High and Low Water 



75 



this book about such lakes, or about their population, 

 but they constitute an interesting class. Life in their 

 waters must meet conditions physiologically so different 

 that few organisms can live in both fresh and salt water. 

 Large lakes in arid regions are continually salt; 

 permanent lakes of smaller voliime aremade temporarily 

 fresh or brackish by heavy inflowing floods; while 





Fig. 20. Marl pond near Cortland, N. Y., at low water. The whiteness of the 

 bed siuTounding the residual pool is due to deposited marl, largely derived 

 from decomposed snail shells. The marl is thinly overgrown with small 

 freely-blooming plants of Polygonum amphibium. Tall aquatics mark the 

 vernal shore line. (Photo by H. H. Knight). 



strand lakes (called by the Spanish name playa lakes, 

 in the Southwest) run the whole gamut of water con- 

 tent, and vanish utterly between seasons of rain. 



Complete withdrawal of the waters is of course fatal 

 to all aquatic organisms, save a few that haye specialized 

 means of resistance to the drought. Partial withdrawal 



