How much snow does it take to make a gallon of water

Depending on the type of snow, about 10 gallons of snow makes 1 gallon of water. The snow volume will be about 10 times the volume of water.

http://www.answers.com/Q/How_much_snow_does_1_gallon_of_water_make

If the snowfall amounts were translated into equivalent volumes of water - then how much water would that be? Using a rule of thumb that each 10 inches of snow, if melted, would produce one inch of water, then each inch of snow produces about 2,715 gallons of water per acre. Of course, the actual amount can vary considerably depending on whether the snow is heavy and wet or powdery and dry, so this is based on the 'average' water content of snow.
Heavy, wet snow has a very high water content and 4 or 5 inches of heavy, wet snow can contain about one inch of water, while it may take 20 inches of dry, powdery snow to equal one inch of water. The 10=1 equation also assumes a 'perfect' snowmelt without evaporation or other losses.

http://www.scienceiq.com/facts/howmuchwaterinaninchofsnow.cfm