What is the River Discharge Calculator?
Calculate river discharge (stream flow) with this free river discharge calculator. Use the velocity–area method Q = A × v with channel width, mean depth, and mean velocity — the standard field technique used in hydrology. Model trapezoidal river cross-sections with bottom width, depth, and bank side slope (H:V). Estimate discharge with Manning's equation when channel slope and bed roughness are known. Results in m³/s, L/s, CFS, daily volume, and river size classification. Metric and imperial units. Runs instantly in your browser.
How to use the River Discharge Calculator
- Choose Velocity–Area, Trapezoidal Section, or Manning (Slope) method.
- Select metric (m, m/s) or imperial (ft, ft/s) units.
- For velocity–area, enter channel width, mean depth, and mean velocity.
- For trapezoidal or Manning, enter bottom width, depth, and side slope (H:V).
- For Manning, add channel slope % and roughness n; review Q, river class, and steps.
Common use cases
- Estimating stream discharge from width, depth, and current-meter velocity
- Hydrology field lab with trapezoidal cross-section geometry
- Comparing Manning-based discharge to velocity–area measurements
- Classifying whether a reach is a brook, creek, or small river
- Converting field notes to m³/s and CFS for a gauging report
Frequently asked questions
- What is river discharge?
- River discharge (Q) is the volumetric flow rate of water passing a cross-section, usually in m³/s or cubic feet per second (CFS). It equals cross-sectional area times mean flow velocity: Q = A × v̄.
- What is the velocity–area method?
- Hydrologists measure channel width and mean depth to get area A, then multiply by mean velocity (from a current meter or float) to estimate Q. It is the most common field method for stream gauging.
- How do I model a trapezoidal river cross-section?
- Enter bottom width b, water depth d, and side slope z (horizontal:vertical, e.g. 2 for 2:1 banks). Area A = d(b + zd). Top width = b + 2zd. Wetted perimeter includes sloped banks.
- When should I use Manning's equation for rivers?
- Manning's formula estimates Q from geometry, slope, and bed roughness when you cannot measure velocity directly: Q = (1/n) × A × R^(2/3) × S^(1/2). Use measured or estimated n for sand, gravel, cobble, or vegetated channels.
- How is river size classified?
- This tool gives approximate classes by discharge: brook (<0.1 m³/s), stream (0.1–10 m³/s), small/medium/large river up to major rivers (>10,000 m³/s). Ranges are illustrative; local context varies.
