The increasing use of detailed detrital-zircon provenance data calls for an understanding of the controls that sedimentary processes exert on changing detrital-zircon provenance signatures in time and space in addition to the well-established influence of tectonics. Our U-Pb detrital-zircon data from seventeen samples taken across a series of modern second-order rivers to deep-marine depositional systems in central California show that marine mixing processes and sediment pathway partitioning exert important controls on detrital-zircon provenance signatures. The second-order rivers that input sediment into this system produce differentiable detrital-zircon provenance signatures as a function of the unique geology of their drainage areas. The mixing of sediment through the marine realm can be traced through the evolution of distinct detrital-zircon signatures as sediment is transported from rivers down system through longshore drift into submarine canyons. Sediment input from second-order streams is mixed in the littoral and upper submarine-canyon environments, yielding provenance signatures that are not thoroughly mixed until the lower reaches of submarine canyons. Sediment partitioning in these dispersal systems results in distinct detrital-zircon provenance signatures for different dispersal systems over short length scales (< 50 km). These data suggest that sediment mixing in the marine realm is likely an important control on detrital-zircon provenance signatures in any system in which sediment is transported through longshore drift and sediment pathways are distinctly partitioned. Changes in the way sediment pathways are partitioned through time has the potential to create heterogeneous detrital-zircon provenance signatures in stratigraphic successions through time and between closely spaced deposits of the same age. When seen in an ancient system, this heterogeneity may be mistakenly attributed to tectonic controls without the recognition of the influence of sedimentary process.