Southern Coastal Systems

Transcription

1 APPENDIX 7-2: INTER-CONNECTIVITY OF SOUTHERN COASTAL SYSTEMS WITH UPSTREAM ENVIRONMENT The region is inextricably linked to the upstream environment of the Greater Everglades. This linkage provides the fresh water that establishes and maintains brackish salinities that are critical to the native estuarine flora and fauna of the region. This appendix describes the physical inter-connectivity between the southern estuaries and the upstream watershed to provide a better understanding of factors controlling freshwater movement between the regions. Both historical and present day connectivity are provided, and each subregion of the is treated separately as each subregion has different connectivity characteristics. Biscayne Bay The Biscayne Bay watershed region can be characterized by four primary physiographic zones situated contiguously from east to west: the mangrove and coastal wetlands, the Atlantic Coastal Ridge, the Rocky Glades, and interior Everglades (Sonntag 1987, Fish and Stewart 1991) (Figure A7-2-1). The bay and its associated coastal glade wetlands are mostly separated from the Everglades by the Atlantic Coastal Ridge. This ridge generally parallels the coast and has a width of about two to ten miles. The ridge varies in elevation from eight feet above sea level in the south to 22 feet in the north and forms a natural barrier to drainage of the interior Everglades, except where breached by canals, rivers, or sloughs (also called transverse glades). East and south of the ridge are the Coastal Glades and mangrove wetlands. This zone was formerly characterized by low-lying wetlands but was drained for farming and urban development. West of the ridge, there is a flatland area of freshwater marshes over thin rocky soils about four miles wide (Rocky Glades), then the interior Everglades extends 40 or so miles inland. The interior Everglades elevations range from four to thirteen feet above sea level. Historically, water levels at and just west of the Atlantic Coastal Ridge were several feet higher than they are today (Parker et al. 1955). This drove flow eastward to the bay through the rivers and transverse glades (Figure A7-2-1). Once on the east side of the ridge, the water spread out into broad sheet flow across the extensive Coastal Glades wetlands, then rechannelized somewhat into a complex network of creeks near the bay. Since the morphology of the creeks was shallow, narrow, and sinuous, flow velocity was not large and did not ordinarily vary greatly from day to day. The continuous flow resulted in a range of salinity patterns from fresh to brackish near the coast. Also, the sheet flow across the Atlandic Coastal Ridge and Coastal Glades was relatively slow and steady, which maintained brackish salinity conditions in the nearshore waters of the bay well into the dry season. Groundwater is an important component of the Biscayne Bay regional hydrology and it is greatly influenced by the highly permeable surficial Biscayne Aquifer. Under pre-development conditions in western Miami-Dade County, water entered the limestone aquifer by lateral movement from Broward and Collier counties and by downward seepage from the Everglades and the Biscayne Aquifer, and moved southward and southeastward into Miami-Dade County to coastal discharge areas. Groundwater flow direction in the Biscayne Aquifer inland was primarily to the south and southeast. In eastern Miami- Dade County, the seasonal groundwater ridge that formed under pre-development conditions 2014 System Status Report Final

2 supported both easterly and westerly groundwater flow away from the ridge axis (Fish and Stewart 1991). Overall, surface water makes up the largest input of fresh water to Biscayne Bay; however, groundwater influence becomes proportionally greatest at the end of the dry season, typically in April and May. Groundwater discharge to Biscayne Bay occurs in two ways: seepage from the aquifer and flow through subsurface leakage channels (Parker et al. 1955). Figure A Schematic showing the four primary physiographic zones of southeastern Everglades and adjacent area, including historic freshwater flow pathways. The present day canal system (blue lines) is included for reference System Status Report Final

