1904] MAC DOUGAL—DELTA AND DESERT VEGETATION 47 
THE DELTA. 
The expedition cast loose from the shore at Yuma at noon on 
January 28, and within a short distance below the sand bluffs on 
either hand curved away from the stream, and we were fairly in the 
great delta which extends from this point to the Gulf of California, 
a distance of about 140*™; while the coastal plains on the western side 
of the Gulf embrace mud flats that constitute an actual extension 
of the delta 5o'™ further. This delta probably offers more varied and 
striking features of natural history than any other watercourse in 
North America. The river which has formed it rises in the perpetual 
snows of Utah, Wyoming, and Colorado, and runs 2500'™, chiefly 
through arid regions, before it empties into the upper end of the sub- 
tropical Gulf, into which it carries sixty million tons of sediment 
yearly, building up the delta and extending it seaward at a rate visible 
to common observation within a single lifetime.’ Numerous wit- 
nesses among the Cocopa Indians, Mexicans, and river men are 
agreed that the various distinct associations of plants characterized 
by salt grass, willow, and poplar, have advanced about 12-14*™ to the 
southward during the last fifty years. 
The portion of the delta near the present course of the river con- 
sists of an alluvial plain, not more than 4™ above the low-water 
mark, subject to constant bank erosion, shifting, and remaking of the 
soil, cut in all directions by old channels existing as bayous and 
sloughs, and- flooded at high water in May, June, and July. Almost 
pure formations of willow and poplar (Populus mexicana) cover 
many square kilometers and furnish food for thousands of beavers 
that burrow in the banks. The poplar is thickly infested with a 
mistletoe (Phoradendron), and fungal parasites are abundant. 
arge areas are occupied by the arrow-weed (Pluchea sericea), and 
Mesquite (Prosopis velutinea), and the screw-bean or “tornilla” 
2 af pubescens). Two or three species of Atriplex are also to be 
found in sections in which the action of the water prevents the estab- 
lishment of the woody perennials of greater size. In the upper part 
of the delta a cane (Phragmites) fringes the channel, and its closely 
interwoven roots act materially in preventing erosion of the banks. 
: Forzgs, R. H., The Colorado river of the west. Univ. of Ariz. Monthly 6: 112. 
1904, 
