Partially-Mixed

Circulation in Estuaries Circulation in Estuaries

• Instantaneous tidal exchange (flood & ebb) • Net circulation (mean over many tidal cycles) – Tidal flood & ebb at all depths at (roughly) the same – Tidal flood & ebb average almost to zero time – Surface layer: ebb is stronger than flood (rivers) – Subsurface layer: flood is stronger than ebb – Flood • Replace water lost to entrainment & tidal pumping

Entrainment Tidal pumping

– Ebb

Types of Estuaries Salt wedge estuaries • 4 types of estuaries • Dominated by high river runoff – Classified by pattern of vertical stratification – Large river mouths – Salinity is the most important factor • Columbia & Mississippi • Unlike temperature dominance in most oceans – Smaller river mouths entering larger estuaries • Change in salinity between upper and lower layers • Duwamish entering Puget Sound – See table in lab manual p. 38. – Named for sharp boundary between river & sea – Vertical salinity gradient is a balance water • River flow creates the stable vertical stratification • Strong halocline moves back and forth with tides • Mixing due to tidal action disrupts stratification. • Strong vertical stratification Ocean 200 Spring 2008 Puget Sound Estuary © 2008 University of Washington Ocean 200 Spring 2008 Puget Sound Estuary © 2008 University of Washington

Salt wedge Fjord estuaries estuaries • Dominated by high river runoff • Like a salt wedge but some special traits – Weak vertical mixing, strong stratification – Carved by glacier: Deep, narrow, straight – Strong halocline “wedge” moves with tide • U-shaped cross section vs. V-shaped river valleys • Moves up river channel for some distance on flood – Shallow area called a sill at the mouth • Moves out into open water on flood – Weak entrainment of higher-S water

Fjord estuaries Well-mixed estuaries

– Sill blocks exchange of deep water with ocean • Low river input & strong tidal mixing • Little water movement below sill depth – Strong vertical stratification – Far from river mouth • River water runs off with little mixing – In areas of large tidal exchange, fast tidal • Nearly horizontal isohalines currents, and/or turbulence – Often shallow Ocean 200 Spring 2008 Puget Sound Estuary © 2008 University of Washington Ocean 200 Spring 2008 Puget Sound Estuary © 2008 University of Washington Partially-mixed Well-mixed estuaries estuaries • Weak vertical stratification & halocline • Rough balance between rivers & mixing – Isohalines almost vertical – 2 layers & 2-layered circulation • Move back & forth with tide • Halocline weaker & broader than salt wedge or fjord • No 2-layered net circulation • Mixing, entrainment, tidal pumping stronger – Net outflow at all depths

Flushing in Estuaries

• Gradual replacement of water in estuary – Inflowing river & ocean waters flushed to sea

– Removes pollutants, replaces nutrients & O2