32 THE FUTURE OF ARID LANDS 
ruins were not really influenced much by land use. Other cities 
were located in places where their destruction was inevitable. 
Scientists in this field are only now getting into good position to 
sort out the unlike situations and the reasons back of them. 
[See, as a good example, Leopold and Miller (4).] Obviously, re- 
search in geomorphology is most productive when associated 
with parallel researches in climatology, soil science, hydrology, 
botany, and archeology. 
We must avoid building dams and irrigation canals, for ex- 
ample, in places where they will soon become choked with sedi- 
ment or be undercut, regardless of our soil management practices. 
Other less critical areas can be used successfully only if precau- 
tions are taken to insure no weakening of the surface that would 
initiate severe erosion. Still other areas are comparatively safe 
and no costly precautions are necessary. 
I should like to emphasize that such situations cannot be evalu- 
ated in general or by superficial examinations. They are not 
directly related to any one of the factors taken by itself. The only 
reliable course is detailed investigation of all the features of each 
drainage basin, one by one. 
As a detail in this connection I should like to add this further 
caution: We cannot always assume that all the soil erosion we see 
is more or less equally responsible for the silting of our streams 
and reservoirs; commonly it is the sediment from a few critical 
stream banks or gullies that is doing most of the damage. In 
such areas control measures need to be pinpointed, not generalized. 
In areas of greatest erosion potential grazing may need to be 
avoided altogether, although commonly light grazing of grasslands 
gives a better cover than none at all. 
Ground Water 
Advances are being made in our knowledge of ground water 
storage and recharge. Although a good vegetative cover is essential 
to soil stability in many landscapes, it does not follow, as many 
formerly assumed, that maximum vegetation gives maximum 
water yield, either in the ground water or in surface storage. 
Especially where the main source of mountain water comes as 
