Most ideal environmental flow regimes are designed by hydrologists or ecologists to meet a set objective(s), such as improving the ecological condition of a particular ecosystem. This involves determining a volume likely to achieve the objective, designing a release hydrograph and developing a monitoring program to assess the impact of the flows relative to the objective.

Moving from such an idealised design to opening the flood gates to deliver an actual environmental flow event is constrained by a large number of factors other than how best to meet environmental objectives. Policy and political constraints can have a large impact on the amount of water that may be available and the manner in which water is shared among a wide range of users, including across political boundaries. Socioeconomic constraints can also be relevant: for example, the timing of irrigation flows can influence when channel capacity is available. Finally, water-delivery constraints such as channel capacities can all influence how an environmental flow event may be possible.

We compare the impact of such constraints on the objectives and flow delivery profile of the first environmental flows in the Colorado River, USA/Mexico with the planning process associated with such flows in the Murray-Darling Basin, Australia. Differences between the case studies in the existing flow regimes, allocation of water rights, competing interest of multiple states and countries and in the maturity of the science regarding potential ecological impacts of flows result in very different planning processes and potentially very different environmental and other outcomes.