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Water
The place where the lower end of a river meets the ocean is called an estuary (ES-choo-AIR-ee). What do you think happens when the rush of freshwater collides with the gush of the ocean's tides? Do the two kinds of water instantly mix? Does the freshwater float on top of the salt water? Or does the salt water float on top?
So What are Salinity and Density?
Before you begin, it is necessary to understand two important differences between freshwater and seawater.
One is salinity (suh-LIN-ih-tee), which means how salty a solution is. As you'd probably guess, ocean water has greater salinity than freshwater.
Another important difference between freshwater and seawater is their density. Density refers to the amount of "stuff" in a given space or packed in a space. For example, a 10 inch round pan containing a fluffy angel food cake would have a lower density than the same size pan filled with a fudge cake.
The salinity of water affects its density. The saltier water is, the denser it is. Which is denser — a cup of ocean water or a cup of freshwater? To help you answer this question, look at the pictures to the right.
Freshwater is made up of water molecules with a little bit of salt in it. Ocean water is made up of water molecules with a lot of salt. When put in the same container, denser materials fall to the bottom and less dense materials sit on top.
What happens when freshwater and saltwater collide?
Super Salty Sea
The Dead Sea, located in the Middle East, between Jordan and Israel, contains some of the saltiest water in the world. It's almost six times as salty as the ocean. Because of the Dead Sea's high salinity, no plants or animals live there. Since human bodies have a lower density than the water of the Dead Sea, people can float in it effortlessly. The extremely salty water holds people up instead of letting them sink.
Image Credits:
Photos: Dead Sea: courtesy of Frieda. All other photos courtesy of AMNH.