Estimate a stream’s flow by multiplying its cross-section area by its surface speed (timed with a float over a measured reach) and by a correction of about 0.85, because water at the surface moves faster than the average. A 3 m wide stream averaging 0.4 m deep, with a float covering 10 m in 12 seconds, carries roughly 0.85 cubic metres per second, about 850 litres per second.
Field estimation of discharge, bank condition and flood marks for a stream or river that may supply a settlement.
Discharge Is Area Times Velocity, Corrected for the Surface
Flow (discharge) is the volume passing a point each second: Q = A x v. Area A is the cross-section of the water, width times average depth. Velocity v must be the average speed of the water column, but a float only tells you the speed at the top. Friction with the bed and banks slows the water below, so average speed is lower than surface speed. A correction factor near 0.85 is commonly used for surface floats (reported range roughly 0.8-0.9, depending on how rough the bed is). Treat the result as an estimate good to perhaps a third, not a measurement; it is for ranking sites and checking the dry-season minimum, not for designing a dam.
What You Need
- Tape, rope with knots, or a marked pole — for width and depth.
- A float — an orange, a stick, or a piece of bark that floats mostly submerged.
- A watch or counted seconds — and a straight reach of 10 m or more with steady flow and no eddies.
- A second person — one to release, one to time at the downstream mark.
Method: Float Gauging
Step 1: Choose a straight reach Pick a section with parallel banks and no rapids or pools. Mark two points 10 m apart along the bank.
Step 2: Measure the cross-section Stretch the rope across the channel at the middle of the reach. Measure the width. Measure the depth at least five evenly spaced points from bank to bank (more if the bed is uneven), count each bank as zero, and average. Area = width x average depth.
Step 3: Time the float Release the float well upstream of the first mark so it is moving at stream speed. Time it between the marks. Repeat at least three times and average. Speed = distance / time.
Step 4: Compute Q = A x surface speed x 0.85.
Worked Example
- Width 3 m, mean depth 0.4 m: A = 3 x 0.4 = 1.2 m2.
- Float over 10 m takes 12 s: surface speed = 10 / 12 = 0.83 m/s.
- Mean speed = 0.83 x 0.85 = 0.71 m/s.
- Q = 1.2 x 0.71 = 0.85 m3/s = 850 L/s.
Compare that with demand. A settlement of 150 people (see Settlement Layout for 150 People) at the 15 L per person per day survival standard needs 150 x 15 = 2,250 L per day = 2,250 / 86,400 = 0.026 L/s. The stream carries about 33,000 times as much (850 / 0.026). A stream does not have to be anything like this large to be enough. What matters is whether the dry-season flow is still comfortably above the need, and whether the water is clean.
Reading the Bank for What a Single Visit Cannot Show
- Stain and algae lines on rocks above the waterline show the usual range.
- Debris lines in trees and fences show the flood height, which this estimate does not give.
- Undercut banks and fresh gravel bars show a channel that moves.
- Dry bed or isolated pools upstream show a stream that stops.
For flood sizing, which a low-flow float cannot do, see Watershed Position & Flood Risk and Flood Planning.
Common Mistakes
- Forgetting the 0.85 — the raw surface speed overstates flow by about 18% (1 / 0.85).
- Using the deepest point as the depth — use the average across the channel.
- Gauging a wet-season stream and quoting it as the supply — repeat at the end of the dry season, which is the number that matters.
Safety
Do not wade swift or murky water
Do not wade a stream where you cannot see the bottom, where the water is above knee height, or where it moves faster than a brisk walk. Stand on the bank and cast the float. Streams rise quickly after rain upstream even when your own sky is clear.
What’s Next
- Learn next: Watershed Position & Flood Risk — how big the same stream can become
- Combine with: Surface Water Sources — scoring the whole candidate