You now obtain a water sample of the same river from a specific depth, 10 cm above

Question:

You now obtain a water sample of the same river from a specific depth, 10 cm above the bed, and have its grain-size distribution analyzed.
There are three primary grain diameters involved (all quartz; assume that they are all essentially spherical), and their sizes and concentrations at this level in the flow are as follows:

D1 D2 D3 0.25 mm 0.17 mm 0.01 mm 45 mg/L 30 mg/L 20 mg/L