A percentile is a value on a scale of one hundred that indicates the percent of a distribution that is equal to or below it. For example, on the map of daily streamflow conditions a river discharge at the 90th percentile is equal to or greater than 90 percent of the discharge values recorded on this day of the year during all years that measurements have been made.  In general,

• A streamflow which is greater than the 75th percetile is considered above normal
• A streamflow which is between 25th and 75th percentiles is considered normal
• A streamflow which is less than the 25 percentile is considered below normal

In some hydrological studies, particularly those related to floods, a variation of the percentile known as the "percent exceedance" is used. It is simply obtained by subtracting the percentile scale value from 100 percent. For example, a discharge at the 75th percentile is the same as a discharge at the 25th percent exceedance (100-75=25).

The flow category "Low" indicates that the estimated streamflow is the lowest value ever measured for the day of the year. Similarly, the flow category "High" indicates that the estimated streamflow is the highest value ever measured for the day of the year.

"Not ranked" indicates that a flow category has not been computed. Common reasons for a "Not ranked" category are insufficient historical data or no current streamflow estimates.