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Oceanography/Cceans Rivers Interface

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Question
Hello,

Why don't oceans and seas  engulf rivers flowing into them as the fresh water of rivers mixes with the salty water of seas and oceans and the mixture which is basically salty overflows and engulfs rivers tracks transforming rivers with time into salty water streams

Thanks in advance

Answer
Hi Mahmoud,

The reason oceans/seas don't mix with rivers, thus turning them salty (which I believe is what you mean by 'engulf') is because a) rivers are at a higher elevation and b) salty water is more dense than fresh.

Take a simple case where water from the mountains flows slowly down to the ocean. Even if the fresh mountain water were the same density as the ocean water (ts actually less dense --> lighter), the ocean water wouldn't have any reason for moving up to a higher elevation, i.e., there isn't any force available to lift it above sea level. Of course, adding river water to the ocean will raise the sea level a little bit, but this is negligible and doesn't really constitute engulfing.

Also, since the fresh river water is less dense than salty ocean water, it will tend to lie on top of the ocean water (not counting sediments). Of course, in the real world, mixing due to wind or just the momentum of the flowing river water will tend to mix things up and create a 'briny' coastal patch of water (eg., as in a tidal basin), but these mechanisms wouldn't cause the ocean water to move up the river channel.

If you ever have the chance to travel to the Yucatan Peninsula in Mexico, there is a place called Xelha (pronounced 'shell-ha') where fresh water from the tropical interior meets the salty water of the Gulf of Mexico in a confined inlet. If you snorkel there, you can rise up and down between the salty lower depths (1-2 meters deep) and the fresh water on the surface. You can tell the difference by the refraction effects in the water (the salty water seems to make things blurrier). I wonder the Nile has any of these effects?

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randy patton

Expertise

Physical oceanography, surface and internal wave characteristics, ocean currents, fluid mechanics, geophysical fluid dynamics, ocean optics, coastal dynamics, modeling and simulation, data analysis, El Nino and related large scale dynamics Not an expert in marine biology (some in bioluminescence) or chemical oceanography

Experience

26 years as professional scientist for research company working mostly on Navy and other government contracts. Projects included modeling, simulations and data analysis related to Non-acoustic Anti-submarine Warfare (NAASW). Other projects included remote sensing of ocean features, statistical analysis of ship tracks, ocean optics instrumentation development, synthetic aperture radar (SAR) and sonar (SAS).

Publications
Journal of Physical Oceanography, 1984, "A Numerical Model for Low-Frequency Equatorial Dynamics" (with M. Cane)

Education/Credentials
MS Physical Oceanography, MIT, 1981 BS Applied Math, UC Berkeley, 1976

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Currently an Expert for All Experts in Advanced Math

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