These deposits are laid down by slackening currents where the velocity of the stream is checked, as on the inner side of curves, and where the slope of the bed is diminished, and in the lee of islands, bridge piers and projecting points of land.  How slight is the check required to cause a current to drop a large part of its load may be inferred from the law of the relation of the transporting power to the velocity.  If the velocity is decreased one half, the current can move fragments but one sixty-fourth the size of those which it could move before, and must drop all those of larger size.

Will a river deposit more at low water or at flood? when rising or when falling?

Stratification.  River deposits are stratified, as may be seen in any fresh cut in banks or bars.  The waste of which they are built has been sorted and deposited in layers, one above another; some of finer and some of coarser material.  The sorting action of running water depends on the fact that its transporting power varies with the velocity.  A current whose diminishing velocity compels it to drop coarse gravel, for example, is still able to move all the finer waste of its load, and separating it from the gravel, carries it on downstream; while at a later time slower currents may deposit on the gravel bed layers of sand, and, still later, slack water may leave on these a layer of mud.  In case of materials lighter than water the transporting power does not depend on the velocity, and logs of wood, for instance, are floated on to the sea on the slowest as well as on the most rapid currents.

Cross bedding.  A section of a bar exposed at low water may show that it is formed of layers of sand, or coarser stuff, inclined downstream as steeply often as the angle of repose of the material.  From a boat anchored over the lower end of a submerged sand bar we may observe the way in which this structure, called cross bedding, is produced.  Sand is continually pushed over the edge of the bar at b (Fig. 42) and comes to rest in successive layers on the sloping surface.  At the same time the bar may be worn away at the upper end, a, and thus slowly advance down stream.  While the deposit is thus cross bedded, it constitutes as a whole a stratum whose upper and lower surfaces are about horizontal.  In sections of river banks one may often see a vertical succession of cross-bedded strata, each built in the way described.

Water wear.  The coarser material of river deposits, such as cobblestones, gravel, and the larger grains of sand, are water worn, or rounded, except when near their source.  Rolling along the bottom they have been worn round by impact and friction as they rubbed against one another and the rocky bed of the stream.

Experiments have shown that angular fragments of granite lose nearly half their weight and become well rounded after traveling fifteen miles in rotating cylinders partly filled with water.  Marbles are cheaply made in Germany out of small limestone cubes set revolving in a current of water between a rotating bed of stone and a block of oak, the process requiring but about fifteen minutes.  It has been found that in the upper reaches of mountain streams a descent of less than a mile is sufficient to round pebbles of granite.