Are the tens of thousands of playa lakes that dot the Southern High Plains key to keeping the Ogallala Aquifer’s dwindling waters clean? That’s the question plant physiologists are asking about North America’s largest aquifer. Playas are ephemeral lakes that form when rainwater fills natural clay depressions in the landscape. Gitz found that this water flows continually downward, if slowly, into the Ogallala, contributing to its recharge.