Bay Floor Morphology

Apalachicola Bay is a shallow estuary however the bathymetry shows that its floor is not uniform in depth (fig. 4). Navigation charts (NOAA, 1996; 2000) show that East Bay, the area off the Apalachicola River delta, is the shallowest part of the estuary with no areas exceeding 2 m. Much of St. Vincent Sound also is less than 2 m deep, although a narrow trough extends approximately 6 km eastward from Indian Pass, and in this trough water depths reach 5-7 m. The data collected during this survey cover the Apalachicola Bay and St. George Sound sections of the estuary, which are its deepest parts. Within the surveyed parts of Apalachicola Bay and St. George Sound, the estuary has a smooth floor that increases in depth to the south from 2 to 4 m (fig. 4). Depth locally exceeds 15 m on the shoreward side of West Pass. St. George Sound is the deepest part of the estuary: much of its floor is smooth with depths increasing southward from 2 to 4m. Depths increase to 4-6 m in the eastern part of this Sound and the sea floor here is more irregular and shaped into a series of broad, low-relief depressions and mounds.

This regional pattern in the bathymetry is interrupted by shoals. These shoals take two forms: shore-attached lobate shoals, and linear shoals. Names for the shoals are shown on figure 6 and are those used by the Apalachicola Bay National Estuarine Research Reserve or from NOAA navigation charts (NOAA, 1996; 2000). The lobe-shaped shoals extend northward into the southern side of the bay from St. George and Little St. George Islands. The largest of these are Pelican Bar in St. George Sound and Cedar Point and Higgins Shoal in Apalachicola Bay. Higgins Shoal is shoreward of a former inlet on Little St. George Island that was open in 1860 (fig. 7) and appears to be the remnant of the flood tidal delta that formed when this inlet was active. The other lobe-shaped shoals along the north side of the islands may have originated as flood tidal deltas shoreward of other abandoned inlets.

The linear shoals are found in the western part of St. George Sound and Apalachicola Bay (fig. 6). These linear shoals are divided into two groups based on their dimensions: the large shoals exceed 4 km in length and the small shoals are less than 1.5 km in length. The large shoals trend roughly perpendicular to the long axis of the bay and four of the six of them have one end attached to the shore. The largest, St. Vincent Bar, is found at the western end of Apalachicola Bay and extends southeastward from the northeastern tip of St. Vincent Island to within 1.5 km of Little St. George Island. A deep channel that extends eastward from West Pass separates the southern tip of St. Vincent Bar from Little St. George Island (fig. 4). The bar is 7 km long, 0.7-1.3 km wide, and rises to within 0.5-1 m of the sea surface. Other large linear shoals include Norman's Bar that is separated from Hotel Bar by the Intracoastal Waterway, Cat Point Bar, East Hole Bar, Platform Bar, and Porter's Bar. Cat Point Bar extends southeastward from the eastern side of Cat Point and connects with Platform Bar. East Hole Bar extends northwest from St. George Island and merges with Cat Point Bar (fig. 6). This complex of bars forms a nearly continuous string of shoals across the bay that is only broken by narrow, deeper gaps. The channel between Pelican and Platform Bars reaches 6 m deep. The northwest trending channel that separates Cat Point Bar from East Hole Bar was not completely surveyed by this study, but, where surveyed, reaches 4.9 m at its northwestern end and 3.2 m at its southeastern end. The navigation chart shows that in the section not surveyed by this study depths reach 1.5-2 m (NOAA, 1996). The Intracoastal Waterway is dredged through East Hole Bar, and has depths exceeding 4 m (fig. 8). The easternmost of the large linear shoals is Porter's Bar, which extends from the northern shore of St. George Sound southeastward for 4 km. The crests of the linear shoals rise to within 1 m of the sea surface with the exception of Norman's Bar whose crest is 2-2.5 m deep. A unique feature of all of the linear shoals is that they are asymmetrical with their west-facing sides being steeper (fig. 9).

In addition to the large linear shoals there are several smaller shoals. These shoals are all less than 1.5 km in length, are isolated from the shoreline, and are more varied in their orientations (fig. 6). One cluster of these shoals is east of St. Vincent Bar, and includes Cable Lumps, North Spur, Sugar Lumps, and West Lumps. A second cluster is south of the John Gorrie Bridge, and includes a discontinuous string of linear to circular mounds immediately north of Norman's Bar as well as the East Lumps. Two small shoals are found east of Porter's Bar: Green Point Bar and a second unnamed shoal. The smaller shoals have varied orientations. The two in the eastern part of St. George Sound trend northwest (Green Point and the unnamed bar), those north of Norman's Bar trend north, and those in the western part of Apalachicola Bay have varied orientations (West, Cable, and Sugar Lumps and North Spur). These smaller shoals mostly have less relief, and crests that are mostly deeper than 2 m. Most of the small shoals also are asymmetrical with their steeper sides facing west (fig. 9). The only symmetrical shoals are those in the central part of Apalachicola Bay between Norman's Bar and the John Gorrie Bridge.

A pronounced man-made feature of the bay floor is the Intracoastal Waterway. This dredged channel starts near the eastern edge of the survey area, weaves its way around the shoals in St. George Sound, cuts across East Hole Bar, extends westward to the central part of Apalachicola Bay, here it turns north and leads to the mouth of the Apalachicola River (fig. 4).

This channel is 3-5 m deep along much of its length. Dredge material has mostly been deposited along the southern side of the channel. Along some sections of the channel it forms a continuous ridge whose crest is as shallow as 1.7 m, while in other areas the dredge material forms discrete circular mounds that are less than 200 m in diameter with crests 1.9-3.5 m deep fig. 8).

Click on figures for larger images.

Figure 4. Bathymetric map of the Apalachicola Bay estuary. See also Mapsheet 1.

Figure 6. Map showing the names of bay-floor and geographic features within the Apalachicola Bay study area. Locations of sediment samples collected by NOAA Coastal Services Center (NOAA, 1999) that were used to verify the sidescan-sonar interpretation are shown.

Figure 7. 2000 NOAA chart # 11402 (top panel) and 1860 NOAA chart # 485 (bottom panel) showing the location of New Inlet, a former tidal inlet near Higgins Shoal that presently is sealed (NOAA, 1860; 2000).

Figure 8. Bathymetric map of the Intracoastal Waterway near the Bryant Patton Bridge showing dredged material south of its channel.

Figure 9. Map showing the slope of the bay floor. The steepest slopes are found along the flanks of the Intracoastal Waterway, along the margins of the bay, and on the western sides of linear bars (eg. St. Vincent's Bar and Porter's Bar).