46 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



floods, and reduce water losses through evaporation. Lakes and 

 ponds tend to modify temperature ranges, hold up the ground 

 water level, and serve as reserve to be redistributed by the great 

 cyclonic movements that obtain their chief supply from the ocean. 

 This redistribution may not find expression in actual rain or 

 snow, but in an increase in relative humidity, thereby reducing 

 evaporation losses. 



It is the opinion of the writer that a hydrometric survey would 

 demonstrate the wisdom of preserving large tracts of land in 

 their natural state. Such reserves might include many of our 

 lake groups and broken lands in the north ct?ntral Mississippi 

 Valley. They would be worth while based on agricultural con- 

 siderations only. Such reservations might ser\'e incidentally as 

 parks and play grounds but primarily as stabilizers of stream flow 

 and water supply, conservers of the soil, and would aid the 

 Mississippi Valley to maintain its position as the Granary of the 



World. 



i 1. 



