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Merrimu Reservoir

Current BGA Bloom

A Blue Green Algae (BGA) bloom warning is current for Melton Reservoir

Warning signs advising people not to swim, fish or come in contact with the water have been erected at the reservoir, and boating is also not recommended.

Blue Green Algae can be dangerous to humans and animals. It can cause skin rashes or itchiness; sore eyes, ears and nose; or if swallowed, gastroenteritis, nausea or vomiting. People who come into direct contact with contaminated water should wash immediately in fresh water.

If you are experiencing a health issue you think is related to contact with water contaminated by BGA, please seek medical advice promptly. SRW will continue to monitor the situation and will provide updates accordingly.



The reservoir was constructed in three stages from 1969 to 1986 and has a capacity of about 32,000 ML.

The reservoir has a small natural catchment area, so to supplement its storage ability, two small diversion weirs and two tunnels from neighbouring catchments were built. These works and the subsequent raising of the reservoir embankment form the three main stages of the Merrimu Reservoir construction.


The water in the reservoir is primarily used to supply drinking water to Melton and Bacchus Marsh with the remaining used for irrigation in the Werribee Irrigation District.

Merrimu Reservoir provides better management and regulation of water flows in the Werribee Basin, consequently increasing the security of supply to Werribee and Bacchus Marsh irrigation districts, urban water and industrial supplies in the Melton and Bacchus Marsh areas.

Capacity 32,516 ML
Dead storage 335 ML
Surface area 420 ha
Full supply level 174.1 m AHD
Catchment area 85 km22
Maximum height 45.3 m
Length 400 m
Crest length 30 m
Discharge capacity 26,000 ML per day

Merrimu Reservoir is located on Pyrites Creek, a tributary of the Werribee River, 10kms north-east of Bacchus Marsh and approximately 50kms from Melbourne – VicRoads Reference Map 328 J6.

Please check the water levels before planning a day out at the reservoir!

Merrimu Reservoir visitors guide

Southern Rural Water (SRW) needs to manage our dams and catchments carefully to maintain a high quality supply of water. We encourage all visitors to work with us to minimise any impact to Merrimu Reservoir and its surrounds, and to ensure everyone has a safe and enjoyable time here.

In the park and picnic areas:

use the park and picnic areas only between 8.30am to 4.30 pm
consider others and keep noise to a minimum
keep the picnic areas and BBQs clean
take your rubbish home
use the toilet facilities provided
keep within the speed limits
obey all signs and notices
keep dogs on leads
protect all native animals and plants
comply with the directions of SRW officers & security personnel

use the park or picnic areas outside of opening hours
drive or ride motorbikes around the reservoir or in fenced areas
enter prohibited areas
vandalise the facilities
ride or walk horses anywhere on site
camp overnight on site
undertake any dangerous activities including
abseiling, paraflying, shooting and archery.

On the water:

Merrimu Reservoir is a closed catchment and reservoir. All water activities are prohibited.

Fires prohibited As part of its commitment to adopting the best enviromental practices, SRW prohibits fires at all of its recreational areas AT ALL TIMES

More information
For more information contact us on 1300 139 510 or visit

Download Factsheet

Bulk entitlements

A Bulk Entitlement Order is a set of operating rules for a reservoir.

At Southern Rural Water (SRW) we deal with the following Bulk Entitlement Orders:

  • Latrobe
  • Maribyrnong
  • Yallourn Energy
  • Werribee
  • Thomson/Macalister

Because more than one organisation or group can have shares (or entitlement) in the water being held in a reservoir, each shareholder must follow rules about:

  • the volumes that can be taken from the reservoir, system or waterway
  • costs of managing the system, and how they are shared

At a number of reservoirs, SRW acts as the storage manager on behalf of all shareholders.
As a storage manager, we: 

  • allocate water (both increases and decreases) according to the inflow and capacity shares held by shareholders
  • ensure that we meet the rules for passing flows downstream for environmental purposes
  • release water to meet a shareholder’s request

Passing flows are a vital part of our Bulk Entitlement Orders and are met before allocation requests are supplied.

Environmental entitlements

Environmental Entitlement Orders spell out a whole pattern of environmental flows that are required downstream of a reservoir – from how long they last, to how frequently they flow and how large they are.

Environmental flows are designed to mimic the natural conditions of rivers. It is not just about the amount of water but also the timing and quality. In each order, environmental flow requirements are low during the summer and autumn, and are much higher during winter and spring.

Environmental flows are vital to supporting the river’s ecological processes. High flows provide triggers for fish breeding and supply water for fish passage, so that they are able to move up and down rivers to appropriate habitat. They also keep estuaries open and provide recreational opportunities. Low flows in summer maintain fish refuges and connect habitats. Spring floods regenerate wetlands and floodplains and replenish the river channel.


To make sure that we comply with the rules about passing environmental flows, we:

  • perform random checks at points along rivers, to ensure that customers are taking only the amount of waterthey have ordered
  • maintain a number of gauging stations with alarms that alert us of unusual changes in water levels.

More information
For more information contact us on 1300 139 510 or visit

Download Factsheet

SRW's role during floods at Rosslynne Reservoir

Rosslynne Reservoir is not designed to manage floods
Southern Rural Water helps to provide river flow information during floods
Please keep clear of dams and spillways during floods

Rosslynne Reservoir capacity is 25,400 megalitres (ML) and is managed by Southern Rural Water.

This reservoir was not designed to manage floods. It simply stores water for various uses including water supplies for the townships of Sunbury and Gisborne and irrigation water for river diverters along the Maribyrnong River.

Operating Rosslynne Reservoir during floods

Rosslynne Reservoir has a fixed crest spillway, so that once it is full any excess water coming into the reservoir will flow over the spillway into Jackson Creek. Therefore Southern Rural Water does not have the ability to deliberately surcharge (over fill) the reservoir.

This reservoir also does not have the ability to pre-release enough water to have any effect on a possible flood, but it can help reduce flood peaks by temporarily storing water if room is available, however, once the lake is full, it has little impact on flows.

When we know high rainfall is possible, our staff work closely with other agencies such as the SES, local councils and the Bureau of Meteorology to provide information on the flows passing through our structures to assist them in their operations.

Southern Rural Water provides daily updates on its website ( about the water levels in local storages and also has a web page which has links to Bureau rainfall forecasts, flood warnings, river levels and emergency assistance provided by the SES.

We strongly recommend that if you are in a flood prone area that you and your family familiarise yourself with the SES’s "Flood Safe information brochure" and take all necessary precautions.

More information
Contact us on 1300 139 510 for more information or to make an appointment to chat to one of our assessment staff.

More information can also be found at

Download Factsheet

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