72 University of California Publications in Zoology [Vol.12 



The paiicity of terrestrial mammals in this association is probably 

 due to the repressive effect of the annual overflow which cannot fail 

 to reduce the food-supply for many days at a time, even if extensive 

 mortality does not directly ensue through drowning of individuals. 



As already implied (p. 58), the willow association varies greatly 

 in width in different parts of the river's course. Where the channel 

 is constricted by rock walls, as in the box canon at The Needles, all 

 trace of it is effaced for many rods. Where the river flows among hills 

 patches of willows in ravine-mouths give detached representations to 

 one or more elements. On the other hand, the broad valleys are occu- 

 pied chiefly by this association which may then be as much as seven 

 miles wide and continuous for many miles on one side or the other 

 of the meandering channel. Taken by and large, the willow associa- 

 tion is the most important one biotically of the entire set of associations 

 dependent upon the presence of the river. 



TrLE ASSOCI.VTION 

 BIRDS 



Fulica americana: max.: winter Melospiza mclodia saltonis: min.; 

 Oxyechus vocifenis vociferus: max.; resident 



winter Geothlypis trichas seirpicola: max.; 

 Circus hudsonii:s: max.; winter resident 



Xanthoeephahis xanthocephalus: max.; Geothlypis trichas oecidentalis: min.; 



winter transient 



Agelaius phoeniceus sonoriensis: Telmatodytes palustris plesiiis: exel.; 



min.; resident winter 



Sigmodon hispidiis ereraicus: min. Ondatra zibethica pallida: min. 



Reithrodontomys raegalotis deserti: Proeyon pallidiis: min. 



max. 



Remarks upon tJie Tnlf Association. — The river's habit of overflow 

 would be expected to result in rather extensive tracts of palustrine 

 flora. As a matter of fact, however, marshes were few and of small 

 size. This was probably due to the rapid rate of evaporation of 

 overflow water so that favoring conditions did not last long, and also 

 to the rapid silting-in of such water basins as ox-bow cut-offs. As 

 a result there were either almost lifele.ss alkali depressions, or lagoons 

 practically identical in biotic features with the main river. But in 

 a few places there were well-defined palustrine tracts kept wet 



