blackwater streams. In Figure 9C, the 

 highest flows are only about twice the 

 lowest flow. Although most of the base 

 flow of the stream arises from the uniform 

 discharge of the spring, hydrographs may 

 also indicate local rainfall (Rosenau 

 et al. 1977). 



Spring-fed streams are influenced by 

 surface and ground-water fluctuations. 

 During flood stages of the Suwannee River 

 (FL), the flow from Falmouth Spring is 

 reversed, and the darker waters of the 

 Suwannee flow into the spring. At the 

 other extreme, these streams may go dry 

 annually, leaving an exposed bed as does, 

 for example, the Alapaha River (FL); or 

 the entire river channel, bed and all, may 

 disappear as the river drops into under- 

 ground corridors (lower Aucilla River, 

 FL). 



BOG AND BOG-FED STREAMS 



Two additional swamp stream types 

 occurring on the Coastal Plain of the 

 Southeast are bog and bog-fed streams. 

 Bog streams have limited distribution and 

 generally occupy the linear depressions or 

 swales between adjacent sand ridges and 

 reworked Coastal Plain relict dune depo- 

 sits. An example is White Water Creek in 

 Georgia, located in Cretaceous residual 

 dune sands. Many bog streams occur within 

 the Florida Panhandle area. Bog streams 

 are characterized by a steady lateral 

 seepage from the surrounding sand ridges. 

 Therefore, substrates of these systems are 

 constantly wet and support fire-resistant, 

 bog-type vegetation. The linear nature of 

 these streams precludes any significant 

 watershed interception of rainfall beyond 

 that falling directly on the stream. 



Bog-fed streams, on the other hand, 

 flow intermittently due to discharge from 

 expansive bog-filled depressions. This 

 intermittent discharge occurs only after 

 significant runoff from rainfall exceeds 

 the water storage capacity of the bog. 

 The depressions which feed these streams 

 are areas of internal perched drainage 

 underlain by clay aquicludes (impervious 

 soil layers that retard the downward move- 

 ment of groundwater). These basins are 

 not incised by streams, water tables gen- 

 erally occur at the surface, and excess 

 flow from precipitation discharges readily 



into the receiving bog-fed stream. The 

 streams receive little or no sediment 

 load; therefore, few have floodplains and 

 most resemble shallow ravines. Their 

 hydrographs exhibit extreme fluctuations 

 in response to rainstorms, with little or 

 no base flow (Figure 9D). Examples are 

 the New and Sopchoppy Rivers in Florida, 

 which drain giant shrub bogs and bay 

 swamps in the Bradwell Bay Wilderness Area 

 (FL). Typically the streams flood rapidly 

 and drain gradually due to the baffling 

 effect of '■.heir dense bay vegetation. 



FLOODING DURATION AND FREQUENCY 



Flooding on alluvial floodplains de- 

 pends on the size and slope of the water- 

 shed, which, together with soil and slight 

 elevation differences, help explain the 

 variability in forest communities on vari- 

 ous floodplains. The duration of flooding 

 also directly relates to watershed drain- 

 age area. Bedinger (1980) concluded that 

 drainage areas in the mid-West with less 

 than 776 km^ (300 mi^) have fast runoff 

 characteristics, with flooding occurring 

 5% to 77o of the year. Flooding occurs in 

 drainage areas ranging between 12,950 and 

 18,130 km2 (5,000 and 7,000 rniO. Flood- 

 plains for rivers with watersheds exceed- 

 ing several tens of thousands of square 

 miles are inundated from 18% to 40% of the 

 year. Flood peaks are significantly lower 

 in basins with lake and wetland areas 

 (Carter et al. 1978). 



Steep watersheds with dense clay 

 soils have "flash" inundations of compara- 

 tively short duration. Rivers with intact 

 floodplain swamp forest slow down the rise 

 and fall of floodwaters (Wharton 1970). 

 Flood heights diminish markedly as soon as 

 alluvial (Wharton 1980) and blackwater 

 (Benke et al. 1979) rivers top bankful 

 stage and begin to utilize their flood- 

 plain swamps. 



Leitman (1978) showed the importance 

 of local rainfall in maintaining saturated 

 conditions at several locations on the 

 Apalachicola floodplain where residual 

 water is often perched in backswamps 1 to 

 2m (3 to 7 ft) higher than river level. 

 Water levels in these floodplain pools and 

 sloughs rise from local rainfall indepen- 

 dently of river stage. 



20 



