Swan Coastal Plain · Perth, Western Australia
Why is there always water flowing into the ocean at Floreat Beach?
The drain at Floreat Beach runs year-round — even in summer, even in drought years. The answer connects you to a chain of lakes stretching across inner Perth, a groundwater system that took 12,000 years to form, and a century-old engineering decision that changed everything.
27 ML
Average daily drain flow
3,084
Hectares of urban catchment
1921
When the drain was built
01 · The Groundwater System
The lakes aren’t separate. They’re all the same water.
Beneath the Swan Coastal Plain lies a single unconfined aquifer — the Gnangara Mound — stretching over 2,200 km² from the Swan River north to Gingin Brook. Winter rain soaks straight through the sandy soil and slowly raises the water table. The lakes you see at the surface are simply places where the land dips below that table. They’re windows into the same body of groundwater.
This is why the system behaves so differently to other Australian cities. There’s almost no surface runoff in the natural landscape — rain goes down, not sideways. But once it’s in the aquifer, it moves slowly westward and south-westward toward the coast at roughly 50–150 metres per year, discharging into rivers, lakes, and ultimately the ocean.
“At low points in the landscape, the water table frequently intersects the land surface to form lakes and swamps.”
The groundwater mound beneath northern Perth forms a subtle dome. Water flows outward from its crest in all directions — east toward the Swan River, west toward the coast, north toward Gingin. The lakes along the western dune systems sit right in the path of that westward flow.
02 · The Lake Chain
Four lakes, one valley, one aquifer — and a lot of stormwater drains.
Running roughly north-west to south-east across inner Perth, Galup (Lake Monger), Jackadder Lake, Herdsman Lake, and Perry Lakes occupy a shallow inter-dunal valley in the Spearwood Dune system. Groundwater flows through all of them in a south-westerly direction. But since the 1920s, urban stormwater has dramatically changed their water budgets — bringing far more water than the natural system ever handled, loaded with nutrients from roads, gardens, and industry.
01
Galup
Lake Monger
The most upstream lake in the chain. Groundwater flows south-westerly through it from the Gnangara Mound. Twenty-three stormwater drains — mostly from the freeway corridor and surrounding residential areas — discharge directly into the lake, making it highly vulnerable to nutrient-enriched urban runoff. Street wash carrying fertilisers, vehicle oils, and leaf litter enters the lake every rain event. One controlled outlet: the Mounts Bay Drain to the Swan River.
23 stormwater drains
Algal bloom risk
02
Jackadder Lake
Joondoorup
A smaller wetland in the chain, sitting between Galup and Herdsman. Like its neighbours, it receives groundwater from the east and urban stormwater from its catchment. Often overlooked in the popular narrative but plays a role as a stepping stone for fauna moving through the corridor.
Stepping-stone wetland
03
Herdsman Lake
Boodjamooling
The largest lake in the chain and the most studied hydrologically. Receives groundwater seepage from the Gnangara Mound along its northern and north-eastern boundary — a year-round input regardless of rainfall. But urban stormwater has become the dominant inflow since the 1920s: a 3,084-hectare catchment covering Osborne Park’s industrial estates, commercial zones, and residential streets feeds multiple branch drains. Fertilisers, detergents, hydrocarbons, and heavy metals all arrive here via street runoff. No natural river outflow — the only exit is the Herdsman Main Drain, built in 1921, tunnelling to the ocean at Floreat Beach.
3,084 ha catchment
Year-round drain flow
04
Perry Lakes
Name not confirmed
East and West Lakes sit at the southern end of the chain, near the edge of Bold Park. Historically they connected to Camel Lake and the Alderbury Flats in high-water years. Now perched above the water table due to decades of groundwater decline, they depend on the Herdsman Main Drain diversion for replenishment. Street runoff from the Perry Lakes Estate, Underwood Avenue, and surrounding streets enters East Lake directly through an urban stormwater catchment.
Replenished since 2023
Phosphorus risk
03 · The Herdsman Main Drain
A 1921 engineering decision that still runs every day.
The Herdsman Main Drain was built to solve a problem that no longer exists. Post-WWI planners wanted to drain Herdsman Lake entirely, reclaim the land for soldier settlement market gardens. The scheme — a 3.2 km tunnel from the lake to an ocean outlet at Floreat Beach — cost the equivalent of $10.7 million today. It failed completely as an agricultural project. The land was unsuitable, blocks sold slowly, and flooding continued every winter.
But the drain was never removed. It remains, a century later, the sole outflow from a system now receiving far more water than it was ever designed to handle.
The drain flows year-round because it has to. It isn’t draining last week’s rain — it’s draining last year’s groundwater recharge, plus the daily runoff from 3,084 hectares of urban Perth.
