The depth of the epilimnion is between 5 m and 20 m and is usually deepest in fall because of cooler air temperatures and an increase in mixing generated by storms. Annually water temperatures do not vary by more than 1 degrees C below a depth of 80 m. The average pH of the entire water column of Crater lake is 7.5. Average near-surface pH ranges from about 7.6 to 7.7, whereas the pH decreases with increased lake depth and is about 7.3 at a depth of 550 m. Both variables increase slightly with increasing lake depth.
Total phosphorus is nearly uniform in concentration throughout the water column.
Secchi disk clarity readings are generally in the mid-high 20s and low 30s each year from June through September. For example, in 1997 the readings from late July through August were the deepest recorded during the study period, whereas Secchi disk readings in late July 1995 were the shallowest readings from 1982-1999.