490 



A. Tylor — Formation of Deltas. 



The heights are not determined from a trigonometrical survey at 

 points above Natchez, and are therefore only approximate. They 

 are derived from the information given by different railway engi- 

 neers, whose lines have crossed the Mississippi. 



By drawing a parabola, the curve will pass near all the chief 

 intermediate points. There is a deviation of twenty-two feet at 

 Eed Eiver Landing, and of three feet at St. Louis. The fall of the 

 Mississippi is six inches per mile at Cairo. This is the case of 

 greatest error south of Cairo. At many points the parabolic curve 

 corresponds exactly with the surface of the alluvium, so that by 

 knowing the distance from any point, you could find the height ; or, 

 by knowing the height, you could tell the distance precisely from 

 Cairo or New Orleans. It will be seen that a similar law holds 

 good for the navigable part of other rivers. 



The heights at some points are probably not correct within several 

 feet, as they are not based upon a trigonometrical survey. 



In a paper published in ^1853, the author considered the proba- 

 bility of the rainfall all over the globe being many times the present 

 average throughout the world, on the evidence of the fineness of 

 the present river sediment compared with the coarseness of the old 

 alluvial and gravel deposits. 



The average maximum depth of the Mississippi between the Ohio 

 and the head of passes in the Gulf of Mexico, determined from the 

 average of forty-five cross sections, is 104 feet. 



The highest maximum recorded is 180 feet, and the lowest maxi- 

 mum is 71 feet. The greatest width of river is 7,800 feet, and 

 smallest recorded 1900 feet. The average area of the cross section 

 is 197,250 square feet. The average width is 3,505 feet ; so that ac- 

 cording to these figures the average depth must be 5%^ feet from 

 the Ohio to the sea, although it is not printed in the report. 



Yearly Amount op Eain in the Valley or the Mississippi. 





Square 

 Miles. 



Downfall of 

 rain in inches. 



Cubic feet of rain. 



Delta 



Red River 



Arkansas and "White Rivers 



St. Francis 



Missouri ... 



Up Mississippi 



Ohio 



Yazoo 



SmaU 



12,300 



97,000 



189,000 



1,000 



518,000 



169,000 



214,000 



13,850 



32,400 



60-9 

 39-0 

 29-0 

 41-1 

 20-9 

 35-2 

 41-5 

 46-3 

 47-8 



1,700,000,000 



8,800,000,000 



13,000,000,000 



1,100,000,000 



25,200,000,000 



13,800,000,000 



20,700,000,000 



1,500,000,000 



3,600,000,000 





89,400,000,000 



In extreme low-water years, as 1839 and 1835, the discharge of 

 water is only about 11,000,000,000 of cubic feet. In ordinary years 

 it is 19,500,000,000 cubic feet. 



In great flood years, as 1823, 1828, 18M, 1849, 1858, it is supposed 

 to average about 27,000,000,000 cubic feet. The discharge is now 



