June 2021

(Please click on photos to enlarge)

Falling Creek Falls is located in a small county park about a mile north of Lake City in North Florida. The park, nestled in the wooded countryside of rural Columbia County, includes a boardwalk that leads to the falls about a quarter mile down the trail. The creek drops 10 feet over a clayey limestone lip at the falls, and continues downstream through a narrowing ravine. Falling Creek, a tributary of the storied Suwanee River, is an intermittent stream fed by seasonal rain and shallow groundwater from a drainage basin extending to the east. It flows west for a few miles, down the falls, then disappears down a swallow hole in the karst topography, flows underground, then reemerges and joins the Suwanee River to the northwest.

Falling Creek’s water level varies drastically depending on the season and recent precipitation. During my visit in June in the midst of an uncharacteristically dry start to the summer, the creek was mostly dry, with the stream bed fully exposed (perfect for viewing the geology), and barely a trickle coming down the falls. When I returned at the end of a rainy July, the entire creek was flooded above its banks and into the floodplain of the surrounding woods. Water was flowing full and fast above the ravine, flooding the boardwalk. The water level was so high you wouldn’t have known there was a waterfall there!

The lithology of the area, exposed at the falls and the stream bed when the creek is dry, consists of the Coosawhatchie Formation of the Hawthorn Group. The Coosawhatchie Formation here is made up of gray-green sandy clays interbedded with phosphatic poorly consolidated limestones and sands. It was deposited during the Miocene Epoch, roughly 5-23 million years ago, in a low-energy shallow marine environment.

The falls are formed where the denser and more consolidated gray member that forms the creek bottom upstream overlies the softer and less consolidated light green member that make up the lithology downstream. The stream cuts down through the softer layers where it meets them, the flowing water eroding them away. Eventually those softer layers break away to the edge of the more resistant rock, and then continue to get cut away under the overlying resistant rock, resulting in an overhanging lip that water cascades down vertically, a waterfall. The waterfall will slowly retreat upstream as the cycle continues.

Karst topography is very evident in the more consolidated, gray, calcareous stream bed upstream of the falls. Small sinks and fractures are abundant in the clay/rock beds where mildly acidic surface water has dissolved the rock. Moving downstream, the walls of the ravine and plunge pool beneath the falls show the less consolidated gray/green/yellow phosphatic clay/sand/rock beds. The stream bed and banks are lined and littered with cobbles and boulders from these eroded beds. Vegetation covers much of the surface of the ravine walls, showing that the creek often has minimal to no discharge. Karst topography is still present below the falls, but the dissolved sinks and fractures are mostly covered by the boulders and cobbles. About a quarter-mile downstream of the falls, the creek disappears into one of those sinks.

The Coosawhatchie Formation here is semi-confining, the low permeability from the tight, clayey, lithology capping the highly permeable and productive Floridan Aquifer below it, slowing flow in and out. I believe the confining nature of the formation plays a role in the highly variable water level of the creek and its susceptibility to flooding. Surface water from the drainage basin flowing into the creek after rainy periods cannot easily infiltrate through the low permeability lithology that makes up the stream bed, so the water level rises and rises. As it drains downstream it may also encounter a choke point at the swallow hole, the heavy discharge backing up as it tries to flow down the tighter sink, like a partially clogged drain. Increased sediment load from the heavy discharge might clog up the swallow hole, further reducing drainage, resulting in increased flooding of the banks. I would like to visit the creek again and see if I can find a way down to the swallow hole to check it out.