116 THE FUTURE OF ARID LANDS 
post-glacial but pre-Columbian periods of erosion followed by 
aggradation. The last period of erosion prior to the present one 
can be dated by pottery buried in the alluvium as occurring 
approximately in the period a.p. 1200-1400 (2). The concurrent 
trenching of alluvial valleys in that period appears to have oc- 
curred at least as far north as Wyoming (6) and south into 
Texas (3). 
The problem is further complicated by difficulty in assessing 
the effect of fluctuations in climatic factors on the recent episode 
of erosion. In the hundred years of rainfall record in central New 
Mexico, no progressive shifts of annual totals are discernible. 
But there has been a progressive change in the number of rains of 
various sizes. The period 1850 to 1880 was characterized by a 
deficiency in small rains and a relatively great proportion of rain 
events of large magnitude. This might be interpreted to mean 
that coincident with the wave of settlement and accompanying 
pressure of livestock in the nineteenth century, climatic factors 
were particularly adverse to maintenance of physiographic equi- 
librium (4): 
The upshot of these considerations is that in the last century 
there has been a repetition of a physiographic episode which had 
occurred more than once in post-glacial time. But the recent 
valley trenching was influenced to more or less extent by activities 
of man and his grazing animals. The presettlement periods of 
valley erosion and subsequent alluviation presumably resulted 
from changes in climatic elements. 
The arroyos cut during the last century have radically changed 
the contribution of sediment which the alluvial valleys provide 
to the master stream. This change of sediment inflow has prob- 
ably contributed to the fact that the bed of the Rio Grande at 
Albuquerque has gradually risen in recent decades and now 
stands only slightly below the level of the flood plain on which 
the center of this city stands. The gullies have dissected the 
valley flats which were the best agricultural parts of the hinter- 
land. The cutting of an arroyo trench lowers the local ground 
water table and cienaga grasses give way to less productive 
vegetation. Water formerly could be diverted from the shallow 
