The velocity of a river current varies with the seasons on account of the different volumes of water carried, but it also varies at different points of the river, during the different hours of the day, through the constant action of the laws of erosion and deposition. The banks not furnishing a constant, even resistance, erode in the looser parts, causing each time a deflection of the current, as well as a change in the velocity. This action soon changes a straight stream with an even current, to a serpentine course with currents of different velocities, as the outside of a curve has always greater speed than the inside; consequently the outer edge is eroding while the inner is depositing sediment. The Colorado River, in the lower part of its course from Yuma down, overflows its flood plain, and deposits thereon a certain amount of sediment during every high water. This flood plain extends back for several miles on the west bank, showing a succession of benches or levees running parallel with the stream on a higher level than the back country. The water, when high enough to reach over these benches, gathers in the back part, where it evaporates, leaving a mud that greatly enriches the soil. These levees are the result of the law quoted above, as the water in the regular channel flows swifter than on the flood plain; consequently on the border of the two a large amount of sediment is thrown down. As these border lands are only covered during the highest floods, they maintain a growth of willows and poplars that gives the banks a greater stability, and preserves them to a certain extent from the erosive force of the current. A source of changes in the direction of the main current is the stranding of logs and tree tops brought down from the upper courses of the river, as the sediment accumulates behind them and forms islands in midstream. Several of these are to be seen in the river channel in passing down the stream. From soundings made across a section of the river at the railroad bridge at Yuma, the depth was found to vary from 2½ feet to 21 feet. At the meter placed in the main channel the indicator showed a depth of 17 feet. At ordinary high water, caused by the annual rising of the Gila River, or of the Colorado in its upper reaches, which events usually occur at different seasons of the year, the meter has not been known to show a depth of over 28 feet, but in consequence of a co-equal rise in both streams during the past season, the meter registered over 33 feet. This excess filled the river swamp on the right bank, where the erosive force of the current was most effective.