Abstract

| THE ICHETUCKNEE RIVER SYSTEM is one of three of the best known and most studied spring and river systems in Florida. The system begins in the Ocean Pond, Lake City and Raiford Ridges geomorphic provinces near Lake City and ends in the Branford Karst Plain geomorphic province. It crosses the Cody Escarpment from recharge areas to the north and the springs and river to the south.

Of interest within the system are (1) an area of minimal recharge, (2) poljes and swallets within the escarpment that contribute water to the drainage basin (springshed), (3) karst windows, (4) a karst trace, (5) glorious springs, and, finally, (6) the Ichetucknee River itself.

This paper describes these features and how they contribute to the springs and river within the Ichetucknee Springs State Park.

Topography and Hydrogeology

A springshed is the drainage basin that supplies water to a spring or spring group. As is the case with the Ichetucknee River system, a springshed may include areas where recharge to the aquifer that hosts the groundwater flow system and springs is minimal and well as areas where recharge is concentrated in karst features, and areas where it is dispersed.

Figure 1 shows the Ichetucknee Springshed with the regional topography. The relatively high areas shown in a greenish yellow tint are within the recently named Ocean Pond, Lake City Ridge, and Raiford Ridges provinces of Williams et al. (2020). The Miocene Hawthorn Group of sediments underlies this area. Because of its clay content and the tortuous flow paths that recharging water must take to reach the Eocene Ocala Limestone below, the Hawthorn Group serves as an aquitard that limits recharge where it is thick. Even so, there is groundwater drainage in the Ocala Limestone that feeds the Ichetucknee River System.

The Cody Scarp (Puri and Vernon, 1954) is a fluviokarst escarpment (Upchurch, 2014) that represents headward erosion of the stream and sinkhole topography as the Hawthorn Group is removed. On Figure 1, the Cody Scarp is represented by the brown colors. Notice how “rough” the topography appears. This is because the escarpment is characterized by sinkholes and larger poljes surrounded by erosional remnants of the Hawthorn Group. The low areas in the scarp are at or near the top of the Eocene Ocala Limestone. As streams drain the highland areas, they encounter these karst features (swallets) and form a region of enhanced recharge. Only the largest rivers (i.e., the Suwannee River) cross the escarpment without going underground. The Cody Scarp is termed the Alachua Karst Hills province by Williams et al. (2020).

The green and greenish brown areas on Figure 1are part of the Branford Karst Plain province of Williams et al. (2020). This karst plain has little or no remnant Hawthorn Group sediments. Where present, they are residual clays that cap the low hills. A thin veneer of Plio-Pleistocene marine sand mantles the Ocala Limestone here. Because there is no aquitard and no streams in this area, it serves as a region of dispersed recharge to the limestone aquifer.

The Springshed

Figure 2 shows a hypothetical springshed based on the Ichetucknee River System (Scott et al., 2004). A springshed is a combination of all regions that contribute to the water discharging from a spring or spring group (Copeland, 2002). In systems like the Ichetucknee System, the springshed includes both a groundwater basin and surface water drainage basins. Streams drain the areas where the aquifer is confined, and recharge is limited. When they reach the escarpment, they recharge the aquifer through a swallet. Because groundwater drains the limestone aquifer under the confined area, the spring’s groundwater basin typically extends upgradient under the area of confinement. The limestone under the confined area has lower permeabilities than in the unconfined karst plain because of limited karst development so the potentiometric surface is typically higher than in the karst plain.

The groundwater basin part of the springshed for the Ichetucknee River System is shown in Figure 3. This map illustrates the potentiometric surface of the upper Floridan aquifer as determined by Upchurch and Champion (2003). If you compare Figures 1 and 3, it is evident that a significant part of the groundwater
basin extends under the uplands, where aquifer confinement is significant. For example, the Hawthorn Group and confinement begin near Alligator Lake in Lake City. However, the groundwater basin extends all the way northeast to Ocean Pond.

Notice also that the potentiometric surface is nearly flat northeast of the 45-foot contour, is steep from the 45- to the 30-foot contour and is again nearly flat southwest of the 30-foot contour. This pattern indicates where recharge is occurring. Under the highlands, recharge is minimal, and the relatively low permeability of the Ocala Limestone retards drainage to the southwest and results in a potentiometric surface high. The area of rapid change of the potentiometric surface is where permeability is elevated, and recharge is enhanced. This area coincides with the Cody Scarp. Where the surface flattens out to the southwest, permeability of the limestone aquifer is relatively high, and recharge is dispersed through the many sinkholes in the area.

Part of the reason that the reentrant of the potentiometric surface extends as far northeast as Ocean Pond is groundwater use in the Lake City area. However, the sinkholes and other karst features in the area are also well developed. The extent of the springshed is sensitive to recharge and water use. It expands and contracts as rainfall patterns and consumptive use vary.

Sinkhole Lakes, Poljes and Swallets

Streams that originate in the highlands flow toward the Cody Scarp, largely because the Hawthorn Group of sediments prevents significant recharge of the underlying upper Floridan aquifer. However, when these streams reach the Cody Scarp, they flow into swallets, poljes, and sinkhole lakes. Swallets are sinkholes that capture streams. Sinkhole lakes are sinkholes that intersect the surficial or upper Floridan aquifer and are water filled. Poljes are large sinkholes that are filled with lake or wetland sediments. All three are present in the Cody Scarp.

Swallets

Figure 1 shows the locations of known swallets in the Ichetucknee Springshed. Note that all the known swallets are within the brown elevations that reflect the Cody Escarpment. Let’s look at two of the best-known swallets – Cannon Creek Sink and Rose Creek Sink (Figure 4).

Cannon Creek drains the Alligator Lake area on the eastern side of Lake City. It includes an overflow channel for the lake and discharges into a pair of swallets that are adjacent to I-75 and State Route (SR) 47. Normally, the creek goes underground in the swallets (Figure 5). When the stream flow exceeds the infiltration capacity of the swallets, stream flow continues to Clay Hole Sink (Figure 4) and then to flooding the dry stream valley downgradient.

The Rose Creek Swallet is in the Community of Columbia in the northeast corner of the intersection of SR 47 and County Road (CR) 240. There are two swallets in this location. The upstream swallet captures water from Rose Creek under normal circumstances and the downstream one is a karst window exposing upper Floridan aquifer water in a sinkhole (Figure 6). During floods, water enters the aquifer via both sinkholes and then flows into the dry stream channel that connects to the Ichetucknee River.