Why are rivers in the Colorado River Basin salty?

Why are rivers in the Colorado River Basin salty?

Background: The Virgin River is a tributary to the Colorado River located in Utah and Arizona. Millions of years ago, the Colorado River Basin was a shallow sea, so the rocks and soils have some naturally occurring salt deposits. When rain and irrigation water filters through the earth, it dissolves some of the salt and carries it to the river. This means that rivers will have different amounts of salt in them depending on where the water is coming from. Snowmelt will have relatively little salt while runoff from rain and groundwater sources will be higher in salt. 

The graph shows the average monthly salt content (salinity) for the Virgin River in each month from 1937-2011. The concentration is in milligrams of salt per liter of river water.

Data Source: http://www.usbr.gov/uc/progact/salinity/

Questions: 

1.  Describe what the graph shows about how the salinity of the Virgin River changes throughout the year.

2. I interpret the graph to mean….  

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