3 Currently, the water elevations at the ridge are controlled at less than three to four feet above mean sea level year round, and flow from the Everglades to the coast is heavily managed through a system of canals, levees, and water control structures. Drainage as a result of extensive canal systems and largescale pumping from municipal well fields has greatly altered the pre-development flow system in eastern Miami-Dade County by (1) eliminating or greatly reducing a seasonal and coastal groundwater ridge; (2) reducing groundwater flow in the lower portion of the Biscayne Aquifer; (3) reducing or eliminating seasonal westward movement of groundwater; (4) changing the pattern of freshwater discharge into the bay from long, slow releases over a broad front to pulse releases from canals following rain events; and (5) lowering the water table and inducing saltwater intrusion. In addition, this management has created an abrupt difference between saline and freshwater habitats. The very low and moderate salinity habitats have become uncommon along the bay s western shore. Rapid drainage afforded by the canals and the reduced water table now maintained in the study area have reduced the functional habitat value of the remaining freshwater wetlands. Hydroperiods that were once as long as an entire year have been reduced to days or weeks. Saltwater intrusion into the aquifer remains a continuing problem due to the introduction of drainage canals and increased groundwater withdrawal to satisfy potable water demand. Water management programs have been implemented to control the intrusion rate. A network of canals and control structures provides for water and salinity control in the area. Salinity monitoring sensors near the coastal structures indicated that the installation of salinity control structures were effective in controlling saltwater intrusion decades ago. Wellfields, which are the source of municipal water supplies, are significantly recharged by water from the Water Conservation Areas that are located northwest of the project area. Water stored in the Water Conservation Areas is used to maintain groundwater levels in the coastal area for public water supply, to irrigate the vast agricultural areas interspersed within the project area, and to maintain a freshwater head along the lower east coast for salinity control. Minimum stages are maintained in lower east coast canals, principally to provide the volume of water needed to protect the Biscayne Aquifer from saltwater intrusion. The head created in the canals raises groundwater levels, recharging the aquifer and the urban wellfields Card Sound, Barnes Sound, Manatee Bay, Long Sound, Little Blackwater Sound and Blackwater Sound Card Sound, Barnes Sound, Manatee Bay, Long Sound, Little Blackwater Sound, and Blackwater Sound are the southeastern-most estuaries on the coast of Florida, and are located between Biscayne Bay and Florida Bay (Figure A7-2-2). Card Sound has an open connection to Biscayne Bay. The others are mostly isolated from each other and from Florida Bay to the south and west (Marshall et al. 2008). Barriers to flow include natural mangrove covered shoals and the Card Sound Road and U.S. Highway 1 causeways. Several relatively small breaks in the shoals and causeways exist allowing some exchange of water, the largest opening being the dredged channel for the Intracoastal Waterway System Status Report Final

