Abstract: The average water level during tidal cycle, also known as the residual water level, is a typical outcome resulting from the nonlinear interaction between tides and river flow. To quantify the contributions of runoff, tides, and their interactions to the change of residual water level, the friction term is decomposed, and the components of mean tidal friction are examined to study the variations in residual water level. A two-dimensional numerical model is employed to simulate the hydrodynamic conditions and spatiotemporal characteristics of the residual water level in the Yangtze River Estuary. The continuous wavelet transform method togerther with the friction calculation formula is used to examine the primary sources of tidally averaged friction. Furthermore, a linear regression model equation is established to explain the variations in the residual water level. The results indicate that the residual water level in the Yangtze River Estuary exhibits distinct characteristics between wet and dry seasons and throughout the spring-neap cycle. Wet season experiences a more pronounced increase in the residual water level, particularly during spring tides. Tidally averaged friction is predominantly influenced by the river flow term and river-tide interaction term, with the contribution from river discharge consistently being greater due to its substantial volume. Tidally averaged friction governs the behavior of the residual water level. The variation in residual water level in the Yangtze River Estuary is primarily controlled by river flow; however, the influence of river-tide interactions cannot be ignored at downstream stations.

Key words: residual water level, river-tide interaction, tidally averaged friction, continuous wavelet transform, linear regression, Yangtze River Estuary