In a preceding chapter we learned that the power of a stream of water to transport sediment depends upon its velocity, which, in its turn, is determined by the slope of the ground and the volume of water. Further, we discovered the very surprising fact that, for the coarser material which is pushed along the bottom, the transporting power increases as the sixth power of the velocity (T =V6). It follows from this that a slight decrease in the swiftness of a stream will cause it to throw down the greater part of its load of sediment, while a slight increase will cause it to carry off what it had before deposited. Thus, great rivers, like the Mississippi, which flow in soft, easily moved deposits, are preeminently whimsical and treacherous. As the volume and velocity of the stream are much subject to change, there will obviously be corresponding changes in the scour and deposition at any given point, but there are certain places where deposition is so constant that extensive accumulations may be formed there. As we trace a river downward from its source in a mountain region, we find that in the upper stream, which is a torrent in swiftness, only large stones remain at rest, everything else being swept along.
Farther down stream, as the slope of the bed diminishes, the coarse gravel is thrown down, next the coarse sand is deposited, and in the lower reaches of a river, which, like the Mississippi, flows over land that has a very gentle slope, and is raised but little above the sea-level, only the finest silt gathers on the bottom. The exact limits of the different kinds of deposit will vary with the stage of water..
Fig. 91. - Manti Creek, Utah; flood of August, 1901.
At points where the velocity of the stream meets a constant check, there will be constant deposition, and thus bars and islands are built up in the channel, which will be permanent unless some change of conditions is brought about. In the sand-bars and gravel-spits the up-stream side is a gentle slope, ending abruptly on the downstream side, the bar or spit advancing by having sand or gravel pushed up the gentle slope by the current and dropped over the steep face, where it forms inclined layers. Flattened and elongated pebbles arrange themselves so as to offer the least resistance to the current, in a slanting position, with their tops down stream. When the stream is subsiding, the material tends to assume a more horizontal direction, giving an irregular and confused stratification to these deposits.
Fig. 92. - Effects of flood; Black Hills, S.D. (U. S. G. S).