A matter of turbidity

Abstract

Have you ever stood by a river swollen after rain? If the river lies in the mountains it is quite likely that pebbles and cobbles are being carried over the bed. Above the rush of the invigorated current, you may hear a rapid succession of hollow-sounding crashes and bangs, caused by frequent impacts between stones in motion and between these and the stationary bed. You would be justified in concluding that you were listening to the stream’s bedload, but it is most unlikely that you would be able to see that load, on account of the turbidity of the water. What makes it turbid? Dip a jar into the stream and let the contents settle. You will find that the turbidity is due to the presence of particles of mainly silt and clay size, perhaps with a little relatively fine sand. The contrast in texture with the bedload is striking. You will also notice that the settled grains form only a thin layer at the bottom of the jar. The fine particles were therefore widely dispersed and at a very low concentration when in the river, in sharp difference from the coarse bedload debris. Finally, your direct observation of the bedload was prevented because, as you will have noticed, the fine sediment was dispersed throughout the whole body of the stream. It constitutes the suspended load of the river.