Width
Extend a tape measure from the point where the dry bank meets the water on one side of the river to the same point on the other side (hold it taut about 20cm above water level). Record the length, viewing the reading from directly above the tape measure.
Average depth (across channel)
Immerse a metre ruler (edge facing upstream to minimize exposure to flowing water) into the water until it just touches the riverbed. Record the distance from the bed to the water surface. Repeat at regular intervals (e.g. every 50cm) across the channel.
Velocity
Measure a set distance of the river (up to 10m). Mark the start and end points. Put the float (i.e. a brightly coloured object that floats in water) slightly upstream of the start point. With a stopwatch, time how long it takes the float to travel from start to end. Repeat at least five times, releasing the float at regular intervals across the stream to measure velocity across the channel. Disregard any anomalous results (e.g. if the float gets stuck).
Discharge (cumecs or cubic metres per second)
Multiply the cross-sectional area* (square metres) by its velocity (m/s)
*width x depth
Annissa's IGCSE & IB Notes 