814 



SCIENCE 



[N. S. Vol. XXXIV. No. 



(except in a few rivers fed by the greater 

 rain and snow of mountains) it tends to 

 spread into debi-is-laden sheetfloods and 

 will not flow down to the sea; lakes, in 

 which water lodges for a time, are essen- 

 tially expansions of streams due to what 

 may be called geologic accidents — e. g., the 

 Great Lakes chiefly to glacial scouring, 

 the Millelacs to the irregular configuration 

 of glacial-drift svirfaces, Great Salt and 

 Winnemucca Lakes originally to warping 

 of the earth-crust; waterfalls, in which 

 power is easily developed, are also due to 

 geologic accidents — e. g., Niagara and 

 Genesee and St. Anthony to conditions at- 

 tending withdrawal of the Pleistocene 

 glaciers, the cataracts of the Susquehanna 

 and Potomac and James and the Dalles of 

 the Columbia to displacement in the 

 earth-crust. 



4. In humid regions (including moun- 

 tains in which rain and snow are more 

 abundant than over neighboring lowlands) 

 the streams carry a part only of the water 

 reaching the surface — i. e., the run-ofi', 

 averaging about one third of the rainfall; 

 about half the aggregate is evaporated, 

 partly from the soil and open waters 

 though more freely from growing vegeta- 

 tion, forming the fly-off; while a smaller 

 fraction (the cut-off) passes deeply into 

 the earth to be absorbed in chemical com- 

 bination or carried subterraneously to the 

 sea. In arid regions thore is (normally) 

 no run-off, and all the water except the 

 small cut-off is evaporated to temper the 

 local climate. 



5. In a state of nature — and also under 

 intensive cultivation — little if any storm 

 water flows over the land surface apart 

 from the streams; the rainfall is absorbed 

 by the soil and its vegetal growth, and the 

 streams are supplied partly by springs but 

 much more largely by seepage directly into 

 their channels — this being the normal con- 



dition, in which streams are generally clear 

 and nearly uniform in flow. 



6. Under certain conditions attending 

 settlement, especially with injudicious 

 clearing and negligent cultivation, a con- 

 siderable part of the rain falling during 

 storms runs off the land surface, erodes the 

 soil, renders streams turbid, gathers into 

 destructive floods, and introduces wide 

 fluctuations in flow (this representing what 

 may be deemed a temporary condition in 

 the history of the country, and one remedi- 

 able by proper classification and use of the 

 lands for purposes to which they are 

 adapted, and by intensive cultivation of 

 areas devoted to the growing of seasonal 

 crops). 



7. All parts of each stream are interre- 

 lated; increase or decrease in volume, in- 

 wash of detritus, the initiation of fluctua- 

 tion, or other changes in regimen at any 

 point eventually affect the stream through- 

 out; especially susceptible to disturbance 

 at the sources are clarity and steadiness of 

 flow at points whence Avater supply is com- 

 monly taken, in the middle course where 

 power development is customary, and in 

 the lower course devoted to navigation. 



8. Normal streams, being derived chiefly 

 from seepage, are maintained directly by 

 the store of water accumulated in the 

 ground as the residuum of rains of preced- 

 ing seasons and decades, and only indirectly 

 by the current rainfall. In the humid 

 part of this country the ground water 

 within the first hundred feet from the sur- 

 face has been estimated at some 25 per 

 cent, of the volume of subsoil and rock, 

 equivalent to 6 or 7 years' rainfall — i. e., 

 it may be conceived as a reservoir of water 

 25 feet deep coinciding in area with the 

 humid region. This reservoir is the chief 

 source of the streams available for water- 

 power and other purposes ; it is also the re- 

 serve agricultural capital of the country. 



