How Salty Are Estuaries?

Estuaries—where rivers meet the sea—are brackish environments with salinity ranging from 0.5 to 30 parts per thousand (ppt). For reference: freshwater <0.5 ppt; seawater ~35 ppt.

Salinity Drivers

• River Discharge: High flow → lower salinity (freshwater dominance).
• Tidal Exchange: Flood tide pushes saltwater upriver; ebb tide flushes it out.
• Evaporation: Increases salinity, especially in hot, arid climates (e.g., Laguna Madre, TX).
• Precipitation: Direct rain and watershed runoff dilute salinity.
• Geometry: Depth, width, sill depth, and stratification affect mixing.

Temporal Variability

• Daily: Tidal cycles cause fluctuations of 5–15 ppt at fixed points.
• Seasonal: Wet vs. dry seasons shift the salt wedge kilometers up/down river.
• Event-driven: Storms/floods freshen the system; droughts allow salt intrusion.

Ecological Significance

Salinity gradients create distinct habitats: oligohaline (0.5–5 ppt), mesohaline (5–18 ppt), polyhaline (18–30 ppt). Species distribute according to tolerance (e.g., oysters: 10–30 ppt; Spartina alterniflora: 0–35 ppt). Estuaries are nurseries for 75% of US commercial fish/shellfish.