• flow management;
  • cyanobacteria;
  • thermal stratification;
  • risk;
  • River Murray;
  • Anabaena spp


Cyanobacterial blooms present significant water quality problems worldwide. Flow management is considered to be one of the most promising approaches for combating the cyanobacterial bloom problem in rivers. In this paper, a new method for evaluating the effectiveness of flow management strategies in reducing the risk of cyanobacterial blooms is developed and applied to the River Murray at Morgan, South Australia. As stratification has been shown to be a necessary condition for significant growth of certain cyanobacteria species, the method uses estimates of the probability that stratification events of various durations will occur in conjunction with estimates of population growth during stratified conditions to determine the probability that blooms of various magnitudes will occur. The results of the case study indicate that the probability that blooms of the cyanobacterium Anabaena circinalis exceeding 15 000 cells/ml will occur in any given year under summer entitlement flow conditions is 11.7%, which is in excellent agreement with results obtained using a Poisson–Bayesian approach applied to 17 years of historical data of cell densities of Anabaena circinalis at the study site. The results obtained also indicate that increasing inflows into South Australia by 10 000 Ml/day, which is the maximum achievable increase given current operational constraints, would not have a significant impact on the occurrence of blooms exceeding 15 000 cells/ml. An additional flow of 19 900 Ml/day into South Australia would be required to reduce the probability of occurrence of blooms exceeding 15 000 cells/ml to 0.01%. Copyright © 2004 John Wiley & Sons, Ltd.