The transporting power of running water is dependent upon the velocity of the current, and both mathematical and experimental treatment of the problem brings out the surprising result that the transporting power varies directly as the sixth power of the velocity. If the rapidity of a stream be doubled, it can carry 64 times as much as before. The destructiveness of sudden and violent floods is thus explained. In the terrible flood which overwhelmed Johnstown, Pennsylvania, in 1889, great locomotives and massive iron bridges were swept off, it is hardly an exaggeration to say, like straws, and huge boulders carried along like pebbles. The formula as to the relation of velocity to transporting power refers more particularly to the coarser materials which are pushed along the bottom of the stream. No relation has yet been determined for very fine particles of silt and clay, some of which remain suspended indefinitely even in still water. Transporting power also increases as the temperature of the water decreases.

It obviously follows from the relation obtaining between velocity and transporting power, that a slight increase in the rapidity of a stream will largely augment the load which it carries, provided the stream obtains as much material as it can transport, while a slight reduction of velocity will cause the deposition of a large part of that load. The buoyancy of water adds, in an important degree, to its ability to sweep along sediment, because when any substance is immersed in water, it loses weight to an amount equal to the weight of an equal bulk of water. The specific gravity of most rocks is from two and one-half to three, so that when immersed they lose from one-third to two-fifths of their weight in air. The shape of the fragments is likewise a factor in determining the velocity requisite to move them; the larger the surface of the fragment in proportion to its weight, the more easily it is carried in suspension. Thus flat grains or scales are carried farther than round ones; while, on the other hand, rounded fragments are more easily rolled along the bottom, when too heavy for the current to lift.

The greater part of the debris or sediment which a stream carries is furnished to it by the destructive activity of the atmosphere; the rains wash in the finer materials, while frost and land-slips bring in the larger masses which are carried down by mountain torrents. To this material the river adds that which is derived from its own work in the cutting away of its banks and bed.