Surveying the bedload of the river is best done while measuring the dept when doing a cross-section of the river. You are already needing to take measurements at intervals across the river, so why get wet twice? Do the bed load sampling at the same time.
The theory says that water will move any bed load that it has the power, or competence, to move. It’s also assumed that faster water can move larger load, so whilst very slow-moving water can move fine sediment, it takes faster water to move small pebbles, and much faster water to move large rocks.
So, in theory, what you find on the bottom can tell you something about the water velocity.
Also, the roundness of the load tells you about its history. Broken pieces of rock start off very angular, but constant bumping into other rocks wears off the edges and makes them smooth. By examining the smoothness or roughness of the load and knowing a little about the type of rocks involved, you can say something about the power of the river, and the erosion rates for the bed load.
Power’s Index of Roundness Chart
Rules or callipers to measure length and width
Notebook and pencil
At set intervals across the river reach down and pickup a sample of bed load directly below your marker. Don’t hunt around the edges of a large chunk though. If you find a huge rock or a tiny pebble, go with it or your survey won’t represent what’s actually there.
Measure and record the length and width of the sample.
Record the dimensions in your notebook’
Compare the sample to the illustrations in the Power’s Roundness Index.
Power’s Index of Roundness divided samples into Classes and also describes them in words.
Record the degree of roundness in your notebook.
A really simple table you can use to record bedload size and roundness across the river