Here’s the mechanism. The Gnangara Mound continuously seeps groundwater into Herdsman’s northern and north-eastern boundary — even in summer, even in drought years. This water was recharged months or years earlier, far to the north and east, and moves at 50–150 metres per year toward the coast. It has no seasonal off-switch.
On top of that, Herdsman receives urban stormwater from a 3,084-hectare catchment covering Osborne Park’s industrial estates, commercial strips, and kilometres of residential streets. Several major branch drains — Osborne Park, Balgay, Flynn Street, Herdsman Parade — channel road runoff, garden wash, and industrial site drainage directly into the lake. The organic material, fertilisers, hydrocarbons, and heavy metals from every road surface and garden in that catchment arrive here.
⚠ STREET RUNOFF: THE HIDDEN PROBLEM
In a normal city on clay soils, runoff stays above ground long enough for some pollutants to settle out. On the Swan Coastal Plain’s permeable sand, water that doesn’t reach a drain soaks straight down — carrying phosphorus, nitrogen, and contaminants with it into the aquifer, which then delivers them to the lakes. Both Galup and Herdsman show elevated nutrient loads from surrounding urban catchments. Herdsman sediments contain arsenic, lead, copper, and zinc at up to four times above government guidelines — accumulated from decades of urban drainage.
1912
Osborne Park swamps drained into Herdsman Lake to improve market garden conditions. First major hydraulic modification.
1921–1925
Main drainage works built; 3.2 km tunnel to Floreat Beach ocean outlet completed 1925, improved 1947–48.
1918–1925
Mean lake level 9.23 m AHD — more than 2 metres higher than it would be 60 years later.
1984–1994
Mean lake level drops to 6.72 m AHD. Over 2 m of water lost through drainage and groundwater decline.
2023–now
Town of Cambridge installs pump infrastructure to divert Herdsman Main Drain water to Perry Lakes instead of the ocean.
27 ML
per day — average drain flow
That’s 27 million litres flowing to the ocean every day on average. The lowest recorded flow was still 1.9 ML/day — year-round.
3,084
hectares of urban catchment
Herdsman’s catchment covers industrial Osborne Park, commercial strips, and dense residential suburbs — all draining to one lake.
3.2 km
length of the ocean tunnel
Built in 1921, the drain tunnel runs underground from Herdsman Lake to the beach outlet at City Beach / Floreat. It has never been removed.
23
stormwater drains into Galup
Lake Monger (Galup) receives stormwater from 23 separate drain inlets, largely from the Mitchell Freeway corridor and surrounding streets.
04 · Perry Lakes
12,000 years of geology.
60 years of decline.
One year to turn it around.
Perry Lakes formed in wind-scoured depressions at the end of the last ice age, about 12,000 years ago. The basins were originally up to 3 metres deeper than today’s land surface. When sea levels stabilised and the climate softened around 8,000 years ago, the rising water table filled them. At their historic peak, the lakes extended across the Alderbury Flats into Bold Park, and what are now McLean Park and Floreat Oval would have been wetlands too.
By the time Perry Lakes Stadium was built for the 1962 British Empire and Commonwealth Games, the lakes were still largely intact. Then the long decline began: groundwater extraction from the Gnangara Mound (for scheme water supply, parks irrigation, and residential garden bores), a drying climate, and pine plantation transpiration all drew down the water table. By the 1990s and 2000s, both East and West Lake were dry for much of summer.
The key concept is the perched lake. As the regional water table dropped below the lake floor, the lakes became disconnected from the aquifer. They could only be replenished by rainfall and surface runoff — a poor substitute for the slow, reliable groundwater supply they had always relied on. Meanwhile, the exposed lake beds baked in summer, releasing stored phosphorus and nitrogen back into the aquifer.
A study found nitrogen in groundwater was greatest downstream of Camel Lake, then West Lake, then East Lake — reflecting their degrees of drying and the nutrients leaching from desiccated lake beds.
Street runoff remains a real concern even now. Catchments draining into East Lake include Underwood Avenue, the Perry Lakes residential estate, Alderbury Street, Brookdale Street, and Grovedale Road — all carrying the standard cocktail of urban pollutants. West Lake has its own catchment, including stormwater from surrounding streets entering via the inlet channel. The reed beds planted in 2022 act as a biofilter — but they’re managing a chronic rather than a solved problem.
Over 700 water level readings have been collected for West Lake since 1962, and around 560 for East Lake. The long-term record shows a clear declining trend interrupted only by wet winters. Current target maintained by pumped replenishment.