4 Figure A Map of Manatee Bay and Barnes Sound System Status Report Final

5 The Everglades and the Coastal Glades (Figure A7-2-1) are sources of fresh water to Card Sound, Barnes Sound, Long Sound, and Manatee Bay. Under natural conditions, the southernmost transverse glades through the Atlantic Coastal Ridge provided a direct connection for the Coastal Glades to the freshwater marshes of the Rocky Glades in the Greater Everglades. Fresh water in the Coastal Glades also comes from precipitation or emergent groundwater. Intruding salt water into fresh coastal water table aquifers can force fresh or brackish water in the water table aquifer to emerge above ground and flow overland to the estuarine receiving water (Nuttle and Portnoy 1992, Nuttle and Harvey 1995). Before the diversion of fresh water for agricultural or urban uses, when water levels were high in Shark River Slough (SRS) within the Greater Everglades and rainfall was abundant, fresh water could stage in the Rocky Glades then flow through the transverse glades into the Coastal Glades. Under natural conditions, the combination of local precipitation and Greater Everglades-sourced fresh water discharged through local, small-scale coastal creeks and streams into Card Sound, Barnes Sound, Manatee Bay, and Long Sound. The very low slope of the Coastal Glades meant that fresh water could pond there under natural conditions as the wet season progressed, to be discharged into Card Sound, Barnes Sound, Manatee Bay, and Long Sound, continuing well into the dry season. The currently managed freshwater flows to the southeastern-most estuaries are dominated by the influence of the C-111 Canal and the extension of the L-31E Canal. In the northern areas of the Card Sound watershed, the management of water for the existing power plant has altered the connection of Card Sound to the hydrology of the watershed. These drainage ditches remove ponded water in the Coastal Glades and groundwater from the porous substrate, collecting like freshwater horizontal wells. The general result throughout most of the Coastal Glades is drier conditions than the historic, nonmanaged system. Under the managed regime, fresh water is concentrated in the C-111 Canal, controlled by the S-18C structure, then directed to the southeast and discharged through the S-197 structure into Manatee Bay. When the S-197 is open, it can rapidly reduce the salinity in Manatee Bay to less than 10 practical salinity units. Some of the water in the C-111 Canal flows over the southern bank towards estuarine Long Sound. The C-111 Canal also collects and diverts ponded Coastal Glades water towards the east that may have naturally flowed into Joe Bay. Fresh water is also trapped behind (west of) U.S. Highway 1 creating much wetter local conditions. In the current managed-flow system, the volume and timing of freshwater flows being discharged to the estuaries is not always based on the amount and pattern of precipitation, as was the case for the natural system. For example, the needs of drainage for the agricultural activities west and southwest of Homestead may require a discharge of fresh water from the C-111 Canal when wet season precipitation is ending, which can release large quantities of fresh water into Manatee Bay and Long Sound over a short period of time. In the meantime, Joe Bay is starved for fresh water. Under natural conditions, fresh surface water in the Coastal Glades would have been stored as ponded water and released slowly into Barnes Sound, Manatee Bay, and Long Sound. Many believe that this unnatural temporal and spatial distribution of fresh water being discharged from the Coastal Glades has negatively- affected the waterbased ecology of the Coastal Glades, Joe Bay, Long Sound, Barnes Sound, and Manatee Bay System Status Report Final

6 Fresh water was also historically stored as a water table aquifer in the sands of the Atlantic Coastal Ridge. Some of this groundwater emerged in the Coastal Glades. Water management has depleted this freshwater source. The effect of loss of this freshwater source is exacerbated by the function of the C 111 Canal because the remaining Coastal Glades fresh water, which now comes primarily from precipitation, is collected and directed away, instead of forming a dispersed, slower moving freshwater front towards Long Sound, Barns Sound, and Manatee Bay. Therefore, in these southeasternmost areas of coastal Florida the natural, relatively slow-reacting, dispersed hydrologic connection between fresh water in the Greater Everglades and the estuaries has been replaced by a short-circuiting flow path for fresh water flowing to the estuaries in a more concentrated manner. This has reduced the storage of fresh water in the Coastal Glades, concentrated fresh water in some areas, and starved fresh water flow to other areas, resulting in altered spatial and temporal patterns of salinity in the estuaries. Florida Bay The natural connection of Florida Bay to the Greater Everglades was primarily through the discharge of fresh water from Taylor Slough and freshwater contributions from the west end of the Coastal Glades into the nearshore embayments of eastern and central Florida Bay (Joe Bay, Little Madeira Bay, and Terrapin Bay; Figure A7-2-3). Freshwater flow from Taylor Slough into Florida Bay has been impacted not only by the reduction of water in the Greater Everglades by regional water management but also by the loss of emerging groundwater from the water table under the Atlantic Coastal Ridge. Joe Bay naturally received some fresh water from the Coastal Glades that is now diverted by the C-111 Canal. Ponded water in the Rocky Glades of the Greater Everglades north of the pineland keys in Everglades National Park (ENP) contribute to the flow in Taylor Slough as a result of high stages in SRS and/or local precipitation on the Rocky Glades. Water management upstream has reduced water levels in SRS within the Greater Everglades, thereby reducing the opportunity for this source of water to contribute to freshwater flows in Taylor Slough, and consequently to discharge into Florida Bay. Loss of the Atlantic Coastal Ridge groundwater source of fresh water to the Rocky Glades has also contributed to lower stages in this wet prairie. It is inferred from paleoecological evaluations in Florida Bay that water levels in SRS before the alteration of natural freshwater flows in the Greater Everglades may have been feet higher than the current observed levels, which would require about two times more flow than is currently being discharged (on average) across Tamiami Trail into SRS (Marshall et al. 2009, Marshall and Wingard 2012, Marshall et al. in press). Paleoecological evaluations also indicate that the flow of fresh water into Taylor Slough may be three to four times less than natural conditions (Marshall et al. 2009, Marshall et al. in press). Even so, recent work by Zapata-Rios and Price (2012) indicates that groundwater discharges into Taylor Slough still contribute about 27% of the freshwater inputs under current conditions. Therefore, Taylor Slough still has a relatively important connection to groundwater flowing from upstream sources System Status Report Final

