In this paper we present an analytical approach for assessing the effect of river discharge on tidal dynamics in convergent estuaries. The analytical model makes use of a hybrid analytical expression for tidal damping derived by subtracting the envelopes of high water and low water, using a new approximation of the friction term. The analytical solutions are compared with observations in the Modaomen estuary in China and proved to be both efficient and accurate, suggesting that the proposed model is a powerful analytical instrument for conducting impact assessments induced by human interventions (e.g., flow reduction). The application to the Modaomen demonstrates that the effect of river discharge on tidal dynamics is significant in the upper part of the estuary, where the ratio of river flow to tidal flow amplitude is substantial. The analytical model is able to reproduce the main tidal dynamics with realistic roughness values in the upper part of the estuary, while a model with negligible river discharge only fits observations with unrealistically high roughness values.