Cascade reservoirs attenuate the seasonality of estuarine tidal duration asymmetry

Study region: The Pearl River Estuary in southern China has strong monsoonal wet-dry discharge contrasts and extensive cascade reservoir regulation in its basin. Study focus: This study quantifies how reservoir-driven discharge re-partitioning affects the seasonality and multi-decadal evolution of tidal duration asymmetry (TDA; the difference between flood and ebb durations). To reveal the linkage between TDA and regulated discharge, we analyze 51 years (1966–2016) of water levels from 21 tide gauges and daily discharge from three upstream stations using a tidal skewness metric, non-stationary harmonic diagnostics, and trend analysis. New hydrological insights for the region: TDA shows significant seasonality, with higher skewness in the wet season (April–September) and lower skewness in the dry season (October–March), and a basin-wide trough in late June–early July when discharge peaks. The discharge-TDA relation is threshold-dependent: normalized skewness increases with normalized discharge up to ∼0.62 (∼12,550 m³/s) and decreases thereafter. Reservoir regulation reduced wet-season peak flows (−76.3 m³/s/yr) and increased dry-season baseflows (+16.5 m³/s/yr), attenuating seasonal TDA amplitude most strongly in river–tide transition reaches (−0.0036 /yr). Spatial differences indicate phase-controlled TDA seasonality upstream but amplitude-controlled seasonality downstream, consistent with shifts in dominant tidal interactions from K₁–O₁–M₂ to M₂–M₄. The threshold rule links managed discharge to seasonal TDA and supports basin-to-estuary flow management in regulated coastal rivers.