Water projects are typically evaluated using benefit cost analysis. Ecosystem services are the direct and indirect benefits that people obtain from ecosystems. Many of these benefits are ignored in benefit cost analysis, because of the absence of markets and the limited information or understanding of how the benefits from ecosystem services are produced. Regional or local government may be interested in learning how the value of ecosystem services associated with projects may change if a project occurs. Ecosystem Service Reviews aim to make ecosystem services explicit and quantifiable so that they can be accounted for in the evaluation of water use projects. Water storage projects can enable land use intensification to occur, and confer environmental benefits in some instances (e.g., flow augmentation) and costs in others (e.g., groundwater contamination and flow‐on effects). Water storage projects can have both positive and negative outcomes for the environment. More flow can lead led to better fishing, better clarity, more contaminant dilution and a healthier aquatic ecosystem. It can also result in loss of braided river‐bird habitat, and regulated flows can result in nuisance growths of potentially toxic algal species. Irrigation can increase productivity of land within the scheme, with attendant benefits to soil quality and other out‐of‐river environmental characteristics. This paper reports the methods used to assess the impact of a water storage dam on the flow of ecosystem services in a river system. We review the range of ecosystem services that are available in a river system and examine how the flow of ecosystem services can be altered by water storage and flow augmentation through the construction of a dam. In order to list and quantify ecosystem services an attempt is made to determine a suitable site specific set of ecosystem service indicators for the Opuha‐Opihi river system case. We draw inferences about shifts in the value of ecosystem services that might arise from water projects in other contexts.