Stream Discharge
Volume of water flowing past a point per unit time.
Formula
Variables
Example
A=20 m², v=1.5 m/s → Q = 30 m³/s
How it works
Stream discharge tells you how much water passes a fixed cross-section of a river or channel each second. It is found by multiplying the size of the flowing area by the average speed of the water because a larger channel or faster flow lets more water move past in the same amount of time. This works when the flow is roughly steady and the velocity used is the average across the whole cross-section.
When to use it
Use this formula when estimating river flow, flood runoff, or water moving through canals and pipes. It is common in hydrology, environmental science, and flood-risk calculations.
Worked example, step by step
A small river has a cross-sectional area of 12 m² and an average water velocity of 2.2 m/s. Q = A × v Q = 12 × 2.2 Q = 26.4 m³/s So the stream discharge is 26.4 cubic metres per second.
Common mistakes to avoid
- Using the river width alone instead of the full cross-sectional area
- Forgetting that velocity must be the average speed across the section, not a single point measurement
- Mixing units, such as using area in cm² or velocity in km/h without converting first
Did You Know?
The Amazon discharges about 209,000 m³/s — roughly a fifth of all river flow on Earth.
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