7 Figure A Map of central Florida Bay. The loss of freshwater flow into Joe Bay, Little Madeira Bay, and Terrapin Bay has increased the salinity throughout central Florida Bay, although the increased flow of managed fresh water into east Long Sound may have moderated this loss of fresh water to some extent. However, managed freshwater flows directed into extreme northeastern Florida Bay typically come at an unnatural time to be beneficial to the ecology of central Florida Bay. The estuarine ecology of Florida Bay benefits from the sustained, dispersed flow of fresh water past the end of the wet season, thereby allowing for the existence of oligohaline to mesohaline conditions well into the dry season. To the west, freshwater flows from western Taylor Slough can pond in the coastal prairie behind the Buttonwood Embankment along the shoreline of central Florida Bay because of the lack of coastal creeks such as those that naturally drain the Coastal Glades to the east. The only locations of discharge into central Florida Bay for this trapped western Taylor Slough water are McCormack Creek discharging into Terrapin Bay and Alligator Creek discharging into Garfield Bight. The residence time for this trapped water in the mangrove areas and open waters of Seven Palm Lake, Cuthbert Lake, Middle Lake, Monroe Lake, The Lungs, Henry Lake, and West Lake is naturally high. During the dry season, and particularly during drought periods, hypersaline conditions have been routinely recorded. The extent to which 2014 System Status Report Final

8 natural freshwater flows from the Greater Everglades via Taylor Slough mitigated the creation of hypersaline conditions in this area in the past is only now being understood (Frankovich et al. 2011, 2012). The oligohaline conditions in the observed data from Roberts River and North River just to the west may indicate that the direct influence of fresh water discharging from SRS on the salinity of the West Lake/Cuthbert Lake/Seven Palms Lake area was historically small. The results of Florida Bay paleoecological evaluations indicate that the salinity in Terrapin Bay and Little Madeira Bay has been impacted greatly (i.e., increased) by the reduction in Taylor Slough flows. Inferred results from these studies suggest that, under natural conditions, Taylor Slough flows may have been two to three times higher than the current conditions (Marshall et al. 2009, in press, Marshall and Wingard 2012). Lower Southwest Coast The largest natural channel for freshwater flows in ENP is SRS, which discharges into the Gulf of Mexico and Whitewater Bay via fresh and brackish water rivers that are characterized by vast stands of mangroves along the banks (Figure A7-2-4). SRS is a dominant feature in the Greater Everglades landscape and freshwater flows into SRS are controlled by the Tamiami Trail causeway and the structures that allow and control flow underneath the roadway. However, once freshwater enters SRS in ENP it is mostly unaffected by any water management features. The mangrove-lined rivers ( mangrove rivers ) of the ENP west coast are wider, deeper, and longer than the coastal creeks that drain the Coastal Glades into the nearshore embayments of northeastern Florida Bay. Reasons for this may be the very large flow volume of SRS compared to Taylor Slough flow, and the addition of emergent groundwater from the Biscayne Aquifer and local precipitation. Compared to the relatively small tidallyinduced flow and stage in Florida Bay due to the dampening effects of mud banks, tidal effects along the lower southwest coast estuaries are larger. These larger tidal effects, combined with larger freshwater outflows from SRS, likely contribute to the lack of a slightly higher elevation belt along the shoreline as is seen along the Florida Bay shoreline. Differing wind patterns and sediment composition may also play a role. In addition, the effect of local rainfall on the estuarine reaches of the mangrove rivers is small due to the small surface area of the stream. For these reasons, there is a more direct effect of Greater Everglades hydrology on the salinity in the estuarine reaches of the mangrove rivers on the Gulf Coast compared to the nearshore areas of Florida Bay. Preliminary paleoecological investigations in the estuarine areas of the ENP Gulf Coast indicate that the salinity regimes in the mangrove rivers of the Gulf Coast have also been affected by the reduction of stages in the Greater Everglades (SRS) due to water management diversions (Frank Marshall, personal communication). Initial results indicate that the effect of water management on salinity is not as severe as the effect of water management on salinity in Florida Bay. These paleoecological analyses are currently underway and should be completed in time for the results to be included in the next System Status Report System Status Report Final