Phosphorus from surrounding streets and gardens is the primary eutrophication trigger. Reed biofilters at inlets partially intercept this load — ongoing monitoring is essential.
05 · The Replenishment Project
Instead of sending 27 million litres a day to the ocean, we redirected some of it back to where it belongs.
The logic is elegantly simple. The Herdsman Main Drain has been carrying freshwater — both natural groundwater discharge and urban stormwater — to the ocean every single day since 1921. Perry Lakes, just downstream in the same hydrological system, were drying out for want of that same water. The Town of Cambridge’s $3 million replenishment project closes that loop.
01
Intercept the drain
A pump station on the Herdsman Main Drain captures water when drain flows are sufficient. A stilling well monitors flow before diversion.
02
Pump to stormwater
Pumped water enters the existing stormwater network at Ulster Road near McLean Oval, travelling through pipes to the lake inlet.
03
Filter through reeds
10,000+ Machaerina articulata (baumea) reeds planted at the West Lake inlet intercept suspended solids, nitrogen, and phosphorus before water enters open water.
04
Fill West Lake first
West Lake is the primary receiving body. As it rises, water flows through a balance pipe and underground to East Lake, which it has already reached.
05
Recharge the aquifer
Once lake beds are inundated, water also percolates down to recharge the local water table — the first step toward restoring the natural groundwater connection.
963
Intercept the drain
May 2023 to May 2024
2.6
ML / day average
Actual pump delivery rate
963
days pumping
94% operating time — stopped only for low drain flows and a Newman College power upgrade
1992
levels restored
Lake levels returned to early 1990s — within one year of operation
The average Herdsman Main Drain flow of 27 ML/day dwarfs the 2.6 ML/day being diverted to Perry Lakes. More than 90% of the drain’s flow still reaches the ocean. There is substantial room to expand replenishment — though water quality at the intake must be carefully managed, and the reed filtration system needs to keep pace.
06 · Ecology & Threats
Returning water is the first step. Keeping it healthy is the harder part.
Perry Lakes now support 83–100+ bird species, oblong turtles, quenda, microbats, banjo frogs, and 127 plant species. The return of open water has been rapid and dramatic — but the ecosystem is in a fragile recovery phase, with genuine threats that need active management.
Oblong turtle
Southwest snake-necked turtle — a species under increasing pressure as wetlands dry. Perry Lakes is now a key refuge. Citizen science turtle sightings help track recovery.
Quenda
Southern brown bandicoot, present at the reserve. Corridor links to Bold Park are a management priority — these animals need connected habitat to maintain viable populations.
Microbats
5–7 species confirmed, including the western false pipistrelle. Open water supports insect prey populations essential for bat foraging.
100+ bird species
More species recorded in summer than winter. The lakes function as a drought refuge when other wetlands across the plain dry out — a critical role that will only grow.
127 plant species
Including paperbarks and native reeds. 4,900 new seedlings planted in 2024. Revegetation creates the shading and structural complexity that suppresses algae and supports fauna.
Western banjo frog
The ‘Pobblebonk’ — water-level management directly affects frog recruitment. Getting the hydroperiod right is a key management variable.
⚠ Active threats to monitor
Eutrophication
Phosphorus in both lakes exceeds trigger levels more than 80% of the time. Incoming drain water, street runoff, and sediment release from exposed lake beds all contribute. Cyanobacteria blooms are an increasing risk — already documented at Galup and Jualbup nearby.
Invasive fish
Gambusia (mosquitofish) confirmed. Carp possible. Carp are particularly damaging because they stir up sediments and mobilise phosphorus back into the water column, potentially triggering the algal blooms the management is trying to prevent.
Polyphagous shot hole borer
An invasive beetle establishing in Perth’s urban trees. Reproductive hosts near the reserve include Moreton Bay Fig and London Plane trees. Active surveillance needed — infested trees may need removal.
Street runoff (ongoing)
Even with reed biofilters, the urban catchments draining to both East and West Lake continue delivering nutrients. Gully trap maintenance, street sweeping, and reducing fertiliser use in adjacent gardens all help reduce the load.
Ready to get involved?
Perry Lakes is a working, recovering ecosystem — and it needs people paying attention. Here’s how you can be part of it.
Membership
Become a member
From $30/yr. Almost all of FoPL’s operating budget comes from membership fees and donations.
Join now →
Hands on
Volunteer with us
Join the weed warriors, planting days and working bees that keep the reserve recovering.
See what’s on →
Science
Citizen Science
Help monitor water quality, levels, birds and biosecurity threats. No expertise required.
Get started →
Community-led. Science-informed. Caring for Perry Lakes.
© 2026 Friends of Perry Lakes Inc.