Side-valley tributary fans within the main axial valley river floodplain system are integral components of the sediment assemblage characterizing floodplains in the high-energy Navarro River basin, California. Laboratory analyses of subsurface core and auger samples from four study sites reveal that sediment-size distributions within the floodplain are controlled by spatial and temporal variation in sediment sources and depositional processes. Over 75% of sediment within individual lower-fan strata are poorly sorted and include gravel clasts inferred to be contributions from the side-valley tributary, whereas remaining strata are composed of fine sediment less than 2 mm without gravel. High-water marks indicate that portions of the side-valley tributary fans are episodically inundated by axial valley overbank flow, a factor that influences fan relief and profile. Thus, two dominant sources supply sediment to this transitional environment: side-valley tributaries and the axial valley river. The transport and depositional processes that supply sediment to the fans operate under three conditions: (1) Tributary debris or fluvial flows, and selective entrainment of fines by fan surface flows, deposit sediment when there is no overbank flow from the axial channel; (2) overbank flow deposits sediment derived from the axial valley river when there is no tributary input; and (3) tributary flows deposit sediment while the floodplain is inundated by floods from the axial valley river. The mixing and interlayering of sediment from these disparate sources within the active floodplain illustrate the significance of both hillslope-floodplain and channel-flood-plain interactions in high-energy fluvial systems with floodplains.