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Physical Characteristics of the North Fork St. Lucie River Quick Topics

Topography and Geomorphology

The North Fork St. Lucie River (SLR) watershed is defined by four regions with differing land characteristics. The river itself is located in the Eastern Valley region which is composed of long, low narrow ridges ranging from 15 to 30 feet in elevation. The natural topography of the watershed is generally flat with few natural rises. Most of the preserve is between one and five feet of elevation and consists of wetland communities including tidal and floodplain swamp and forest. Taller features in the watershed all are manmade and include bridges, roads, high rise buildings, communication towers, spoil piles and water control structures such as pump stations, canals and levees.

Map of historic rivercourse for St. Lucie River's North Fork

Sections of the North Fork SLR were straightened between the 1920s and 1940s for navigation and flood control. The associated spoil was piled as much as 25 feet high and 50 feet wide along the newly-created channel. In addition to increasing bank elevations along the North Fork, flood control expanded the North Fork watershed and altered associated topography and drainage patterns. Prior to these changes, wet season rains pooled broadly across the SLR watershed and moved toward the naturally lower elevations surrounding the river. The construction of the drainage canal system has resulted in controlled discharges of water from west to east through a canal system. Historical flows from the North and South Forks of the SLR have decreased, and large volumes of water now enter the Middle Estuary. Historically, most rainwater was absorbed by the region�s wetland system, which reduced the amount of nutrients and sediment reaching the river during the wet season and increased the amount of groundwater reaching the preserve during the dry season.

Geology

The SLR and its watershed are comprised of coastal lowlands which formed during the advance and retreat of glaciers during the most recent ice age. The region contains four surface geologic formations:

  • Holocene - consists primarily of sand, clay, and organics and formed over 10,000 years ago. It occurs near the coastline at elevations lower than five feet.
  • Holocene/Pleistocene - composed primarily of sand located along the beach ridge and dunes and is associated with the Atlantic Coastal Ridge.
  • Pleistocene (Anastasia formation) - composed of limestone, coquina, and sand and formed over 1.8 million years ago. Lies under the Atlantic Coastal Ridge from St. John�s County south to Palm Beach County. It can be exposed along the coast, and extends up to 20 miles inland.
  • Pliocene - contains some of the most abundant and diverse fossils in the world and dates back to over 5.3 million years. These complex sediments, composed of shells, sand, and clay, confound the origin of this formation, but it is now known as the Tertiary- Quaternary shell unit.

These four surface geologic formations overlie basement rocks of the Florida Platform, including Precambrian- Cambrian igneous rocks, Ordovician-Devonian sedimentary rocks, and Triassic-Jurassic volcanic rocks.

Sediment analyses of the SLR have documented the transition from coarse, well-sorted sands along the shallow margins of the river to fine silt and clay particles (muck) in the deepest areas of the North Fork. In comparison to other sites in the SLR, the North Fork showed the least amount of sand and the most mud, silt/clay, and ooze (soft decaying organic matter). The high concentrations of fine grain sediments negatively impact the health and abundance of the seagrass and oyster reefs within the preserve.

Map of sediments for the St. Lucie River

Map: Chris Perry

 

 

 

 

 

 

 

 

Last updated: April 06, 2015

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