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📘 At the river mouth, fresh water meets the sea

Stand on a river delta and watch fresh water meet salty seawater. The river brings invisible dissolved minerals, not just visible mud. The ocean receives both.

4
lessons
~20 min
to learn
Adults
level
Start the course →

What you’ll learn

  1. A drop takes a long routeTrace how weathering, rivers, evaporation, and seafloor chemistry make the ocean salty.Dissolved minerals enter from land and seafloor sources while water cycles away and removal processes keep salinity in balance.
  2. Most lakes have an exitExplain why connected lakes usually remain fresh despite receiving dissolved minerals.Outlets, seepage, and ongoing inflow export dissolved material from most lake basins.
  3. Closed basins tell the exceptionUse terminal lakes to test the rule about water exits and salt accumulation.Closed lakes lose water mainly by evaporation, so dissolved minerals can concentrate into brines.
  4. Salinity is a moving balanceCompare salinity changes caused by evaporation, freezing, inflow, chemistry, and mineral storage.Salinity is a changing measure of dissolved ions shaped by the whole water cycle and Earth system.

Questions this course answers

Why do rivers contribute to ocean salinity even when the rivers taste fresh?

Weathering puts dissolved minerals into river water, which can carry them to the ocean.

A typical freshwater lake contains no dissolved minerals.

Freshwater is relatively low in dissolved solids, not free of them.

Why can a terminal lake become saltier than the ocean?

A closed basin has little or no surface outflow, so evaporation concentrates salts.

How can sea ice formation affect the salinity of nearby seawater?

Phase changes redistribute salt and freshwater, changing density and local salinity.

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