True: River Mouths Have Lower Salinity

True. Where rivers discharge fresh water into the ocean, surface salinity drops measurably.

The Mechanism

• River water: ~0.5 PSU (practical salinity units) or less.
• Open ocean: ~35 PSU average.
• Mixing zone (estuary/plume): Gradient from <5 PSU near the mouth to ~30+ PSU offshore.

Real-World Examples

• Amazon Plume: Freshwater lens extends 100s of km into Atlantic; salinity <10 PSU over vast area.
• Mississippi Delta: Seasonal low-salinity plume across Louisiana shelf.
• Baltic Sea: Effectively a giant estuary; surface salinity 2–10 PSU due to high river input + restricted exchange.

Ecological Impact

Low-salinity plumes create unique habitats: nursery grounds for fish, zones for estuarine species, barriers for stenohaline (salt-requiring) organisms. They also stratify the water column (fresh over salt), affecting oxygen and nutrient cycling.

So yes—river mouths are the ocean's "freshwater lenses," and salinity maps show them clearly.