The velocity of a stream depends chiefly on three elements—its gradient, its volume, and its load, (i.e., the sediment it is moving). The higher the gradient the greater the volume, and the less the load the greater the velocity. The relation between gradient and velocity is evident; that between volume and velocity is illustrated by every stream in time of flood, when its rate of flow is greatly increased. The relation between velocity and load is less obvious, but none the less definite. Every particle of sediment carried by a stream makes a draught on its energy, and energy expended in this way reduces the velocity. The draught on a stream’s energy of a particle carried in suspension is measured by its mass into the distance it would fall in a unit of time in still water. It follows that a large particle makes a stronger draught on a stream’s energy than the same amount of material in smaller pieces. It follows also that the comminution of sediment facilitates transportation in much more than a simple ratio, for not only can a given amount of energy carry more fine material than coarse, but a larger proportion of a stream’s energy can be utilized in the transportation of the fine. [Illustration: +Figs.+ 97–99.—Diagrammatic representation of the effect of increased velocity on transporting power.] =How sediment is carried.=—Coarse materials, such as gravel stones, are rolled along the bottoms of the swift streams which carry them. Their movement is effected by the impact of water. The same is true to a large extent of sand grains, especially if they be coarse. So far as concerns the material rolled along the bottom it is to be noted that a stream’s transporting power is dependent on the velocity of the water at its bottom. This is much less than the surface, or even the average velocity. The particles of fine sediments, such as silt and mud, are frequently carried by streams quite above their bottoms, as shown by the roiliness of many streams. A particle of mud is usually a small bit of mineral matter, the specific gravity of which is two or three times that of water. Why does it not sink through the water and come to rest at the bottom of the stream, or suffer transportation as the gravel does? [Illustration: +Fig.+ 100.—Diagram to illustrate the relative strength of the two forces acting on a particle in suspension. The arrows represent the relative strength of the two forces when the stream’s velocity is 5 miles per hour. No account is taken in the diagram of the viscosity of the water, or of the acceleration of velocity of fall.]