A spatial survey of iodine and its long-lived isotope, 129I, in 40 rivers of the USA, Canada, and western Europe, reveals that the ratio of 129I/I is a sensitive indicator for the influence of nuclear fuel reprocessing facilities. Rivers with point sources for 129I in their watersheds are drastically affected, while all rivers sampled show evidence for atmospherically transported 129I from the world's major nuclear fuel reprocessing facilities. Varying mixtures of oceanic cyclic salt and soil-derived iodine account for the observed spatial variation in iodine concentrations. A comparison of 129I concentrations in river and rainwater reveals concentration effects due to evapotranspiration.