What gives it is bright blue color?

Scientists have investigated Crater Lake for more than a century. They have plumbed its depths, scaled its cliffs, and explored it with sonar, submarines, and scientific instruments. The eruption that collapsed Mount Mazama was the largest in North America for hundreds of thousands of years. The eruption was 100 times as powerful as the eruption of Mt. St. Helens in 1980. It spewed enough ash to cover the state of Oregon in 8 inches of ash. The lake that formed in its shattered remnants is one of the purest and most pristine in the world. The clarity of the lake allows the refraction of light to go to depths of over 120 feet, thus giving the lake its constant bright blue color. Public boating is not allowed on Crater Lake in order to keep it pristine. We did not realize that the clarity of the lake was its most significant feature. It is said to be the clearest lake in the world!