Endmember samples were collected and analyzed to determine the sources of organic material in the river suspended sediment. Endmembers include soil, leaf litter, and floodplain sediment. Soil and leaf litter samples were collected using an ethanol-cleaned hand trowel. Floodplain sediment samples were collected using an Edelman-type hand auger drilled down to a maximum of ~5 m. Samples were stored in paper bags, and then oven-dried at 40°C.n-alkanes were identified and quantified using an Agilent gas chromatograph (GC 7890-A) with flame ionization detection (FID) coupled to a single quadrupole mass spectrometer (MS 5975-C). We quantified n-alkane concentrations relative to the peak response of the internal standard, and then normalized the abundance to the sediment mass. We measured n-alkane d13C via GC-C-IRMS (gas chromatography/combustion/isotope-ratio mass spectrometry) with helium as a carrier gas (Agilent 7890N, ThermoFisher Delta V Plus). All compounds were measured in triplicate with a standard deviation of =0.5‰. Measurement quality was checked regularly by measuring n-alkane standards (nC15, nC20, nC25) with known isotopic composition (provided by Campro Scientific, Germany). d13C values were normalized to the Vienna Pee Dee Belemnite (VPDB) standard. We measured n-alkane d2H via GC-IRMS using a ThermoFisher Scientific Trace GC 1310 coupled to a Delta-V isotope ratio mass spectrometer. All d2H measurements were made in duplicate, and measurement quality was checked with d2H values were normalized to the Vienna Standard Mean Ocean Water (VSMOW) standard using an n-alkane standard mix with known d2H values (nC16 - nC30, from A. Schimmelman/Indiana University).