Current understanding of blackwater events relating to the Edward-Wakool River System

"June 2009".

Project Number: Understanding blackwater events and managed flows in the Wakool River system: a scoping study - M/BUS/319.

MDFRC item.

1 of 2 reports associated with project see (Understanding the relationship between low dissolved oxygen blackwater events and managed flows in the Edward-Wakool River system).

The Edward-Wakool River System is a complex network of inter-connecting rivers, creeks, flood runners and artificial channels located north of the Murray River between Tocumwal and downstream of Swan Hill. The flow in this system is highly regulated by a series of control structures located throughout the system. Flows in the system have significantly changed since European settlement (Green 2001). Naturally, flows in the river system would have been high in spring and very low in summer and autumn. Flow modelling, using the former Murray-Darling Basin Commission's Monthly Simulation Model for stations on the Edward River at Deniliquin and the Wakool River at Kyalite both show that the flow pattern in the Edward-Wakool system has undergone substantial change in both the timing and volume of flows. Under 'natural conditions' the median monthly flows at both sites would have been lower (by up to 70%) during the period January to May, and up to 3-5 times higher in the period June to December, compared to modelled 'current conditions' (Green 2001). The pattern of river regulation in the system has been altered since the onset of the on-going drought conditions in south-eastern Australia and this has had a marked effect on flows in the Edward-Wakool River System. Figure 2 shows the calculated river flow at Stoney Crossing (towards the lower end of the Wakool River) for the last 30 years. Prior to 1996 a significant (regulated) flow event occurred in the river system on a roughly annual basis. Since 1996, there has only been one significant flow event on the lower Wakool River, in December 2000 - nearly 8 and a half years ago. Previously these large flow events helped to mitigate poor water quality in the lower Wakool River; as well as elsewhere in the system (Green 2001). The Edward-Wakool River System currently has periodic issues with water quality (Green 2001), including salinity, sulfidic sediments and blackwater events (Baldwin 2009). Baldwin (2009) recently identified blackwater as an important water quality issue in the Edward-Wakool River System.