THE GLACIEE LAKES 105 



down, only the finest of the mud-particles being 

 carried through the highest of the series to the 

 next below. Then the next higher, and the next 

 would be successively filled, and the lowest would 

 be the last to vanish. But this simplicity as to 

 duration is broken in upon in various ways, chiefly 

 through the action of side-streams that enter the 

 lower lakes direct. For, notwithstanding many of 

 these side tributaries are quite short, and, during 

 late summer, feeble, they all become powerful tor 

 rents in springtime when the snow is melting, and 

 carry not only sand and pine-needles, but large 

 trunks and boulders tons in weight, sweeping them 

 down their steeply inclined channels and into the 

 lake basins with astounding energy. Many of these 

 side affluents also have the advantage of access to 

 the main lateral moraines of the vanished glacier 

 that occupied the canon, and upon these they draw 

 for lake-filling material, while the main trunk stream 

 flows mostly over clean glacier pavements, where 

 but little moraine matter is ever left for them to 

 carry. Thus a small rapid stream with abundance 

 of loose transportable material within its reach may 

 fill up an extensive basin in a few centuries, while 

 a large perennial trunk stream, flowing over clean, 

 enduring pavements, though ordinarily a hundred 

 times larger, may not fill a smaller basin in thou 

 sands of years. 



The comparative influence of great and small 

 streams as lake-fillers is strikingly illustrated in 

 Yosemite Valley, through which the Merced flows. 

 The bottom of the valley is now composed of level 

 meadow-lands and dry, sloping soil-beds planted 



