Piracy may occur where the material in which the valleys are cut is homogeneous; but, as will be seen later, heterogeneity of material, by determining unequal rates of erosion, stimulates the piratical proclivities of streams.
An actual case of piracy is shown on [Plate XI]. North and South Lakes formerly drained westward to the Schoharie Creek, the present head of which is in the extreme northwest corner of the map. The head of Kaaterskill Creek, which had a much higher gradient, worked back and captured the head of the westward-flowing stream, diverting the drainage from North and South Lakes to itself. Schoharie Creek was thus beheaded.
Plaatekill Creek, near the south limit of the map, appears to have beheaded the creek flowing west and northwest, similarly diverting its head waters. The Dells, Wis., quadrangle (U. S. Geol. Surv.) affords an illustration of domestic piracy.
RATE OF DEGRADATION.
The amount of mechanical sediment which the Mississippi River carries to the Gulf of Mexico is estimated to represent a rate of degradation for the Mississippi basin of about one foot in 5000 years. But the mechanical sediment carried to the Gulf does not really represent the total degradation of the basin, for the water which sinks beneath the surface is dissolving more or less rock substance, especially lime carbonate. This material is carried to the sea in solution, and does not appear in the sediment on which the above estimate is based. Taking into account the matter dissolved by the water and carried to the sea in solution, the average rate of degradation for the Mississippi basin is estimated at one foot in 3000 to 4000 years.
It is not to be inferred that this rate is uniform, or even that erosion at any rate whatsoever is taking place in all parts of the basin. Such is not the fact. On the whole the rate of erosion is doubtless greatest toward the margins of the basins where the land is in its topographic youth or early maturity. It is notably less in the middle courses of the valleys, and erosion is locally exceeded by deposition along the lower courses of the Mississippi and some of its main tributaries.
The average elevation of North America is not accurately known, but it is probably not far from 2000 feet. If the present rate of degradation, say one foot in 3500 years, were to continue, it would take something like 7,000,000 years to bring the continent to sea-level. But this rate of degradation could not continue to the end, for as the continent became lower streams would become sluggish and erosion less rapid. Long before the continent reached base-level the rate of degradation, so far as dependent on mechanical erosion, would become so slow that the time necessary to bring the continent to sea-level would be almost inconceivably prolonged. Furthermore, it is quite possible that the land is suffering, or is liable to suffer, uplift, relative or absolute. If the rate of rise were equal to the rate of degradation the average height of the continent would of course not be affected.
The amount of sediment carried by streams in suspension varies notably according to the stage of the water. During a year when the stream was under careful study the Mississippi at Carrollton (Miss.) was found to carry ¹⁄₆₈₁ of its weight of sediment during the high-water stage of June, and ¹⁄₆₃₈₃ during the low-water of October, the average for the year being ¹⁄₁₈₀₈. The average of a greater number of records gives about ¹⁄₁₅₀₀ as the average ratio between the weight of the sediment and the weight of the water. This corresponds to about ¹⁄₂₉₀₀ by volume, the average specific gravity being about 1.9. The amount of material carried in the upper part of the water was notably less than that carried at greater depths, but that carried midway between top and bottom was about the same as that carried at the bottom.[32]
The discharge of the Mississippi River is about 19,500,000,000,000 cubic feet of water per year, and the sediment it carries in suspension is estimated to weigh about 812,500,000,000 pounds. This is equivalent to about 6,714,694,400 cubic feet. It is estimated that about 750,000,000 cubic feet of sediment is rolled along the bottom, giving a total of 7,468,694,400 cubic feet as the aggregate annual load carried to the Gulf by the river. This would be adequate to cover an area one square mile in extent to the depth of 268 feet per year.
PLATE XI.
