49 



A precise biotic study of the Colorado Delta and its adjacent 

 flood-basins, such as I had no opportunity to attempt, would 

 yield data for further elaboration of the associational areas 

 into which Dr. Grinnell has so graphically and comprehen- 

 sively divided the valley of the river along the whole extent 

 of the California-Arizona boundary. His groupings are worth 

 quoting for the bearing they have upon researches in the delta : 



1. River Association. 



2. Willow-Cottonwood Association. 



3. Tule Association. 



4. Arrowweed (Pluchea sericea) Association. 



5. Quail-brush (Atriplex lentifomiis) Association. 



6. Mesquite Association. 



7. Saltbush {Atriplex polycarpa) Association. 



8. Creosote (Larrea) Association (Mesa). 



9. Wash Association. 



10. Saguaro (Cereus giganteus) Association. 



11. Encelia (E.farinosa) Association (Rocky Hills). 

 Between Needles, California, and the vicinity of Yuma, 



Arizona, each of these belts is more or less continuous and 

 definite, and each can be characterized in terms of position 

 and botanic features, as well as by lists of the birds, and more 

 particularly the resident mammals, that favor it. Along such 

 a great straightaway valley as this reach of the river, a detailed 

 ecologic system is rather easy of application, but in the 

 Mexican portion of the Colorado Desert the physiographic 

 conditions produce so high a degree of complexity that inter- 

 pretation is perhaps possible only through taking account- 

 of the simpler state so well described by Grinnell. Thus ini 

 the delta, the enormous areas of marshy, occasionally flooded 

 land, with a network of streams and sloughs, fresh and brackish 

 lakes, and abundant aquatic vegetation, vastly increase the 

 potential scope of the River Association. The same circum- 

 stances also cause the four succeeding areas of Grinnell's list,, 

 namely the Willow, Tule, Arrowweed, and Quail-brush Asso- 

 ciations, to become involved almost beyond discrimination. 

 Further complexity in the southern Colorado Desert, requiring 

 a replacement or new combination of the associations appli- 