9 Figure A Map of the Lower Southwest Coast embayments System Status Report Final

10 Ten Thousand Islands The Ten Thousand Islands area of the upper southwest Florida coast receives drainage primarily from Big Cypress National Preserve, although there is no defined freshwater slough similar to SRS (Figure A7-2-5). The hydrologic connection between the upstream Big Cypress Preserve is still relatively unchanged from historic conditions. Although the freshwater drainage patterns of this area have not been widely studied, there are some small coastal creeks that direct fresh water into a series of open water areas within the mangrove zone that ultimately flow through other creeks and rivers to discharge into the Gulf of Mexico. The connection between the upland drainage area in Big Cypress and the downstream estuarine portions of the discharging rivers and bays in the Ten Thousand Islands area is direct but dispersed. Emergent groundwater may also be an important freshwater source in this part of the Gulf coast. The Picayune Strand Restoration Project area lies upstream from and discharges into the Ten Thousand Islands area. The connectivity between the project area and the downstream estuaries has been significantly altered by man. Picayune Strand is an upland/wetland area with a historically persistent high water table that is located north of U.S. Highway 41. Drainage for this area was created by a series of canals that were intended to reduce the water table to the point where a large urban development project could be sited. However, the development failed, and the land went into public ownership and became the Picayune Strand Restoration Project. The Picayune Strand water management system includes Faka Union Canal that provides a point source discharge into Faka Union Bay. Discharges from this canal can drastically reduce the salinity in Faka Union Bay. Therefore, there is a direct connection between the freshwater flows from the Picayune Strand water management system and the salinity in Faka Union Bay and the adjacent, connected estuarine areas. The water management system in Picayune Strand is in the process of being retooled to reduce the impact of the freshwater discharge on the estuarine areas of Faka Union Bay and adjacent areas System Status Report Final

