Transport of sediment across riverine flood plains contributes a significant but poorly constrained
fraction of the total chemical weathering fluxes from rapidly eroding mountain belts which has impor-
tant implications for chemical fluxes to the oceans and the impact of orogens on long term climate. We
report water and bedload chemical analyses from the Ganges flood-plain, a major transit reservoir of
sediment from the Himalayan orogen. Our data comprise six major southern tributaries to the Ganga,
31 additional analyses of major rivers from the Himalayan front in Nepal, 79 samples of the Ganga
collected close to the mouth below the Farakka barrage every two weeks over three years and 67 water
and 8 bedload samples from tributaries confined to the Ganga flood plain,. The flood plain tributaries
are characterised by a shallow d 18 O - dD array, compared to the meteoric water line, with a low dD excess
from evaporative loss from the flood plain which is mirrored in the higher dD excess of the mountain
rivers in Nepal. The stable-isotope data confirms that the waters in the flood plain tributaries are domi-
nantly derived from flood plain rainfall and not by redistribution of waters from the mountains. The
flood plain tributaries are chemically distinct from the major Himalayan rivers. They can be divided
into two groups. Tributaries from a small area around the Kosi river have 87 Sr/ 86 Sr ratios > 0.75 and
molar Na/Ca ratios as high as 6. Tributaries from the rest of the flood plain have 87 Sr/ 86 Sr ratios <0.74
and most have Na/Ca ratios <1. One sample of the Gomti river and seven small adjacent tributaries
have elevated Na concentrations likely caused by dissolution of Na carbonate salts. The compositions
of the carbonate and silicate components of the sediments were determined from sequential leaches of
floodplain bedloads and these were used to partition the dissolved cation load between silicate and car-
bonate sources. The 87 Sr/ 86 Sr and Sr/Ca ratios of the carbonate inputs were derived from the ace-
tic-acid leach compositions and silicate Na/Ca and 87 Sr/ 86 Sr ratios derived from silicate residues from
leaching. Modelling based on the 87 Sr/ 86 Sr and Sr/Ca ratios of the carbonate inputs and 87 Sr/ 86 Sr ratios
of the silicates indicates that the flood plain waters have lost up to 70% of their Ca (average ~ 50%) to
precipitation of secondary calcite which is abundant as a diagenetic cement in the flood plain sedi-
ments. 31% of the Sr, 8% of the Ca and 45% of the Mg are calculated to be derived from silicate miner-
als. Because of significant evaporative loss of water across the flood plain, and in the absence of hy-
drological data for flood plain tributaries, chemical weathering fluxes from the flood plain are best
calculated by mass balance of the Na, K, Ca, Mg, Sr, SO 4 and 87 Sr/ 86 Sr compositions of the inputs,
comprising the flood plain tributaries, Himalayan rivers and southern rivers, with the chemical dis-
charge in the Ganga at Farakka. The calculated fluxes from the flood plain for Na, K, Ca and Mg are
within error of those estimated from changes in sediment chemistry across the flood plain (Lupker et
al., 2012, Geochemica Cosmochimica Acta). Flood plain weathering supplies between 33 and 48% of
the major cation and Sr fluxes and 58% of the alkalinity flux carried by the Ganga at Farakka which
compares with 24% supplied by Himalayan rivers and 18% by the southern tributaries