Flushing of the Waitaki River next week will give the river "a good clean out", Fish and Game officer Graeme Hughes says.
The full spilling amount - 950cumecs - will be reached about midday on Monday and maintained for 24 hours.
Mr Hughes hoped the high flow would cut through the shingle bar at the mouth of the river and also give didymo a "good pounding".
While didymo would come back again, it was being removed just prior to expected salmon runs.
The only negative aspect was access to known fishing spots would be difficult during the high flow, but that would not be for long and it would be worth the wait.
The last major flush of the river was 800cumecs in 2004, he said.
South Island hydro lakes are at maximum capacity after weeks of heavy northwesterly rain.
With more northwest rain forecast and low demand for power due to summer holidays, spilling is likely to continue from Benmore and Pukaki dams.
Environment Canterbury is advising people who will be near rivers and waterways throughout the region to phone 0900 RIVER (74837) or check conditions online at www.ecan.govt.nz.
