The levee.—When the snows thaw upon the mountains at the headwaters of large rivers, freshets result and the delta regions are flooded. At such times heavily charged with sediment, a thin deposit of fertile soil is left upon the surface of the delta plain, and in Egypt particularly this is depended upon for the annual enrichment of the cultivated fields. Though at this time the waters spread broadly over the plain, the current still continues to flow largely within the normal channel, so that the slack water upon either side becomes the locus for the main deposit of the sediment. There is thus built up on either side of the channel a ridge of silt which is known as a levee, and this bank is steadily increased in height from year to year ([Fig. 452]).
To prevent the danger of floods upon the inhabited plains, artificial levees are usually raised upon the natural ones, and in a country like Holland, such levees (dikes) involve a large expenditure of money and no small degree of engineering skill and experience to construct. So important to the life of the nation is the proper management of its dikes, that in the past history of China each weak administration has been marked by the development of graft in this important department and by floods which have destroyed the lives of hundreds of thousands of people.
Fig. 173.—“Bird-foot” delta of the Mississippi River.
Wherever there has been a markedly rapid sinking upon a delta region, and depressions are common in delta territory, no doubt as a result of the loading down of the crust, the river may present the paradoxical condition of flowing at a higher level than the surrounding country. Between the levees of neighboring distributaries there are peculiar saucer-shaped depressions of the country which easily become filled with water. At the extremity of the delta the levee may be the only land which shows above the ocean surface, and so present the peculiar “bird-foot” outline which is characteristic of the extremity of the Mississippi delta, though other processes than the mere sinking of the deposits may contribute to this result ([Fig. 173]).
The sections of delta deposits.—If now we leave the plan of the delta to consider the section of its deposits, we find them so characteristic as to be easily recognized. Considered broadly, the delta advances seaward after the manner of a railroad embankment which is being carried across a lake. Though the greater portion of the deposit is unloaded upon a steep slope at the front, a smaller amount of material is dropped along the way, and a layer of extremely fine material settles in advance as the water clears of its finely suspended particles ([Fig. 174]). Simultaneous deposits within a delta thus comprise a nearly horizontal layer of coarser materials, the so-called top-set bed; the bulk of the deposit in a forward sloping layer, the so-called fore-set bed; and a thin film of clay which is extended far in advance, the bottom-set bed ([Fig. 174, 2]). If at any point a vertical section is made through the deposits, beds deposited in different periods are encountered; the oldest at the bottom in a horizontal position, the next younger above them and with forward dip, and the youngest and coarsest upon the top in nearly horizontal position ([Fig. 174, 3]).
Fig. 174.—Diagrams to show the nature of delta deposits as exhibited in section.
It has been estimated that the surface of the United States is now being pared down by erosion at the average rate of an inch in 760 years. The derived material is being deposited in the flood plain and delta regions of its principal rivers. Some 513 million tons of suspended matter is in the United States carried to tidewater each year, and about half as much more goes out to sea as dissolved matter. If this material were removed from the Panama Canal cutting, an 85-foot sea-level canal would be excavated in about 73 days. The Mississippi River alone carries annually to the sea 340 million tons of suspended matter, or two thirds of the entire amount removed from the area of the United States as a whole. It is thus little wonder that great deltas have extended their boundaries so rapidly and that the crust is so generally sinking beneath the load.