ORIGIN OF LAKE BASINS. 27 



Beaver dams afford still another illustration of the manner in wliieh 

 drainage is obstructed and lakes formed by organic agencies. Beavers 

 formerly lived over nearly the whole of North America, and are still 

 found in limited numbers in the Northern states and Canada, and extend- 

 ing southward along the Cordillei-as at least as far as New Mexico. The 

 dams they constructed with great intelligence and skill, across small 

 streams, retained drift logs and floating leaves, thus leading to the 

 accumulation of deposits which obstructed the drainage foi' a long time 

 after they had been abandoned by the animals that built them. The 

 ponds and swamps due to the work of beavers number tens of thousands, 

 and have produced important changes in the minor features of the surface 

 of the continent. Many of these ponds, after becoming choked with 

 vegetation and converted into peat swamps, have been drained and 

 furnish rich garden-lands. 



Where brooks and creeks flow through forested regions, it fre- 

 (^[uently happens that large trees fall across them and retain the sticks 

 and leaves swept along by the current. When such a start is made, the 

 nnid carried, especially during freshets, is lodged among the leaves and 

 branches, and tends still farther to obstruct the drainage and lead to the 

 formation of swamps and lakes. This process has been observed espe- 

 cially in Red river, Louisiana, where timber rafts several sqiuire miles in 

 area, and covered with living vegetation, form floating islands and dam 

 the streams so as to cause their waters to spread out in shallow lakes 

 twenty to thirty miles in length.^ 



Numerous instances in the Yukon river, in Alaska, were observed by 

 the writer, where lateral branches of the stream and the })assage ways 

 between islands, Avere closed by accumulations of drift logs that greatly 

 obstructed the flow of the Avaters. In some instances these accumulations, 

 called " Avood yards " by steamboat men, are several acres in extent. 



Still aiiotlicr way in Avhicli organic agencies lead to the formation of 

 basins may be observed in swani[)s where vegetable matter buried beneath 

 mud and clay is undergoing decomposition. Openings similar to those 

 produced in alluvial deposits by the violent esca})e of Avater during carth- 

 ([uakes, but not necessarily connected Avith such disturbances, are formed 

 in marshes by the violent escape of gases from beloAV. Instances of this 

 occurrence have come under the Avriter's notice on Smoke Creek desert, 



1 riiark-s Lyell. " rriiicii.lfs of (u'(il()i:y."" lltli (■«!.. A'ol. 1. ]). 411. llumplircys & Abhcitt. 

 " Ucport (111 the Physics ami Ilyilraulics of tlii' Mississippi." I'lotVssional Papers, Corps of 

 Eugiufcrs, U. S. A., 1861, p. ;]7. 



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