Thalweg variability at bridges along a large karst river: the Suwannee River, Florida

Formation of extensive phreatic caves in eogenetic karst aquifers is widely believed to require mixing of fresh and saltwater. Extensive phreatic caves also occur, however, in eogenetic karst aquifers where fresh and saltwater do not mix, for example in the upper Floridan aquifer. These caves are thought to have formed in their modern settings by dissolution from sinking streams or by convergence of groundwater flow paths on springs. Alternatively, these caves have been hypothesized to have formed at lower water tables during sea level low-stands. These hypotheses have not previously been tested against one another. Analyzing morphological data and water chemistry from caves in the Suwannee River Basin in north-central Florida and water chemistry from wells in the central Florida carbonate platform indicates that phreatic caves within the Suwannee River Basin most likely formed at lower water tables during lower sea levels. Consideration of the hydrological and geochemical constraints posed by the upper Floridan aquifer leads to the conclusion that cave formation was most likely driven by dissolution of vadose CO 2 gas into the groundwater. Sea level rise and a wetter climate during the mid-Holocene lifted the water table above the elevation of the caves and placed the caves tens of meters below the modern water table. When rising water tables reached the land surface, surface streams formed. Incision of surface streams breached the pre-existing caves to form modern springs, which provide access to the phreatic caves. Phreatic caves in the Suwannee River Basin are thus relict and have no causal relationship with modern surficial drainage systems. Neither mixing dissolution nor sinking streams are necessary to form laterally extensive phreatic caves in eogenetic karst aquifers. Dissolution at water tables, potentially driven by vadose CO 2 gas, offers an underappreciated mechanism to form cavernous porosity in eogenetic carbonate rocks.

Section: Discussionmentioning

confidence: 99%

Formation of phreatic caves in an eogenetic karst aquifer by CO₂ enrichment at lower water tables and subsequent flooding by sea level rise

Gulley¹,

Martin

Moore

et al. 2013

“…They found the greatest water surface differences between 1956 and 1995 (~1.5 m) near the Chattahoochee gauge at RM 106, decreasing downstream until there was no difference at RM 65, then increasing again to RM 35 (~1 m decrease), then with no difference at RM 23 to Sumatra. Although the water surface data generally corresponds to degradation zones, the combination of hydrographic bed surface data with water level data better captures spatial variations and channel changes on this and other rivers (Mossa & Chen, 2021; Mossa & Konwinski, 1998).…”

Section: Discussionmentioning

confidence: 99%

“…Although the water surface data generally corresponds to degradation zones, the combination of hydrographic bed surface data with water level data better captures spatial variations and channel changes on this and other rivers (Mossa & Chen, 2021;Mossa & Konwinski, 1998).…”

Section: Sand Bar Areamentioning

confidence: 99%

Geomorphic response to historic and ongoing human impacts in a large lowland river

Mossa

Chen

2022

Lowland coastal plain rivers are important for navigation and ecosystems, having unique concerns regarding water and sediment management. The Apalachicola River in Florida, the lower end of the ACF (Apalachicola–Chattahoochee–Flint) system, had historical modifications beginning in the 1800s. Most impacts are attributable to the Apalachicola Navigation Project in the latter half of the 20th century, which included dredging, disposal, rock and snag removal, artificial cutoffs, and wing‐dike building. As one of few studies using paired hydrographic surveys over a long reach and time (170 km, 50 years), we defined four zones of bed elevation change between 1960 and 2010 based on the cumulative thalweg elevation plots. The river has four bed change zones: (1) uppermost, marked degradation from river mile (RM) 106 to 85 because of dam construction; (2) middle, minimal degradation from RM 85 to 34; (3) artificial cutoff zone, highest degradation, from RM 34 to 27; and (4) lowermost zone, aggradation from RM 27 to Apalachicola Bay because of upstream bed and bank sediment disturbances. Artificial cutoffs and lithology influence lateral migration, with rates being highest in cutoff zones and least in areas with some bedrock upstream and finer sediments downstream. Length changes from 1941 to 2019 are stable, except for the middle reach, which lengthened nearly 10%, and the lower non‐tidal reach, where length shortened with cutoffs then regained in subsequent decades. Minimizing disturbance has led to 18% of the sand bar area revegetating since a decade of dredging ended. This framework helps in understanding historic and ongoing human activities, notably upstream water consumption, connectivity changes, sediment inputs, and climate change, that will affect the water and sediment management of the river, its floodplain, and bay, thus varying approaches for restoration.

“…High degrees of calcite undersaturation are maintained along the entire length of the Suwannee River by hydraulic damming of groundwater inputs, which prevents inflow of water that is saturated with respect to calcite to the river, and armoring of the Suwannee River bed with siliciclastic sand shed from the confining unit (cf. Mossa and Konwinski, ), which reduces the contact of allogenic runoff with carbonate minerals. Flow of this undersaturated water into conduits connected to springs that reverse during floods causes phreatic conduits in the Suwannee River Basin to enlarge through dissolution (Gulley et al ., ).…”

Section: Discussionmentioning

confidence: 99%

Dissolution in a variably confined carbonate platform: effects of allogenic runoff, hydraulic damming of groundwater inputs, and surface–groundwater exchange at the basin scale

Gulley

Martin

Spellman

et al. 2013

In variably confined carbonate platforms, impermeable confining units collect rainfall over large areas and deliver runoff to rivers or conduits in unconfined portions of platforms. Runoff can increase river stage or conduit heads in unconfined portions of platforms faster than local infiltration of rainfall can increase groundwater heads, causing hydraulic gradients between rivers, conduits and the aquifer to reverse. Gradient reversals cause flood waters to flow from rivers and conduits into the aquifer where they can dissolve limestone. Previous work on impacts of gradient reversals on dissolution has primarily emphasized individual caves and little research has been conducted at basin scales. To address this gap in knowledge, we used legacy data to assess how a gradient of aquifer confinement across the Suwannee River Basin, north-central Florida affected locations, magnitudes and processes of dissolution during 2005-2007, a period with extreme ranges of discharge. During intense rain events, runoff from the confining unit increased river stage above groundwater heads in unconfined portions of the platform, hydraulically damming inputs of groundwater along a 200 km reach of river. Hydraulic damming allowed allogenic runoff with SI CAL < À4 to fill the entire river channel and flow into the aquifer via reversing springs. Storage of runoff in the aquifer decreased peak river discharges downstream and contributed to dissolution within the aquifer. Temporary storage of allogenic runoff in karst aquifers represents hyporheic exchange at a scale that is larger than found in streams flowing over non-karst aquifers because conduits in karst aquifers extend the area available for exchange beyond river beds deep into aquifers. Post-depositional porosity in variably confined carbonate platforms should thus be enhanced along rivers that originate on confining units. This distribution should be considered in models of porosity distribution used to manage water and hydrocarbon resources in carbonate rocks.