WORK OF RUNNING WATER 



RATE OF DEGRADATION 



The amount of mechanical sediment which the Mississippi 

 River carries to the Gulf of Mexico was estimated many years ago 

 to represent a rate of degradation for the Mississippi basin of about 

 one foot in 5,000 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. More recent studies, based on 

 fuller data, indicate that the average rate of degradation for the 

 United States is about i foot in 9,000 years. 1 



The sediment carried to the Gulf by the Mississippi River is 

 gathered from nearly all parts of its basin, but much more of it 

 comes from some places than from others. On the whole, the 

 rate of erosion is probably greatest toward the margins of the basin, 

 where the land is in its topographic youth or early maturity. It is 

 notably less in the middle courses of the valleys, and is exceeded by 

 deposition in some places along the lower courses of the Mississippi 

 and some of its main tributaries. 



The average elevation of North America is probably not far 

 from 2,000 feet. If it is being degraded at the rate of one foot in 

 9,000 years, and if this rate were to continue, it would take some- 

 thing like 18,000,000 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, the 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 prolonged almost indefinitely. 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. 



FEATURES RESULTING FROM SPECIAL CONDITIONS OF EROSION 



Running water develops many striking topographic and scenic 

 features. Some of them depend primarily on the conditions of 

 1 Water Supply Paper 234, U. S. Geol. Surv., pp. 78-83. 



