Salt wedge, tidal mixing, stratification, and salinity intrusion
Estuaries are governed by the competition between river discharge (which pushes fresh water seaward) and tidal mixing (which stirs the salt wedge upward). The Estuarine Richardson number (Ri_E ∝ Q_river / tidal energy) classifies the system: salt-wedge estuaries (Ri_E > 20, e.g. Mississippi) have near-zero mixing; partially mixed (1 < Ri_E < 20, e.g. Chesapeake Bay) show strong vertical salinity gradients; well-mixed (Ri_E < 1) show mostly horizontal gradients. Salinity intrusion length scales inversely with river discharge — doubling discharge halves the intrusion, critical for water supply management.