The diameter, in millimeters, of suspended sediment or bed material. Particle-size classifications are: (1) Clay: 0.00024 - 0.004 millimeters (mm); (2) Silt: 0.004 - 0.062 mm;(3) Sand: 0.062 - 2.0 mm; and (4) Gravel: 2.0 - 64.0 mm