How many gallons of water are in the Great Salt Lake?

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To answer this, a couple of assumtions are made. The average depth is 4.3m, and the surface area is 4400 km². Assuming these numbers are static (which they are NOT due to rainfall and heat evaporation), and ignoring the fact that the shoreline changes with depth of the water, the computation follows.

4.4(6) * 4.3 = 18.92(6) cubic metres
1 gallon = 3.78l
1 litre = .001 cubic metre
18.92(6) cu. metre = 18.92(9) litres

18.92(9) litres equals just a hair over 5 billion gallons, close enough that the rounding to 2 sigfigs wipes it out.
Sorry, I am not as versed in metric, but the answer is actually 5 trillion (not billion) gallons. Wikipedia gives the volume as 18.92 cubic kilometers, which is 4.5 cubic miles. A cubic mile contains 1.1 trillion gallons, which would make the volume of 4.5 cubic miles about 5 trillion gallons. Wikipedia also gives the volume as 15.3 million acre-feet. 15.3 million x 43,560 square feet in an acre = 666 billion cubic feet. 1 cubic foot of water = 8.1 gallons. 666 billion cubic feet x 8.1 gallons = 5.4 trillion gallons. Caveats regarding the changing volume of the lake due to weather conditions apply.
I indeed make a mistake. The conversion from 4400 km² to m² was inaccurate. It is 4.4(9) instead of 4.4(6). This makes the final answer 18.92(12) which does come to 4.998 trillion gallons.
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Why is the water in the Great Salt Lake so salty?

It was part of Lake Bonneville, which was part of the sea. As land grew higher the water gathered down. There is no outlet to the lake; it's virtually an inland sea. The sa