If we watch where a torrent enters a mountain river, we observe that the main stream in a way sorts over the waste contributed to it, bearing on only those portions which its rate of flow will permit it to carry, leaving the remainder to be built into the bank in the form of a rude terrace. This accumulation may not extend far below the point where the torrent which imported the _debris_ joins the main stream; a little farther down, however, we are sure to find another such junction and a second accumulation of terrace material. As these contributions increase, the terrace accumulations soon become continuous, lying on one side or the other of the river, sometimes bordering both banks of the stream. In general, it can be said that so long as the rate of fall of the torrent exceeds one hundred feet to the mile it does not usually exhibit these shelves of detritus. Below that rate of descent they are apt to be formed. Much, however, depends upon the amount of detritus which the stream bears and the coarseness of it; moreover, where the water goes through a gorge in the manner of a flume with steep rocky sides, it can urge a larger amount before it than when it traverses a wide valley, through which it passes, it may be, in a winding way. At first sight it may seem rather a fine distinction to separate torrents from rivers by the presence or absence of terraces. As we follow down the stream, however, and study its action in relation to these terraces, and the peculiar history of the detritus of which they are composed, we perceive that these latter accumulations are very important features. Beginning at first with small and imperfect alluvial plains, the river, as it descends toward the sea, gaining in store of water and in the amount of _debris_ which comes with that water from the hills, while the rate of fall and consequent speed of the current are diminished, soon comes to a stage where it is engaged in an endless struggle with the terrace materials. In times of flood, the walls of the terraces compel the tide to flow over the tops of these accumulations. Owing to the relative thinness of the water beyond the bed, and to the growth of vegetation there, the current moves more slowly, and therefore lays down a considerable deposit of the silt and sand which it contains. This may result during a single flood in lifting the level of the terrace by some inches in height, still further serving to restrict the channel. Along the banks of the Mississippi and other large rivers the most of this detritus falls near the stream; a little of it penetrates to the farther side of the plains, which often have a width of ten miles or more. The result is that a broad elevation is constructed, a sort of natural mole or levee, in a measure damming the flood waters, which can now only enter the "back swamps" through the channels of the tributary streams. Each of these back swamps normally discharges into the main stream through