We measured nutrients and dissolved organic matter (DOM) in surface water and floodplain sediments to evaluate biogeochemical linkages between streams and floodplains in 3 streams of increasing size in a tropical catchment in northwestern Australia. We hypothesized that stream–floodplain biogeochemical connectivity, measured as similar DOM concentrations and spectroscopic properties of streamwater and floodplain sediment leachate, would decrease with increasing stream size. We expected decreasing connectivity to shift support of aquatic foodweb structure from mainly allochthonous C in small streams to algal C in larger streams. Streamwater and sediment leachate concentrations of NH₄-N, NO₃-N, soluble reactive P, and DOM varied several-fold across the 3 sites but showed no strong pattern in stream–floodplain connectivity with increasing stream size. Fluorescence index, specific ultraviolet absorbance, and δ¹³C-dissolved organic C (DOC) for stream water and sediment leachate were most similar to each other at the mid-size Adcock River, indicating tight river–floodplain biogeochemical connectivity via input of allochthonous DOM and NH₄-N to stream water. δ¹³C and δ¹⁵N signatures for key producers and consumers for all 3 streams showed that invertebrate consumers probably were supported by a range of in-stream (e.g., benthic and filamentous algae) and floodplain (allochthonous leaf litter) sources, whereas fish strongly reflected δ¹³C-enriched sources (e.g., benthic algae). Allochthonous C was probably only a minor energy source for metazoa, but we propose that river–floodplain biogeochemical linkages may be important for other aspects of ecosystem productivity, such as input of inorganic nutrients to support in-stream primary production. Terrestrial–aquatic biogeochemical linkages may be critical but poorly quantified components of river–floodplain ecosystems.