Percentile Rank Explained: Is the River High or Low Right Now?

The most useful question you can ask about a river is not "what's the CFS?" — it's "is the flow high or low compared to normal?" Percentile rank answers that question directly.

How Percentile Rank Is Calculated

For each USGS gauge, statisticians have computed the distribution of daily mean discharge for each calendar day, based on the full period of record (often 50–100 years). The percentile rank of today's flow tells you what percentage of historical years on this calendar date had lower flow.

A percentile rank of 50 means today's flow is right at the median — perfectly normal for this time of year. A rank of 25 means 75% of historical years on this date had higher flow — the river is running low. A rank of 90 means only 10% of years were higher — the river is very high.

The Five Categories

StreamFlowData.com uses five categories based on percentile rank:

• Very Low: Below the 10th percentile. Extremely low conditions, often related to drought.
• Low: 10th–25th percentile. Below-normal conditions.
• Normal: 25th–75th percentile. Typical range for this time of year.
• High: 75th–90th percentile. Above-normal conditions.
• Very High: Above the 90th percentile. Exceptionally high conditions, may indicate flood risk.

Practical Applications

For fishing: "Normal" to "Low" flows often provide ideal conditions for wade fishing. "Very High" may wash out holding areas and make wading dangerous. For kayaking: check the specific river's recommended flow range — Class III rapids might become Class V at "Very High" flows. For flood monitoring: watch for stations moving from "High" toward "Very High" and monitor gage height relative to flood stage thresholds.