National Geographic : 1972 Jan
4 Stealthy invader-the sea. In normal times, southern Florida's canals run two feet above sea level, and the resulting pressure in the porous subsurface rock holds back the heavier salt water (left diagram). During droughts, when canal levels may fall to sea level or below, the heavier salt water intrudes and turns fresh pump water briny (right diagram). Some coastal cities have moved wells far inland to escape salt-water encroachment. Jungle on stilts, mangroves (left) grow from a tangle of arch ing roots in Biscayne Bay. Once such stands fringed much of the coastline of southern Florida, but increasing urbanization sees them give way to man-made seawalls. A boat in every backyard, boasts Fort Lauderdale. Gone are the mangroves; in their place stand the concrete dikes of a 20th century Venice. Naturalists deplore the loss of the trees, whose roots shelter shrimp and oysters, and serve as a nursery area for fish of many species. But the pressure for more housing persists as the appeal of Florida living continues to lure new residents.