MEASURING STREAMFLOW

Southwestern Crown of the Continent Community Stream Monitoring Project

Remember, to measure flow we need:

Area (Depth x Width) x Velocity

We will measure depth, width, and velocity at a bunch of different spots all across the stream, and sum their products for the total flow (discharge).

Measuring Water Velocity

We’re going to use a flow meter attached to a top-setting rod that measures speed by recording how fast the water makes a propeller turn:

Start by putting stakes on each side of the stream and attaching the tape measure to it so that it stretches across the entire stream. Record the stream width.

Note that this special tape measures in tenths of feet, rather than inches!

Where to measure? How many measurements?

• Generally, there should be at least 20 measurements across the stream.

• Divide the stream width by 20 to calculate the distance between points. If the stream is relatively uniform in depth and velocity, or if it is a very narrow stream, we can use fewer. In very small streams we can measure at 1-foot, or one-half-foot intervals even if we only have five or ten measurements.

• Measuring points should be closer together wherever there is a lot of variation in the depth or velocity of the cross section.

Start at the same side of the stream each time (for example, the west bank (“west water’s edge”). At this first spot at the very edge of the stream the depth should be zero. Move to the next measuring point. At each measurement point, record the distance from shore, the water depth, and the velocity on the data sheet.