Late 2019, the Northern Basin was in a state of severe drought. Mehi River had ceased to flow for over 100 days, prompting the DPlE – Biodiversity & Conservation Division and the Commonwealth Environmental Water Office, to collaborate on a delivery of water for the environment. NSW and Commonwealth accounts were utilised to sustain critical refuge pool habitat for fish and other aquatic fauna in the Mehi.
Inherent risks exist during such deliveries, including the formation of hypoxic blackwater. Despite the risks, one thing was known with certainty, if flows did not occur to replenish refuge pools, there would likely be a total loss of native fish in the Mehi.
The delivery was made from Tareelaroi Weir and replenished refuge pools. Water quality issues were observed upstream of Moree, where blackwater formed due to organic matter accumulation. A small number of fish mortalities were recorded. A more significant blackwater event occurred downstream at Combadello Regulator where approximately 200 native fish died.
During the early stages of the delivery, an interagency group formed to respond to potential water quality issues. The group identified the significant risk of a further large fish kill downstream at Gundare Regulator, the last refuge pool in Mehi River before Barwon River. The group developed three response actions to attempt to address the event; the upstream flow rate was raised to improve downstream water quality, a portion (100 ML) of the blackwater was removed via an irrigation pump and weir pool manipulation was used at Gundare Regulator to direct blackwater down the dry Mallowa Creek.
These actions proved successful and were important for promoting recovery of native fish in Mehi River. As a result of lessons learnt, we now have a suite of new tools for restarting dry rivers and managing environmental water deliveries during severe droughts.