What Is CFS? Understanding River Flow Measurement

CFS stands for cubic feet per second — the volume of water flowing past a fixed point every second. It's the standard unit of streamflow measurement in the United States, used by USGS in all its data products and by water managers, engineers, hydrologists, and river users across the country.

The Math Behind CFS

One cubic foot is about 7.48 gallons. So 1 CFS equals roughly 449 gallons per second — or about 27,000 gallons per minute. For reference, a typical household uses about 100 gallons per day, meaning a stream flowing at 1 CFS is delivering enough water to supply about 270 homes per day.

CFS in Context

Raw numbers are hard to interpret without context. Here are some reference points:

• A small mountain trout stream in summer: 10–50 CFS
• A good paddling creek: 200–1,000 CFS
• A major river like the Colorado at Lee Ferry: typical spring runoff 15,000–20,000 CFS
• The Mississippi at St. Louis: average 175,000 CFS
• The Missouri River during the 1993 flood: over 750,000 CFS

Why Percentile Rank Matters More Than CFS

Because different rivers and seasons have wildly different normal flows, raw CFS numbers are most useful when compared to historical norms for the same station and calendar date. That's exactly what percentile rank provides. A river running at 500 CFS might be "Very High" for a small spring creek but "Very Low" for a major western river. Always interpret CFS alongside the percentile label.