Chemical weathering of silicate minerals in rocks controls the dissolved silica concentration of riverwater. This means that the dissolution rate of silicate minerals is an important factor for dissolved silica concentration. In addition to the dissolution rate, runoff (Δf) is also an important factor controlling riverwater chemistry. Therefore, the relationshipof the dissolved silica concentration, runoff, and dissolution rate of silicate mineral (feldspar) were theoretically derived on the basis of a dissolution kinetics-fluid flow coupling model. This theoretical consideration indicates that the parameter, φk (A/M), in which φ : porosity, h : thickness of reservoir of groundwater, k : dissolution rate constant, together with Δf, affects the dissolved silica concentration. If Δf is low compared to φ hk (AIM), the concentration (C) is equal to the equilibrium concentration. If Δf is high, C negatively correlates with Lf. This theoretical curve showing the relationship between the dissolved silica concentration and runoff is consistent with the dissolved silica concentration and runoff data of a major global riverwater.