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The Road to Flamingo: an Evaluation of Flow Pattern Alterations and Salinity Intrusion in the Lower Glades, Everglades National Park

Ingraham Hwy & Homestead Canal
Airborne Resistivity
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The Culverts from 1961 - 1971

Construction of the levees enclosing the Water Conservation Areas (WCA), north of ENP, the filling of WCA-3, and the excavation of ditches east of the Park shut off much of the water flow into ENP from 1962 until late 1965 when hurricane Betsy flooded the Everglades. The severe water shortages in the Park from 1961-1965 were further aggravated by the lack of rainfall, making it one of the driest times on record. The lack of water flowing into the Park had serious effects on wildlife and plant communities. Craighead estimated in 1965 that the alligator population of the eastern portion in the Everglades National Park was hardly 5% of the population in 1960. Although most of the wildlife data reported by Craighead were based on visual observations and not on systematically collected data, it became clear that the low water levels of the early 1960's stressed every aspect of the wetlands. The low water levels of May 1962 coincided with extensive fires, burning vast areas of deep water sloughs which had been exposed. These fires also destroyed many hammocks with trees over one hundred years old.

graph illustrating water levels for P37
Figure 4: Period of record water levels for P37. [larger image]

Water levels in the southern portion of the Park regularly dipped below sea level as depicted by the hydrograph of station P37 (Fig. 4). Contour maps drawn by [ 9 ] show the spatial extent of the dry conditions. The lack of surface water flows to the southern glades, combined with the low rainfall and high evapotranspiration rates of the late dry season caused the large dryouts. These conditions were not restricted to the early 1960's, when Craighead and colleagues were doing most of their work. The lack of surface water was also endemic during the period of uncontrolled drainage prior to and into the 1940's. The lack of coastal control structures on the canals north and east of the Park allowed drainage to occur well into the dry season, causing overdrainage in many parts of the system. Low water levels in May 1945 were contoured by ([ 15 ]) (Fig. 5), and determined by [ 12 ] to be more severe than the 1962 conditions (Fig. 6).

water level contour map for May 1945
Figure 5: Water level contour map for May 1945. Source: Schroeder et al.,
[1958] [larger image]
water level contour map for May 1962
Figure 6: Water level contour map May 1962. Source: Leach et al.,
[1972] [larger image]
During periods of low water levels, west-northwest winds blowing across Whitewater Bay create wind tides which drives salt water far into the fresh water wetlands of Everglades National Park. In one particular case it is documented that after several days of strong northwest winds with practically no fresh water in the inner mangrove zone west of S.R. 9336, salt water was driven in and banked up along the west side of the road flowing six to ten inches deep through the culverts ([ 3 ]). These storm driven waters were reported to have completely surrounded Mahogany Hammock. Lack of persistent freshwater flow failed to flush the saltwater off the wetlands within a reasonable amount of time causing ecological changes to set in.

Based on his field observations, Craighead made a recommendation to block culverts 76 through 144 with sandbags. The proposed project was reviewed by an advisory group in 1964, and with their affirmative recommendation was authorized by the NPS Regional Director. By blocking the flow, it was hoped that fresh water could be retained in the eastern marshes south of Pumphouse Road to West Lake. The sandbagging of the culverts acts as a barrier to surface-water flows helping to retain fresh water to the east of the road and stem the water losses to the west in much the same way as the natural embankments north of Florida Bay (i.e. the Buttonwood strand) act as dams. Craighead in his 1967 report stated that the road embankment alone, even with the culverts open, had a considerable retarding effect on the westward flow of water immediately after heavy rains. From Craighead's papers, one can infer that he believed the general flow of water was to the west or southwest across S.R. 9336.

The culverts were blocked in November of 1964 by placing sand bags in the west end of each culvert. Three to five bags were used in each culvert. Between 1964 and 1967 the bags had to be replaced several times because the cloth would rot, alligators would move them, or high water events would wash the sandbags from the culverts. Craighead noted that all the remaining bags were flushed out after the heavy rains in June 1967. However, surveys in 1996 showed that several southern culverts were still partially blocked by the remains of sandbags. During times when there was sufficient water in the system, the blocked culverts helped retain fresh water east of the road.

