Why are the Little Colorado River and Havasu Creek blue?
The color of the water is determined by depth and mineral composition. Calcium carbonate is carried in the Little Colorado river water column and coats the river bottom. Calcium carbonate scatters white light (aka, looks white), so combined with water, which scatters blue light, a turquoise color is formed. When the water column is thin, white scattering of the calcium carbonate is more pronounced and the color of the water is lightened. In the deeper parts of the river, the color is a darker blue due to the smaller concentration of calcium carbonate (due to more volume of water) and the increased scattering of blue light through a thicker water column. 

This blog has a great explanation.