Abstract: The Environment Agency provides guidance for run-of-river hydropower schemes in England, but concerns about small effects from multiple single schemes led to a project to determine how these can be assessed and to test an approach on migratory fish populations. We developed a model to assess the impact of multiple hydropower schemes. The model forecasts catchment-scale cumulative effects using three elements: hydropower scheme, spatial fish population and fish life cycle. The scheme element includes effects that are important to migratory fish and those that can be quantified. These are: 1) impediment to up- and downstream migration; 2) alleviation of up- and downstream impediment (e.g. fish pass installation); 3) impingement and entrainment; 4) habitat loss to freshwater stages (via a depleted reach). The model can be modified to include other effects if new research is able to provide better quantification, for example on migration delay causes by barriers. The spatial population element was developed by applying reference juvenile density values to the wetted area of the river network using Atlantic salmon as a demonstration species. The scheme effects element was then applied to the spatial population element, enabling the salmon population upstream of the scheme, and (via the life-cycle element) returning adults to be quantified and associated scheme benefits / dis-benefits to be applied. The sequential evaluation of multiple schemes in this way allowed assessment of cumulative effects. A number of hypothetical scenarios confirm that cumulative effects (positive, neutral or negative) were possible. The extent of effects was dependant on scheme location, the combined net benefit / dis-benefit of all schemes and the population status. The model results are sensitive to the precision and accuracy with which we are able to quantify barrier passability to fish and fish pass efficiencies, both of which can be hard to quantify with confidence.