Explanation:

Highest and lowest month-end water levels are monthly extremes for the period of record
Above-normal is the 75% quartile (25% of month-end water levels were higher)
Below-normal is the 25% quartile (25% of month-end water levels were lower)
Median is the 50% quartile (half of the month-end water levels were higher or lower)
Water levels after September 2005 are provisional and subject to revision.