636 DYNAMICAL GEOLOGY. 



eleven trillions in dry years to twenty-seven trillions in wet years. 

 This amount is about one-quarter of that furnished by the rains. The 

 river is 3,500 feet wide at St. Louis, 4,000 off the mouth of the Ohio, 

 and about 2,500 at New Orleans. 



The mean annual discharge of the Missouri River is about three and three-quarter 

 trillions, or Jifteen-kundredihs of the amount of the rains over the region. The corre- 

 sponding amount for the Ohio is five trillions, which is one-quarter the amount of rain. 

 (Humphreys & Abbott.) 



Floods. — The larger part of the geological work done by rivers is 

 carried forward in times of flood. Streams that are sluggish and 

 impotent in the dry season, or even burrow out of sight, become tor- 

 rents of tremendous power during rains. -The rivers of some dry 

 countries, as Australia, spread out in immense floods in the rainy sea- 

 son, although but strings of pools in the dry. 



Bursting of Lakes. — The floods made, when the banks of a lake 

 suddenly give way, have the character of those arising from a sudden 

 precipitation of rain in the mountains, but sometimes with vaster 

 results, the water ploughing profoundly into the slopes before it, and 

 spreading the gravel or earth, uprooted trees, and the contents of the 

 lake basin, far and wide. 



2. Amount of Pitch or Descent in Rivers. — The average descent of 

 large rivers, excluding regions of cascades, seldom exceeds twelve 

 inches to a mile, and is sometimes but half this amount. 



The following facts on this point are from Humphreys & Abbot's Report on the Mis- 

 sissippi Basin. The descent per mile is given in inches; L. stands for the low-water 

 pitch, and H. for the hiyh-water pitch. 



Mississippi R. Mouth to Memphis (855 m.) 



Mouth to Cairo, at mouth of Ohio (1088 m.) 



Above the Missouri to source (1330 m.) 

 Missouri R. Mouth to St. Joseph (484 m.) 



St. Joseph to Sioux City (358 m.) 



Sioux City to Fort Pierre (404 m.) 



Fort Pierre to Fort Union (648 m.) 



Fort Union to Fort Benton (750 m.) 



Fort Benton is 2,644 miles above the mouth of the Missouri. The whole Missouri, 

 from its highest source, a distance of 2,908 miles, has a descent of about 6,800 feet, — 

 or 28 inches per mile. 



During floods (1), the pitch of the surface of a stream, when the 

 bottom has in the main a regular descent, and the channel is broad 

 and open, is increased in amount and uniformity. But, in the Missis- 

 sippi, below the mouth of the Ohio, it is less than the low-water pitch, 

 because the lower jiart of this river, for 200 miles from the Mexican 

 Gulf, is horizontal, or very nearly so. The waters are raised less near 



L. 



H. 



4'82 in. 



5 - 23 in. 



6-94 



5 - 96 



1174 





9 24 





10*32 





12-12 





13-20 





10-56 





