A NSW Government website

Latest science

Minimising algal blooms in the Barwon-Darling (Barka) River

Our project investigated how much water is needed to flow through the Barwon-Darling River (Barka) pools to keep the river healthy. When sufficient water flows through the pools, thermal stratification is less likely to occur which reduces the risk of toxic algae blooms and possible fish deaths.

Bourke weir.

Minimising algal blooms in the Barwon-Darling (Barka) River

Severe and extensive cyanobacterial blooms are a common occurrence in the Barwon-Darling River. Low flows and persistent thermal stratification are key drivers of bloom events. They may also lead to fish kills when destratification creates anoxic conditions (water depleted of oxygen) throughout the water column.

Thermal stratification is the development of temperature layers in weirs, reservoirs and lakes, which often occurs during summer. The surface layers are warmer than natural river temperatures and bottom waters can be up to 15°C cooler than natural river temperatures (Figure 1).

Figure 1. Thermal stratification in a weir pool. Modified from Chaaya and Miller 2022.

We measured pool depth, volume, flow velocity and temperature profiles at 11 sites on the Barwon-Darling River to better understand the flow velocities and volumes needed to break down persistent thermal stratification in weir pools and natural pools.

Overall, we found that a flow velocity above 0.05 m/s may be sufficient to prevent the formation of persistent thermal stratification.

In the majority of the sites sampled, the discharge needed to prevent stratification is higher than the current flow thresholds for access to water for A Class licence holders (Table 1). This indicates that water use in these areas may increase the likelihood of persistent thermal stratification occurring, particularly in the warmer months. The exceptions were Walgett, Louth and Wilcannia Management Zones, where the discharge estimated to be required to minimise stratification should be protected when A-Class licence holders are permitted to extract water.

An A-Class licence entitles the holder to access a specified volume of river water over a specified time (e.g. 24-hr) and is based on the total volume of water flowing in the river. The river flow trigger for a licence will vary depending on where in the river the licence is held.

When looking at Table 1 it is evident that the estimated flows (discharge) required to prevent stratification may not always be achievable from one weir to another. This is the case for the larger weirs (e.g. Bourke) that are generally deeper, longer and wider and thus need greater velocities of water flow to breakdown stratification. Whilst smaller weirs (e.g. Louth) can receive lower flow velocities to disrupt the occurrence of potential stratification.

This information is an early outcome of a collaborative research project with the University of Technology Sydney. The research will continue to provide data under a range of flow conditions. This will enable further monitoring of the Barwon-Darling River and, in the future, will enable improvement of flow management to reduce algal blooms and the risk of fish kill events. Further work will refine stratification flow thresholds at existing and additional study pools. In particular, it will focus on discharge rates that minimise the development of thermal stratification during the hotter months from October to April when stratification is likely to be more prevalent.

Table 1. Suggested discharge targets to prevent persistent thermal stratification and current A Class cease-to-pump flows.
Site with river gauge number in bracketsEstimated discharge (ML/d) to minimise stratification (0.05m/s)Unregulated river A Class access licences (ML/d)*
Collarenebri {422003}643 - 679>317
Walgett {422001}307 - 341>600
Brewarrina {422002}798 - 906>550
Bourke {425003}887 - 945>605
Louth {425004}160>555
Tilpa {425900}630 - 748>505
Wilcannia {425008}432 - 485>455

*Data from the Water Sharing Plan for the Barwon-Darling Unregulated River Water Source 2012. Accessed 22 March 2022.

Research paper

Learn more about minimising persistent thermal stratification and algal blooms.

Download the paper (.PDF 3020KB)

F C Chaaya and B M Miller (2022) “A review of artificial destratification techniques for cold water pollution mitigation”, WRL TR 2021/17, February 2022, UNSW Water Research Laboratory.