What This Solves
Estimates peak flood flows at ungauged sites using USGS regional regression equations based on drainage area and basin characteristics.
Best Used When
- You need a peak flow estimate at a location without a stream gauge
- You want a quick screening-level estimate of flood flows for planning or preliminary design
- You are comparing regression-based estimates with other hydrologic methods as a reasonableness check
Do NOT Use When
- A stream gauge with adequate record length exists at or near the site — Use Flood Frequency Analysis Calculator
- The drainage area is small enough for the Rational Method and you have IDF data — Use Rational Method Calculator
Key Assumptions
- Regional regression equations are valid within the hydrologic region where they were developed
- Basin characteristics (area, slope, precipitation) are within the range of the calibration dataset
- The regression equations account for average regional conditions and may not capture site-specific factors
- Standard errors of prediction are inherent in regression estimates (typically 30-60%)
- Urbanization adjustments may be needed if the regression equations were developed for rural basins
Input Quality Notes
Drainage area is typically the most important predictor variable. Measure it from the best available topographic data (USGS maps or LiDAR-based DEMs). Check that you are using the correct regional equation set for your state and hydrologic region.
Screening-Level Tool
This calculator provides preliminary estimates only. Regional regression equations give approximate peak flows for ungauged sites. For final design, use state-specific USGS publications or USGS StreamStats.
About USGS Regional Regression
Regional regression equations estimate peak streamflow at ungauged sites based on statistical relationships developed from streamgage data. The USGS has developed equations for all 50 states based on regional hydrologic characteristics.
The general equation form is:
Q = a × DAb × Sc × Pd × ...
Where Q is peak discharge, DA is drainage area, and other variables may include channel slope (S), mean annual precipitation (P), basin storage, impervious area, and forested area.
Hydrologic Regions Overview
| Region | Description | DA Range (sq mi) |
|---|---|---|
| Northeast Highlands | Mountainous terrain with moderate precipitation, mixed forest cover | 0.1 - 1000 |
| Coastal Plain | Low-gradient terrain with sandy soils, higher water table | 0.05 - 500 |
| Piedmont | Rolling hills with clay soils, moderate forest cover | 0.1 - 800 |
| Appalachian Plateau | Mountainous terrain with high precipitation, steep valleys | 0.2 - 1500 |
| Great Lakes / Upper Midwest | Glaciated terrain with moderate topography, abundant lakes | 0.1 - 2000 |
| Central Plains | Low relief agricultural land, seasonal precipitation | 0.5 - 5000 |
| Southwest Semi-Arid | Arid to semi-arid terrain with ephemeral streams | 0.1 - 3000 |
| Pacific Northwest | High precipitation coastal and mountain terrain | 0.1 - 2500 |
Note: These are generalized regions for educational purposes. For actual design, use state-specific USGS publications or StreamStats.
When to Use This Calculator
Appropriate Uses:
- • Preliminary/screening estimates
- • Planning-level studies
- • Ungauged site analysis
- • Comparison with other methods
- • Educational purposes
Not Appropriate For:
- • Final engineering design
- • Heavily regulated streams
- • Highly urbanized areas (>50%)
- • Areas outside DA limits
- • Critical infrastructure
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Last verified: February 2026