298 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



because of the rapid accumulation of sediment brought to the reser- 

 voirs by the turbid waters of the direct run-off. Much careful study- 

 has already been given and much more is needed to determine the 

 rates of sedimentation imder different conditions and to devise fea- 

 sible methods of prolonging the life of reservoirs by bypassing the 

 most turbid waters, by sluicing out the sediments, or by other means. 

 Much progress has recently been made in the appreciation of the 

 great value of the natural storage facilities and the importance of con- 

 serving and utilizing them. The whole complex subject affords a large 

 field for future study and constructive effort. 



Ice and snow storage. — Ice and snow have recently come into the 

 scientific Umehght in different ways. One of these relates to the in- 

 terest of geologists in the relation of the Quaternary and older glacial 

 stages to fluctuations of sea level and to the cyclic character of some 

 of the stratified rocks of marine origin. Thus it is now beheved that 

 in some of the glacial stages enough water was locked up as glacial 

 ice to depress the sea level as much as several hundred feet, and that, 

 on the other hand, many of the ancient sea terraces, such as occur 

 on the Atlantic and Gulf Coastal Plain, were formed during inter- 

 glacial stages when there was even less ice than at present. It has been 

 estimated that if all the ice that exists at present in the polar regions 

 were melted it would raise the sea level at least 100 feet and perhaps 

 200 feet or more. The advance and retreat of existing European gla- 

 ciers have long been recorded, but systematic observations on North 

 American glaciers have only recently been undertaken. Interest in 

 snow relates chiefly to the snow in the mountains, which supports the 

 summer flow of many streams, and to recently developed methods of 

 estimating the annual snowfall and predicting the resultant stream 

 flow. 



Storage oj water in the soil. — A soil may be regarded as a water 

 reservoir, its water being in the form of moisture adhering to the soil 

 particles. This water is under comphcated stresses produced by com- 

 binations of the molecular attraction of the soil, the downward pull 

 of gravity, the absorptive energy of the plants, and the energy in- 

 volved in the relation between the soil moisture and the atmospheric 

 vapor. The slow movements of the soil moisture in response to these 

 stresses are of much importance in plant growth and in recharge of 

 the water-bearing formations, and they have properly been the sub- 

 ject of much study. 



The value of a soil for producing crops depends largely on its 

 capacity to hold its water supply against the pull of gravity and yet 

 to yield this retained water to the roots of the plants. A clean dune 

 sand retains so Uttle water that even in a humid region it may support 

 only cactus and other drought-resistant plants; on the other hand, a 

 clay soil has a large water-retaining capacity but may hold most of 



