Fig. 453.—Saucer lakes upon the bed of the former river Warren (from the Minneapolis sheet, U. S. G. S.).
Crescentic levee lakes.—As we approach the delta of a river, the size and importance of the levee increases, and here a new type of levee lake may develop in series ([Fig. 454]). At flood time the levee is breached near the point of sharpest curvature on the convex side ([Fig. 454] a). When the waters are subsiding, the current is kept away from the old channel by the rising grade of the levee as well as by the inertia of the current, and an entrance to the old channel is first found below the next change in curvature of the meander, since here scour becomes effective in cutting through the levee. The new channel is thus established in the form of a loop inclosing the old one, and the process of levee building now erects a wall about the territory newly acquired by the meander. This territory has the form of a crescent, and when occupied by water produces a crescentic levee lake often joined to its neighbors in series. The abandoned channel now closed at both ends by levees may be occupied by water to produce a subordinate ribbon type of curving trench ([Fig. 454] b, c).
The importance of levees in obstructing drainage to form lakes is only beginning to be appreciated. It has quite recently been shown that when trunk streams are greatly swollen and burdened with sediment while flowing from a receding continental glacier, they may build such high levees as to aggrade their tributary streams above the junctions, even producing reversed grades and so impounding the waters to form extensive lakes. During the “ice age” lakes of this type were formed in Illinois and Kentucky rivers just above their junctions with the Ohio. The old lake floor with its eastern shore line and its protruding islands is easily made out upon the new topographic maps of Kentucky.
Fig. 454.—Levee lakes developed concentrically in series within meanders of a stream tributary to the Mississippi and flowing upon its delta plain. b and c are examples of the ribbon type of levee lake due to occupation of the abandoned river channel. The larger number of lakes, of which Sip Lake and Texas Lake are examples, have the form of crescents and lie between abandoned levees (from recent map of U. S. G. S.).
Raft lakes.—Within humid regions the flood plains of our larger rivers are generally forested, and as the river swings from side to side in its perpetual meanderings, the timber which grows upon the convex side of each meander is progressively undermined by the river and felled upon its bank. The prostrate trees remain upon the banks during the low water of the summer season, to be gathered up at the time of flood in the next spring season. It is log jams thus acquired which so generally block the main channel of a river and turn the current across the neck of the meander when cut-offs occur with the formation of ox-bow lakes. When the mass of timber thus gathered up by the river is excessive, as, for example, within the flood plain of the Red River of Arkansas and Louisiana, huge log rafts are produced which dam up the river so effectively as to produce temporary lakes. The impounded waters soon find an outlet over the levee at some point higher up the river, and the waters flowing off through the timbered bottom lands, other logs are caught by the standing timber as in a weir. A second dam is thus formed which is separated from the initial one by open water, and in this way the driftwood dam acquires enormous proportions as it gradually moves up the river. After a period of perhaps a century or more, the lower sections of the jam become decayed and dislodged so as to float down the river.
Fig. 455.—Raft lakes along the banks of the Red River in Arkansas and Louisiana at their fullest recorded development (after A. C. Veatch, U. S. G. S.).
In the lower Red River a great raft of alternating jams and open water reached a length of about one hundred and sixty miles and moved up the river at the average rate of something less than a mile per year. Within the limits of the dam all tributary streams were blocked, so that secondary lakes were formed in a double fringe about the main river ([Fig. 455]). The great raft which formed here in the latter part of the fifteenth century has now at the beginning of the twentieth been largely removed and measures have been adopted to prevent its re-formation.
