The silicon isotopic composition (δ30Si) of the headwaters of the Ganges River, in the Himalaya, ranged from +0.49±0.01 to +2.17±0.04 at dissolved silicon (DSi) concentrations of 38 to 239μM. Both the concentration and isotopic composition of DSi in the tributaries increased between the highest elevations to where the Ganges leaves the Himlayas at Rishikesh. The tributaries exhibit a linear correlation between δ30Si and DSi that may represent mixing between a low DSi, low δ30Si (e.g., 40μM, +0.5 ) component potentially reflecting fractionation during adsorption of a small fraction of silicon on to iron oxides and a high DSi, high δ30Si component (e.g., 240μM, +1.7 ) produced during higher intensity weathering with a greater proportional sequestration of weathered silicon into secondary minerals or biogenicsiica. On the Ganges alluvial plain, in the Ganges and the Yamuna, Gomati, and their tributaries, DSi ranged from 122 to 218μM while δ30Si ranged from +1.03±0.03 to +2.46±0.06 . Highest values of δ30Si occurred in the Gomati and its tributaries. In general, the lower DSi and higher δ30Si of DSi in these rivers suggests control of both by removal of DSi by secondary mineral formation and/or biogenic silica production. A simple 1-dimensional model with flow through a porous medium is introduced and provides a useful framework for understanding these results.