To clarify topographic effects on dissolved SiO2 flux produced by chemical weathering in a graniticmountainous catchment, stream discharge were measured and water samples were collected from fifteen streamsin Ikuchijima Island, Seto Inland Sea. The results show that the dissolved SiO2 flux is controlled by streamdischarge, due to high amount of runoff compare to dissolved SiO2 concentration. However dissolved SiO2 fluxof catchment was not affected by horizontal two - dimensional features, such as catchment area, alluvial fanarea, and the catchment shape. On the other hand, streambed gradient of the catchment affects runoff and dissolvedSiO2 flux, but have no influence on dissolved SiO2 concentration. Useful water convergence degree ofthe catchment was calculated by multiplication of the inversion of streambed gradient by prolate degree ofcatchment. The estimated water convergence degree shows good correlation with dissolved SiO2 flux and runoff.