11 Figure A Map of the Ten Thousand Islands System Status Report Final

12 Summary There is significant evidence from the observed data, the output from hydrologic and salinity models, vegetation maps, and the results of numerous studies that there is a connection between the hydrology (stage and flow) in the Greater Everglades and the salinity in the estuarine areas of south Florida. Natural connections in the east, where urban and agricultural development has occurred, have been altered by water management. Significant alterations to the connection between the Greater Everglades and the salinity in Barnes Sound, Manatee Bay, Long Sound, and eastern Joe Bay include the loss of underflow from the water table aquifer that once existed near the Atlantic Coastal Ridge and the diversion of large volumes of water from the Coastal Glades by the C-111 Canal. In western Joe Bay, Little Madeira Bay, and Terrapin Bay (and perhaps Garfield Bight) the large reduction in Taylor Slough flows and stored fresh water has increased average salinities, which translate downstream to elevated average salinity values in central Florida Bay. Under current conditions, the connection between the Greater Everglades and Taylor Slough remains but is significantly altered, and the impact appears to be the greatest in the Seven Palms Lakes region of ENP where hypersaline conditions occur regularly. On the ENP west (Gulf) coast, the connection of SRS to the mangrove rivers is still relatively strong, although paleoecological studies indicate that water levels and flows in SRS are significantly lower than pre-water management times. However, the hydrologic connection between surface water in SRS /Rocky Glades and surface water in Taylor Slough, which may have only been intermittent historically, is now weak and mostly associated with groundwater. In the Ten Thousand Islands area, the hydrologic connection between the upstream Big Cypress National Preserve is still relatively unchanged from historic conditions. In Picayune Strand, the manmade drainage system creates a streamlined surface connection between the hydrology in Picayune Strand and the salinity in Faka Union Bay. Historically, that connection would have been primarily through groundwater and perhaps slow meandering surface flow via small creek networks. Any increase or decrease in discharges from the drainage system can rapidly and significantly affect the salinity in the downstream estuarine waters. The complexity and importance of restoring the original hydrological and salinity regimes of Greater Everglades-Florida Bay ecosystem necessitates coordination and cooperation between researchers and managers of the Greater Everglades and regions for the success and sustainability of the Comprehensive Everglades Restoration Plan and the ecosystem as a whole. Performance measures and targets for both regions should be implemented with both freshwater and estuarine ecology objectives in mind. The alternative could result in the loss of the unique, interconnected water-based ecology that provides so many ecosystem services to all System Status Report Final

13 References Fish, J.E. and M. Stewart Hydrogeology of the surficial aquifer system, Dade County, Florida. Water Resource Investigations Report , United States Geological Survey, Washington, DC. Frankovich, T.A., J.W. Fourqurean and D. Morrison Benthic macrophyte distribution and abundance in estuarine mangrove lakes: Relationships to water quality. Estuaries and Coasts 34: Frankovich, T.A., J.G. Barr, D. Morrison and J.W. Fourqurean Differing temporal patterns of Chara hornemannii cover correlate to alternate regimes of phytoplankton and submerged aquaticvegetation dominance. Marine and Freshwater Research 63(11): Marshall, F., W. Nuttle and B. Cosby Final Report: Biscayne Bay Freshwater Budget and the Relationship of Inflow to Salinity. Environmental Consulting and Technology, Inc., New Smyrna Beach, FL, submitted to South Florida Water Management District, West Palm Beach, FL. Marshall, F.E., G.L. Wingard and P. Pitts A simulation of historic hydrology and salinity in Everglades National Park: Coupling paleoecologic assemblage data with statistical models. Estuaries and Coasts 32(1): Marshall, F.E. and G.L. Wingard Florida Bay salinity and Everglades wetlands hydrology circa 1900 CE: A compilation of paleoecology-based statistical modeling analyses. Open-File Report , United States Geological Survey, Washington, DC. Marshall, F.E., G.L. Wingard and P. Pitts. In Press. Estimates of natural salinity and hydrology in the Southern Everglades, Florida: Implications for management. Estuaries and Coasts. Nuttle, W.K. and J.W. Harvey Fluxes of water and solute in a coastal wetland sediment, 1. The contribution of regional groundwater discharge. Journal of Hydrology 164(195): Nuttle, W.K. and J.W. Portnoy Effect of rising sea level on runoff and groundwater discharge to coastal ecosystems. Estuarine, Coastal and Shelf Science 34: Parker, G.G., G.E. Ferguson and S.K. Love Water Resources of Southern Florida with Special Reference to the Geology and Groundwater of the Miami Area. Water Supply Paper 1255, United States Geological Survey, Washington, DC. Sonntag, W.H Chemical characteristics of water in the surficial aquifer system, Dade County, Florida. Water Resources Investigations Report , United States Geological Survey, Tallahassee, Florida. Zapata-Rios, X. and R.M. Price Estimates of groundwater discharge to a coastal wetland using multiple techniques: Taylor Slough, Everglades National Park, USA. Hydrogeology Journal 20(8): System Status Report Final