MOVEMENTS OF LAKE WATERS. 39 



effects of a sudden rise would be cheeked, and even entirely averted, if 

 a lake of sufficient size existed in its middle course, or if there were a 

 number of lakes on its tributary streams. 



The modulating influence of lakes on the flow of streams is wtdl 

 known to hydraulic engineers ; and it has been proposed to regulate the 

 flow of tlie Mississippi by building storage reservoirs on its head waters. 

 Such reservoirs could l)e filled during floods and the water allowed to 

 escape when the danger stage had passed. In this manner the disasters 

 resulting from annual freshets could be averted and navigation improved 

 during the seasons of low water. 



The effect of gales in heaping up the waters of lakes on the shores 

 against which they blow has already been noted, and an instance cited 

 where the waters of St. ]Mary*s river were suddenly raised by a gale on 

 Lake Superior. A rise of the water in streams flowing from large lakes, 

 due to this cause, is exceptional, however, and by no means as destructive 

 as the fluctuations produced by storms and melting snow on water courses 

 that are without the regulating influences of large lakes. 



The sudden escape of lakes held by dams of ice also causes floods in 

 the streams below, as in the case already cited of the Rhone, when ]Mar- 

 jelen lake is drained, and of the Stickeen, where the glacial-held lakes on 

 its tributaries break their icy bands. 



The rise of Lake Bonneville until it found an outlet and then rapidly 

 cut doAvn its channel of discharge through unconsolidated material, as 

 will ])e described in advance, is supposed to have caused a great rise in 

 Snake river, to wliich it became tril)utar3\ L'l these and other ways that 

 might 1)6 cited, it appears that lakes may cause floods in their draining 

 streams as well as avert them. 



Lakes as settling' basins. — The streams flowing into lakes are fre- 

 quently turbid and heavy with sediment, especially after storms, but the 

 rivers flowing from them are usually clear and free from all but possibly 

 the finest of material in suspension. T)uring the slow passage of the 

 waters through a lake which has an outlet, the material in suspension 

 falls to tlie bottom and contributes to the filling of the basin, while the 

 clarified waters flow on. 



The fact that bodies of standing water retain the mineral matter 

 brought to them in suspension, is illustrated more or less perfectly in 

 nearly every lake and pond, and even by ephemeral pools by tlie wayside, 

 but is especially marked in great seas like those drained by the St. Law- 



