At midnight tonight Meridian Energy will be releasing a 24 hour 'flushing flow' from the Mararoa Weir, in response to a rapid rise in algal cover over the lower Waiau. 

This follows a recent periphyton (algae) survey on January 6 which indicated an undesirable level of algal coverage on the river, predominantly didymo. 

A traffic light system is used to indicate the overall level of algal cover at three specific sites on the lower Waiau. These have now crossed into the 'red' status threshold having been assessed 'green' on December 17.

Southland Fish and Game representative Bill Jarvie says the rapid increase in algae is not unusual during summer and is caused by rising temperatures and nutrients in the river. Didymo is the dominant species, and it is difficult to effectively control. 

“The effectiveness of these flushing flows is variable. The main algae we are dealing with is didymo, which is well adapted to fast water. It will do some good though, and give a general freshening up of the riverbed.” 

Meridian Energy hopes to flush around 120 cubic metres of water per second over the 24 hours, with a peak of 160 cubic metres per second

With the release starting midnight Tuesday and increasing in small increments, it is anticipated that river users are unlikely to be negatively impacted.