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Description: Stormwater control measures—such as raingardens, tanks, or wetlands—retain, treat or divert runoff from roofs and pavement, to protect downstream waters from the polluted, enlarged, and more frequent flows in stormwater drains after rain. Design of control measures has typically aimed to reduce the amount of pollutants draining downstream over a long period, but it has not been possible to demonstrate or predict if the health of rivers and streams will improve as a result. On the other hand, river health is predicted well by effective imperviousness, the proportion of the river’s catchment covered by roofs or pavement that are connected to the drainage network. We propose a new way of calculating effective imperviousness, weighted by a performance index for stormwater control measures, to predict how streams respond to stormwater control across catchments. In an accompanying paper (Walsh et al. in press, PLOS Water: https://doi.org/10.31219/osf.io/mjz9h) demonstrate its use in an experiment aiming to test if stream health is improved by dispersed stormwater control measures in several catchments. This project stores the paper's data and code.