34 The Plant Wori.d. 



bridge, flowing at the rate of 9 feet per second. When thus 

 swollen by rain it is a turbid, impetuous stream, bearing along 

 branches of trees and debris of various kinds to be deposited at 

 different points as the water lowers. At these times of flood 

 its power of erosion is very great, and in the course of a few hours 

 the banks in places are deeply cut away, acres of fertile land 

 being swept into the devouring current. 



In the course of a few days a small and harmless stream, 

 a few inches in depth, is quietly flowing in its channel, and later, 

 after the rains are over, even this disappears, leaving the bed 

 of the river, as far as appears on the surface, only a collection of 

 variously assorted and mingled masses of sand and gravel. 

 Beneath the surface, however, is still at some depth a body of 

 water, and from this, ever in the driest times, the cottonwoods 

 and willows along its banks draw an abundant supply (Fig. 5). 



From what has just be^n stated, it is evident that the condi- 

 tions are in general unfavorable for the growth of aquatic vege- 

 tation. The rapidity of the current when the river is "up", the 

 turbidity of the stream, and the long period when no water 

 appears above ground, are not conducive to the normal develop- 

 ment of water-plants. Yet there are places where these do 

 secure a foothold and grow with remarkable vigor. Especially 

 is this true of the irrigating ditches, in which green algte fre- 

 quently accumulate in such quantities that they have to be 

 cleaned out to prevent the channels from becoming choked. 

 Great masses of Cladophora are often thrown out on the banks 

 for long distances. Hydrodictyon is also abundant. Potamogeton 

 pusillus is frequently conspicuous, and in pools of the river chan- 

 nel, where the force of the current is lessened, the water-cress 

 (Radicula Nasiurtium-aquaticum) grows in luxuriant masses. 



(2) The River Banks; Association of Cottonwoods and 



Willows. 

 The banks of the river are lined with willows {Salix nigra) 

 and cottonwoods (Populus Fremontii), which constitute their 

 conspicuous vegetation, while the arrow- weed (Pluchea sericea) 

 is of frequent occurrence. Another plant conspicuously present 

 where the banks are sandy is Aster spmosus, which also extends 

 out on the flood-plain to some distance. As in similar situations 

 elsewhere, a considerable number of annual plants succeed in 



