MadSci Network: Earth Sciences

Re: Does rainwater mix with saltwater in the ocean?

Date: Mon Oct 22 15:59:50 2001
Posted By: David Kopaska-Merkel, Staff Hydrogeology Division, Geological Survey of Alabama
Area of science: Earth Sciences
ID: 1003058852.Es


Fresh water is lighter than salt water, and so fresh water floats on sea
water. Gradually the two mix and most "pools" of fresh water on the sea do
not persist for long. The most long-lived are probably the effluent from
major rivers, where large amounts of fresh river water flow out onto the
sea.  Glacial ice is also fresh, and melting ice bergs produced short-lived
freshened parts of the sea's surface.

Blue holes, as I understand it, do not open on the sea floor, but open onto
the land surface of Bahamian islands. The fresh water in the blue holes
extends below sea level because the fresh ground water within an island
forms a lens. The top of the fresh water lens is above sea level, and the
weight of this "extra" water pushes the bottom of the lens below sea level.
However, below the fresh ground water in an island there is salt water.

Fresh water can be denser than sea water. The most common way this happens
is to have a lot of sediment suspended in the fresh water, making it
turbid. Turbid water can flow along the bottom of the sea, or even at some
intermediate level within the sea, depending on the exact density
relationships among the different water masses in the sea.

Temperature also affects water density, but sea water is so much denser
than clean fresh water that temperature by itself is not enough to make
fresh water denser than sea water.

You can find discussions of these questions in a sedimentology or
oceanography textbook and many physical geology textbooks.

This is one sedimentology textbook:

Friedman, G. M., Sanders, J. E., and Kopaska-Merkel, D. C., 1992,
Principles of Sedimentary Deposits, Macmillan Publ. Co., New York, 717 p.

You can find many other books that will help you with this question at any
university library and many public libraries.

David Kopaska-Merkel
Geological Survey of Alabama
PO Box 869999
Tuscaloosa AL 35486-6999
(205) 349-2852

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