SHIFT OF EVERGLADES DISCHARGE IN RESPONSE TO LATE HOLOCENE COASTAL BUILDUP, SOUTHWEST FLORIDA

A small scale rise in sea level 2,500-2,400 years before present initiated dramatic changes in coastal geomorphology and drainage patterns of southwest Florida. Relict geomorphic features in lower Everglades National Park, Florida, are interpreted to represent the original channel paths of Everglades discharge prior to 2,400 ybp and coastal mud ridges that blocked and infilled these paleochannels between 2,400 and 2,000 ybp. These paleochannels, south of the present Everglades drainage, extend from Roberts, North, and Watson Rivers across Whitewater Bay and through Cape Sable. The paleochannels became blocked and began to infill as coastal mud ridges built up along the seaward margin of Cape Sable 2,400 to 2,000 ybp, following a small (less than 1 meter), rapid rise in sea level. Blockage resulted in the abandonment of the original channel complex and the shifting of the Everglades drainage system northward toward the present outlets through Shark River and Harney River.

Aerial photograph analysis indicates five primary and numerous smaller relict channel features across the region. These paleochannels are revealed through the presence of three primary features: large relict channel segments within Whitewater Bay that tie directly into Roberts, North, and Watson Rivers; relicts of channel features extending across Whitewater Bay into the landward margin of Cape Sable; and channel features disrupting the earliest coastal mud ridges on the seaward side of Cape Sable. These latter features have been progressively revealed as the interior freshwater marsh of Cape Sable has historically collapsed in response to saline water intrusion.

Cores, collected from just inside the mouth of one of the paleochannels within Cape Sable, record the progressive channel restriction and infilling as expanding coastal mud ridges blocked flow. In this mud-dominated system, abandoned channels were infilled with organic and carbonate muds. Molluscan fauna record an upward isolation from marine influence as paleochannels were blocked and infilled, culminating in a highly stressed Hydrobia spp. assemblage in the upper portion.