Operation of a hydropower dam will impact river hydrology downstream of the turbines where water is returned to the river and, if the dam includes a tunnel, in the section of river that is bypassed. The degree of hydrologic alteration and the consequent ecological impacts depend on the how the power generation facility is operated. Proposed hydropower projects would be expected to assess the environmental flow requirements (EFRs) of the bypassed reach, and also the river downstream of the water return point. These EFRs could have a major impact on the financial viability of the project. We developed a hydropower operation model for use in pre-feasibility assessments that predicts the downstream river flows and hydropower production at an hourly time-step. A routing function modifies the flow as it travels downstream. The model assumes that the environmental flow requirements and other existing human demands are set by non-negotiable river management objectives, and the dam must be operated within those constraints. This allows for testing of various permutations of dam design and operation, combined with various environmental flow options, on power production potential. The model determines the optimum power generation profile for each day, depending on a range of variables, including dam inflows, environmental flows released from the dam, environmental flow rules constraining maximum rates of rise and fall in discharge to the river, dam water level at the start of the day, and the power profile that was implemented the previous day. Both run-of-river and hydro-peaking modes of operation are considered.