Observations during the first year after blocking the culverts from November 1964 to May 1965 showed water levels remaining between 0.2 ft to 0.5 ft higher on the east side as compared to the west side of the road. This enabled the area east of S.R. 9336 to retain water further into the dry season. Only a few observations were made on both sides of the road, but Craighead's water level data clearly showed the differences between east and west. After the results of the first year were analyzed, it was felt by Craighead and others that this was an effective way to impound fresh water for wildlife use further into the dry season.

During the early 1960's the area west of the highway flooded with salt water unimpeded because of the dry glades. Marine tides combined with favorable winds pushed water from Whitewater Bay and Coot Bay into the wetlands and backed up west of the road to a depth of 0.5 to 1.0 foot ([ 2 ]). Fed by the East River into Whiskey Creek the saline water flowed east killing fresh water flora and fauna. In addition to the natural creeks, the Buttonwood Canal allowed saline water to flow west along the highway. Coot Bay was changed from a brackish-fresh water system to a marine environment causing the loss of foraging habitat for thousands of ducks and coots.

The Buttonwood Canal extends 3.2 miles from Florida Bay to Coot Bay. Although the canal was dug beginning in 1922, the connection to Coot Bay was not made until August 1957. Objections to the connection were made by Park ecologists, but the demand for enhanced visitor experience outweighed ecological concerns. At that time a temporary earthen dike at the mouth in Flamingo was removed and the saline waters from Florida Bay were free to interchange with Coot Bay. The Homestead Canal had also been excavated along the eastern edge of Coot Bay, which allowed both Coot Bay and Whitewater Bay to contribute saline water to the canal.

Current Condition of Culverts

Although water level measurements continued from 1965 to 1971, little attention was paid in the following decades to the culverts below the S.R. 9336 intersection with Royal Palm Road. During the planning for the C-111 project, culverts 9 through 22 and the Taylor Slough bridge were evaluated and redesigned. Drought conditions in 1989 and 1990 again caused water levels to drop below sea level in the wetlands and hypersaline conditions prevailed in the upper estuaries of Florida Bay. No detailed investigations were made of the effects of the drought situation along the road near Flamingo. Recently, the South Florida Natural Resources Center initiated a study to re-examine the culverts and again evaluate what role the road plays in altering natural flow patterns. Observations tend to confirm that some impoundment of water on the eastern side of the road still occurs, at least in the beginning of the 1996 wet season.

To determine if there is a notable stage difference between the north/south and east/west sides of S.R. 9336 and the effect it has on the flow of water, discontinuous flow and stage measurements are being conducted. These measurements will help quantify the flow patterns that currently exist between Royal Palm Road and Mrazek Pond. Assessments on the condition of the culverts show that some of the culverts blocked by Craighead still contain sandbags on the west end of the culvert, and many of the approaches to the culverts below Nine Mile Pond are overgrown with vegetation and partially blocked with detritus which may retard flow.

map showing locations of staff gauges
Figure 7: Staff gauges. [larger image]

More recent water level and flow measurements

Staff gauges (Fig. 7) have been installed in and near seven culverts along the main Park road and at two locations on the Pay-hay-okee overlook access road. The recorders have been installed in upstream/downstream pairs. Along with the discontinuous staff gauges (88NGVD), two stations (SR-1 and SR-2) measuring continuous stage have been installed between culverts 57 and 58. SR1 and SR2 have been operational since November 1997. Fifteen culverts have been selected for routine flow measurements along the main Park road along with two culverts on the Pay-hay-okee overlook access road and one on the road leading to Mahogany Hammock (see Tables). Point velocity measurements were taken at each observation culvert using a Marsh McBirney Portable Water Current Meter beginning in 1996. These measurements are aiding in developing a representation of the flow patterns between Royal Palm and Mzarek Pond. The data confirms most of Craighead's observations. The culverts in the Pinelands predominately flow south, but are subject to periodic reversals of flow, after localized rain events. South of Pay-hay-okee there is a predominant westward flow. Discontinuous salinity measurements have been recorded during 1997 at key points between Paurotis Pond and Coot Bay.

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