The implementation of many fisheries management-related activities in fresh water depends on habitat area inventories over extensive geographical scales. While river lengths are readily available, representative widths, necessary for area calculations, are difficult to obtain. As field surveys to collect this information are resource intensive, a predictive model was developed to enable the calculation of river wetted width using GIS-derived values for catchment and river descriptors. A model containing upstream catchment area and the Shreve river drainage network index accounted for 88% of the variation in field measured river wetted width. Comparisons in Irish and Scottish rivers between modelled and measured widths were highly correlated and suggest that the model may be transferable to neighbouring geographic areas. As an example, the model is applied to provide an estimate of the usable fluvial habitat available to Atlantic salmon in Ireland.