Floodplain-derived resources play a vital role in sustaining aquatic life in low-gradient rivers. The availability of both particulate and dissolved resources is strongly influenced by floodplain geomorphology and river-floodplain connectivity. However, river-floodplain connectivity has been suppressed in many riverine ecosystems, hindering our understanding of how it shapes resource dynamics. Here, we aimed to (1) evaluate spatiotemporal drivers of variability in particulate organic matter (POM), dissolved organic carbon (DOC), and dissolved nutrients, specifically nitrogen (N) and phosphorus (P); and (2) determine the relative contributions of material across sites along a longitudinal gradient within the watershed on DOC, N, and P. Our study took place in the Sipsey River, Alabama, U.S.A., an undammed fifth-order alluvial river system with a large swamp complex in its floodplain along our study reach. For two years (February 2021 - February 2023), we collected monthly particulate organic matter (POM), DOC, and dissolved nutrient samples from nine sites across the watershed. We found that sampling year, season, site, and discharge drove variations in POM, DOC, and dissolved nutrient concentrations. To estimate how site-specific contributions of DOC, N, and P changed over time and space, we calculated subcatchment leverage values for each of our sampling sites, representing a distributed mass balance across the watershed. The leverage analysis revealed that the floodplain-swamp complex acted as a persistent net source of DOC to the river, which contains the widest portions of the floodplain and relatively low elevation. Conversely, net nitrogen (NH₄⁺ and NO₃⁻) uptake increased notably as the river flowed through this swamp complex. Sites upstream of the swamp acted as net P sources, with conditions switching to favor net P uptake upon entering the swamp. Our findings highlight the importance of spatiotemporal controls on POM, DOC, and dissolved nutrient variability underscoring the role of floodplain-swamp complexes in modulating resource availability in low gradient river ecosystems.