DATA AND UNDERSTANDING 115 
available excellent maps, knowledge of the soils and of the rocks, 
both at the surface and below the ground, measurements of 
precipitation, descriptions of the vegetation, data on the flow of 
streams, experience in the use, if not the husbandry, of the land. 
It is true that for the purposes of our complex civilization, the 
need for additional data has far outstripped the programs of 
fact-finding. But it appears that an indefinite expansion of the 
collection of routine measurements would still leave something 
lacking. I draw the distinction between measurement data and 
understanding; between the collection of facts and knowledge of 
processes and interrelationships. Although we have a wealth of 
data, our understanding of the semi-arid environment is poor. 
Understanding the physical and biologic processes operating in 
an environment is important for living in and with the land. As an 
example, let us look briefly at the interrelation of the water and 
sediment in ephemeral streams, and the problem of valley trench- 
ing or arroyo cutting. 
The Problem of Arroyo Cutting 
Many of the alluvial valleys of New Mexico are gutted by 
trenchlike gullies. The rapid growth of arroyos in southwestern 
valleys in the United States accompanied the livestock boom of 
the late nineteenth century. It is clear that the pressure of 
livestock on the vegetation has materially contributed to the 
growth of arroyos. But the problem is more complicated. The 
first American reconnaissance teams of the Army of the West 
traversed New Mexico in the fall of 1846, some 20 years before 
American settlers and their livestock had any appreciable effect 
on local vegetation. In August of 1846, Lt. Simpson marched 
from Santa Fé to the Navajo country. He crossed the Rio Puerco 
near Cabezon. So deep was the arroyo at that place that he had 
to cut down the 30-foot banks to get his brass cannon across. 
There is enough evidence of this kind (1, 5) to indicate that in 
certain valleys large gullies existed before American settlement, 
even in places far removed from heavy grazing by Spanish 
livestock. It must be supposed that these arroyos were the result 
of natural rather than human causes. 
Geologic and archaeologic studies have demonstrated several 
