Water samples were filtered to 0.2 micron prior to measurement. Samples for cation analysis were acidified in the field to pH < 2 using 6N HNO3. Cation concentrations were measured with a Varian 720 inductively coupled plasma optical emission spectrometer (ICP-OES) at the GFZ Helmholtz Laboratory for the Geochemistry of the Earth Surface (HELGES), using SLRS-5 (Saint-Laurent River Surface, National Research Council - Conseil National de Recherches Canada) and USGS M212 and USGS T187 as external standards. We corrected for instrument drift by measuring an internal standard (GFZ-RW1) every 10 samples and we determined measurement uncertainty using calibration curve uncertainty. Anion concentrations were measured with a Dionex ICS1100 Ion Chromatograph, using USGS standards M206 and M212 as external standards for quality control, with uncertainty determined from triplicate analysis. We corrected cation concentrations for cyclic salt inputs following Bickle et al. (2005, doi:10.1016/j.gca.2004.11.019).

Abbreviations: #1 = N/A - not applicable#2 = n.m. - not measured#3 = <det - indicates standard deviation of major element concentration is not availabile because the measurement was less than the instrument detection limit.(††) HCO3- and TDS data are calculated with the cyclic-corrected data (see text for data), all other measurements are raw data, uncorrected for cyclic inputs.() Water source column indicates the source of samples, Headwater Tributaries are separated by small individual rivers, the Rio San Francscio, and the Rio Bermejo upstream of its junction with the Rio San Francisco (referred to as the "Upper Rio Bermejo"), the Rio Bermejo downstream of its junction with the Rio San Francisco is listed as "Lower Rio Bermejo".(*) Projected distances is the (straight line) distance of samples downslope of the mountain front (equivalent to the junction of the Rio San Francisco), negative values indicate samples which are upstream of the mountain front.