Technical Abstract: The transport and transformation of phosphorus (P) in riverine systems fundamentally affects the outcome of watershed mitigation strategies aimed at curbing downstream eutrophication. Phosphorus transport and transformations in streams and rivers are mediated by physical (sediment deposition and resuspension and flow regimes), abiotic (P sorption and desorption), and biotic (microbial and plant uptake) processes. Most importantly, P entrained in riverine systems can play an important role in modifying or delaying societal efforts to curb eutrophication. This paper synthesizes literature on P transport processes in riverine systems, providing insight into how these systems may alternatively serve as sources, sinks and stores of P.