Environmental watering plans, including those associated with the Basin Plan for the Murray-Darling Basin, frequently include quantitative environmental outcomes for the distribution or abundance of key taxa. For example, the Basin-wide environmental watering strategy includes outcomes for the distribution of, and number of seeds present for Ruppia tuberosa in the Coorong, to be achieved in 2019 and 2029, respectively. However, variation in local weather, changes in water availability and other factors interact to complicate any assessment of the trajectory of change through time. Thus, in many instances there is no objective method for assessing plausible future trajectories of change to determine whether the targets are likely to be met or not. In a collaboration with the Murray-Darling Basin Authority, we combined a number of existing tools in a novel approach to develop plausible future trajectories to determine whether Ruppia in the Coorong is on track to meet those targets. We used an existing hydrodynamic model for the Coorong linked to an updated life-history model for Ruppia (developed by CSIRO and SARDI Aquatic Sciences). We developed plausible baseline, wet, dry and moderate future scenarios and linked the likelihood of completing each life-history stage to existing monitoring data collected by the University of Adelaide. This enabled us to calculate an approximate extent and number of seeds for Ruppia in each future year to 2042. We then incorporated variability in response associated with local weather conditions using a Markov Chain approach. This approach resulted in a set of plausible future trajectories for Ruppia in the Coorong, enabling a scientifically-credible, transparent and objective method for assessing progress towards the relevant quantitative ecological objectives.