Rivers provide many social and environmental services that benefit humanity. A critical role of water mangers is to prioritize water allocation options that trade off socio-economic and hydro-ecological benefits in rivers. Methods for multi-criteria decision analysis (MCDA) provide a structured and systematic manner for researchers to aid in this process. In this paper, we describe a new MCDA method that prioritizes large multi-dimensional sets of tradeoffs to support well-informed water management in rivers. The method was developed based on an environmental flows planning study in the Goulburn-Broken River catchment, Victoria, Australia. A combined simulation and heuristic optimization procedure was previously integrated into a hydrological catchment network model. That process resulted in a large set of viable daily water allocation schedules that traded off long-term irrigation and hydro-ecological benefits at the catchment outlet. We provided new guidance procedures to identify priority tradeoffs that can be used in stakeholder deliberations and catchment decision-making. Our MCDA method included combined multi-dimensional ordination and cluster analysis to spread the water allocation alternatives onto a two-dimensional plane to discover alternatives with similar criteria tradeoffs. A geometric distance-based method was performed on the full set of alternatives and on the identified clusters to rank the alternatives in accordance with minimizing the distance of the alternatives to an ideal but non-feasible reference point in multi-dimensional space. This method complements the use of elicitation procedures when water manager or other stakeholder interaction is not an option or when objectivity is desired.