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Monitoring the effects of environmental flows on hypoxic blackwater in the Murray and Murrumbidgee Rivers

Commonwealth of Australia, 2014

Prepared by:  Darren S. Baldwin and Kerry L. Whitworth
This monitoring project was commissioned by the Commonwealth Environmental Water Office.

Executive Summary

Hypoxic blackwater is characterised by a high concentration of dissolved organic carbon and low concentration of dissolved oxygen in the water column. Hypoxia can be stressful or fatal to many aquatic organisms.

Hypoxic blackwater was generated on the Murrumbidgee River floodplains during summer and autumn flooding (from rainfall) in 2012. Very low dissolved oxygen was recorded in the river channel downstream of Balranald for upwards of a month. Inflows of this water into the Murray River also posed a risk to water quality in the Murray downstream of the confluence, especially when Murray discharge began to recede while Murrumbidgee discharge remained high in April 2012.

In order to provide and maintain oxygenated refuge habitats for aquatic animals along the main stem of the Murray River, the Commonwealth Environmental Water Office delivered an environmental flow through the Murray River system of up to 120,000 megalitres.  This was in addition to pre-existing environmental flows in the system.  Examination of mixing patterns at and downstream of the junction revealed that Murray River water mixed completely with the hypoxic Murrumbidgee water within 5 river kilometres. Dissolved oxygen profiles also showed that a corridor of oxygenated water persisted past the junction, which would have created oxic refuges and facilitated the passage of biota upstream to oxygenated water.

The impact of the additional environmental water delivery on downstream water quality could be predicted with reasonable accuracy using simple mixing models.  Under a worst case scenario the base flow in the Murray River during the hypoxic blackwater event was predicted to fall as low as 1800 megalitres per day.  Modelling showed that the additional environmental water delivered in this study would have substantially improved water quality downstream of the confluence of the Murray and Murrumbidgee Rivers if base flows had fallen to 1800 megalitres per day.