Read more about the biodiversity in our estuaries.