While the southern part of the Florida peninsula is flat, it is not entirely level. At its highest point near Orlando, it is less than 150 feet above sea level, and most of the Kissimmee Valley north of Lake Okeechobee is less than 50 feet above sea level. The Lake itself rarely reaches 20 feet above sea level. South of the Lake the maximum elevation is less than this, and in many places, as one moves south, the gradient measures only about two inches per mile. This means that under historic conditions water flow in the region was extraordinarily slow and wide—giving rise to the terms “sheet flow” and “river of grass.” In other words water did not flow south in streams but in a single slow sheet from Lake Okeechobee to Florida Bay.
In the wet season, this flow helped sculpt the landscape creating the ridge and slough topography and freshening the estuaries on all sides. Historically, there was more water flowing south than there is today, but the everglades was twice its current extent. This flow was only in the wet season. For half the year output exceeded input and the glades dried, north to south, east and west to the core. In some years water remained in the core, in some years it did not. In fact, this year-to-year difference was important too, as animal populations responded differently and this variation was needed for all to have their place. In the dry years the glades and pinewoods burned. If sawgrass does not burn periodically it will die off. So there needed to be both a wet and dry season for the unique Everglades ecosystem to survive.