Seasonal Patterns of River Connectivity and Saltwater Intrusion in Tidal Freshwater Forested Wetlands

Abstract

Hydrology can be difficult to interpret in tidal freshwater forested wetlands because of several influencing factors including river discharge, tidal stage, local precipitation, evapotranspiration, groundwater and prevailing winds. River discharge and tidal stage are considered the primary factors affecting these wetlands and both vary on a seasonal basis and have potential implications for wetland saltwater intrusion. We examined the hydrologic patterns of tidal freshwater swamps along the lower reach (20 km) of the Apalachicola River system in northwest Florida, USA. Water level recorders were installed in five wetlands along this reach and recorded ground and surface water levels at 15-min intervals from December 2006 to April 2009 (non-continuously). Seasonal trends related to flooding frequency and duration were compared to long term (2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) records of creek and river salinities measured in the tidal freshwater forested zone. Tidal freshwater swamps tended to maintain a narrow water level range (often AE20 cm of the ground surface) except during high river flows (>680 cms or 24 000 cfs) when levels increased 1 to 2 m above ground and dampened tidal effects. All tidal forested wetlands had the highest mean water levels and most days tidally pulsed during the Jul-Oct season which coincided with peak river/creek salinities and low river-flows associated with drought conditions. We explored the implications for the timing of salt water intrusion in these swamps and potential changes related to sea level rise and water management in the Apalachicola River drainage basin.

Forested Wetland Communities as Indicators of Tidal Influence along the Apalachicola River, Florida, USA

“…We found that wetlands in the upper study reach (>20 river-km) were seasonally flooded, with water levels corresponding closely to changes in river stage. Along the lower study reach (<20 river-km) wetlands had a much narrower water level range (normally ±20 cm of the ground surface) except during high river flows (>680 cms or 24,000 cfs), at which point river flow exerted greater influence and water levels increased with rising river stage (Anderson and Lockaby 2011). Unlike non-tidal forests, these wetlands maintained water levels and exhibited tidal fluctuations even during low river flow periods.…”

“…1). Further details of our hydrologic analysis are provided in Anderson and Lockaby (2011) and summarized here. Water levels were monitored in shallow monitoring wells using In-Situ MiniTroll vented pressure transducers (5 PSIG, Model Standard P, In-Situ, Inc., 2000).…”

“…Water level data have shown that in the upper part of the study reach, flooding and groundwater levels are closely linked to changes in river stage. Closer to the river mouth, hydrology is influenced by daily and seasonal changes in sea level (Anderson and Lockaby 2011). The astronomical tidal cycle and other climatic factors (e.g., prevailing winds) cause sea levels to vary on a daily, weekly, and seasonal basis.…”

“…This flooding does not normally become so deep or prolonged to exclude all trees (Hook et al 1970, Hook 1984, Brinson et al 1995, but it can produce extremely anaerobic conditions that prevent many tree species from becoming established, especially in lower lying areas. The extent of anaerobic soils is obscured somewhat by the temporal variability of tidal flooding (daily, weekly, seasonally, and annually; Day et al 2007, Hackney et al 2007, Anderson and Lockaby 2011, and by a hummock-hollow microtopography that can also influence vegetation and edaphic conditions ). In the non-tidal zone, hydrology is controlled primarily by river stage, which typically exhibits seasonal highs and lows.…”

Hydrological modification, saltwater intrusion, and tree water use of a Pterocarpus officinalis swamp in Puerto Rico

“…Tidal freshwater forested wetlands are of particular concern since they are likely to occur in a narrow fringe between freshwater and saltwater inputs, near their tipping point and influenced by multiple, simultaneous stressors (Conner et al, 2007). In addition, tidal influence on freshwater forested wetlands is not well understood (Anderson and Lockaby, 2012), mainly because of a lack of data on how saltwater intrusion into rivers affects adjacent forested wetland soils and what is the primary mechanism of these saltwater inputs (e.g. surface or groundwater storm surges or abnormal spring tides; Conner et al, 2007).…”