Crater Lake does not have feeder creeks or rivers so the water is pure, blue and largely made up of snowmelt and rainwater.  Crater Lake is one of the clearest lakes in the world because the volcanic terrain limits the amount of nutrients and particles within the lake.  The volcanic terrain does not allow for streams to come in or out of the lake.  The currents distribute the few nutrients.   Very little organic material washes in from the sides of the lake.
 

The divers can see almost 150 feet away. Light goes down to about 600 feet within the lake. Scientists are able to record clarity readings of 120 feet using a Secchi disk which is a type of reflector.  On June 25, 1997, scientists were able to obtain a clarity reading of 142 feet, which is a clarity reading.
 

The water remains at a constant level because the evaporation and seepage from the lake equals the annual snowmelt and rainfall.  Most of the rain falls directly onto the lake.  The snowmelt comes down the sides of the crater and runs over nutrient depleted volcanic walls. 
 

 

The volcanoe is not dead but is sleeping.  Scientists have discovered pools of super-heated water on the lake floor and molten material lurks below the bottom of the lake.  Huge extensive hydrothermal vents are down at the bottom of the lake and some rise up 50 ft from the bottom of the lake.
 

Scientists closely monitor the water for chemicals, and any alarming changes in the aquatic environment of the lake.