1914] Grinnell: Mammal.s and Birds of the Colorado Valleij fil 



willows and eottonwood ; the second bottom is of sufficiently hi<i:hpr 

 elevation to be chiefly above high-water mark, and is characterized 

 by mesquite. salt-bush, and rank chimps of creosote bush. The second 

 bottom may be altoi^ether wantiiifr. or it may constitute a bi'oad bench- 

 like tract. The surface is often modified by alluvial deposits at the 

 mouths of washes leading down from the adjacent desert, and by 

 wind-blown sands which heap up about bushes, especially alon? the 

 southeastern borders of the valleys. 



The Laguna Dam has had a pronounced modifying intluence on the 

 flora and fauna of the vicinity. The dam was built to a height of 

 twelve feet above the mean level of the river at that point at the 

 time of beginning construction. As soon as it was completed (in 1909) 

 the retarded waters above began to deposit silt, and by May, 1910. the 

 valley above had been silted in to a depth determined by the top 

 of the dam. The water level had been raised conspicuously for at 

 least ten miles, and we saw evidences of deepening of the first bottom 

 deposits and slowing of current for fully thirty miles, above the dam. 

 The cottonwoods of the first bottom within eight miles above the 

 dam had all been killed, evidently by the raising of the general surface 

 around their trunks; and the mesquites and other vegetation of the 

 second bottom had all been drowned out, there thus being no trace of 

 second-bottom conditions except for dead stalks. These were replaced 

 by vast mud flats growing up to arrowweed. All this change, of course, 

 involved the birds and mammals of the areas affected, in addition 

 to the plant life. 



Below the dam reverse changes took place. The water, having 

 dropped a considerable portion of its sediment above the dam because 

 of the slowing of its current, was able to pick up sediment at a cor- 

 respondingly accelerated rate below the dam. This, and the fact of 

 a new cut-off having been found by the river in the delta in 1909, thus 

 temporarily .shortening its channel, resulted in a deepening of the 

 channel seven feet below the previous level immediately below the 

 dam. Thus the former flood-bottom was, in 1910. far above flood 

 level, and in a way to become good second bottom, with apjiropriate 

 metamorphosis in vegetation and fauna. 



Although these changes were local, and due to man's interference, 

 similar ones, due to natural causes, have doubtless occurred from time 

 to time in various parts of its course in the river's history, thus 

 repeatedly shifting the riparian .strips both in position and total width, 

 with corresponding variability in the powers of the river at different 



