424 Peterson. PROBLEM OF GULLYING [Ch. 23 



CLIMATIC CHANGES 



Inquiry into changing climatic conditions, as a prerequisite to ero- 

 sion, is beset by many uncertainties. To begin with, there is no precise 

 standard for identification of such a change. It could be thought of 

 as a reduction in rainfall, an increase in temperature, a change in 

 rainfall distribution, or merely the occurrence of unusually severe 

 droughts or exceptional floods. Recognition and evaluation of changes 

 affecting any item or combination of items is difficult or impossible 

 with the meager data available. Weather observations in the West, 

 extending back to the 1880's or earlier, have been made at only a few 

 stations. In general these show the expected yearly fluctuations in 

 precipitation with no defined trend toward wet or dry cycles. Droughts 

 and periods of excess precipitation, occurring during the past several 

 centuries, have been identified and dated through tree-ring chronology 

 (Schulman, 1945, pp. 42-47), but the indices used fail again to show 

 definite cyclic trends. 



Despite the lack of positive evidence supplied by precipitation data, 

 investigations at various localities in the Southwest indicate erosion 

 periods which can be attributed only to a climatic change toward 

 drier conditions. In the gullied Jeddito Wash, located in the Hopi 

 Indian Reservation, Arizona, Hack (1942) has identified three periods 

 of deposition and erosion. Remains of extensive sand dunes, and other 

 features closely associated with the erosion periods, lead to the con- 

 clusion that erosion occurred during a dry cycle. Albritton and Bryan 

 (1939) describe a similar recurrence of deposition and erosion in west- 

 ern Texas, the dates of which can be rather closely correlated with those 

 of Hack (1942, p. 68). Sayles and Antevs (1941, p. 39) recognize 

 former periods of erosion in the Whitewater Creek in southern Arizona, 

 and, in summarizing the evidence pointing to climatic change, Antevs 

 (1948) states, "Thus the same climatic evolution from a moist-Pluvial 

 which has grown slightly drier, is recorded by various conditions in 

 several regions distributed from Trans-Pecos Texas northwestward 

 to the Sierra Nevada and Oregon." Illustrations of previously filled 

 gullies are shown in Fig. 5. 



Thornthwaite et at. (1942, pp. 88-89), in discussing the evidence 

 used by these investigators in identifying former erosion periods, argue 

 that climatic change is unnecessary to explain them. Their inter- 

 pretation is that upstream migration of discontinuous gullies within a 

 valley would result in the same features as widespread erosion, includ- 

 ing both terraces and buried channels. It is assumed that development 



