The ability to understand and predict the flux and fate of sediment delivered to the sea by rivers remains an outstanding scientific challenge. Approaches to this challenge are necessarily synthetic, spanning wide ranges in spatial and temporal scales. Here a conventional sediment transport theory used by engineers and sedimentologists at reach and channel scales is applied at the basin scale. Specifically, a straightforward expression proposed by Bagnold and modified accordingly predicts the observed importance of combined wetness and steepness of a source basin as a control of sediment supply to the sea. The reasonable, key assumption underlying the application of sediment transport theory in this context is that the river-mouth sites for which suspended-sediment loads are reported are alluviated, and thus characterized by transport-limited flux of sediment. This analysis also indicates the potential significance of additional, as yet poorly documented factors constraining sediment supply to the sea. These factors, some of which appear to covary systematically with climate, include river-profile concavity, river-mouth channel width and friction, and the characteristic size of sediment in transport.