CHAPTER 2. HYDROLOGY 



Water is the driving force of the 

 bottomland hardwood community. As has 

 been shown, water plays a crucial role in 

 forming and maintaining the floodplain by 

 transporting and redistributing sediments 

 within the system. The rivers and their 

 floodplains are fluctuating water level 

 ecosystems. Their high flows are brought 

 about by winter-spring rains (peak flow is 

 in the summer in Florida). Their low flows 

 correlate with high evapotranspiration 

 during late summer and dry fall months 

 (Wharton and Brinson 1979a). Sources of 

 water to bottomlands include precipitation 

 and runoff from mountains and Piedmont 

 (alluvial rivers), groundwater from local 

 convective and storm-front rainfall (lower 

 Coastal Plain blackwater streams), under- 

 ground aquifers (spring-fed alkaline 

 streams), continuous seepage from sand 

 aquifers (bog and bog-fed streams), and 

 tidal flow. 



Before development, when intact for- 

 ests with thick, organic soil layers cov- 

 ered the landscape of mountains and Pied- 

 mont, almost all water to alluvial streams 

 was derived via subsurface (ground water) 

 flow. Today exposed subsoil horizons in 

 the Piedmont lead to surface runoff which 

 is now the primary source of water to 

 tl^ese streams. On the flat Coastal Plain 

 terrain, surface runoff occurs only spo- 

 radically except when the soils are satu- 

 rated (water table at or near the sur- 

 face); therefore, rainfall in the Coastal 

 Plain reaches blackwater streams via 

 ground water (base flow) in fall and by 

 ground water and surface runoff in winter 

 and spring. Base flows become the primary 

 source to streams during low water or 

 drought conditions. 



Surficial aquifers (Hawthorne, Creta- 

 ceous) may contribute markedly to base 

 flow. During a fall drought Thompson and 

 Carter (1955) computed base flow discharge 

 from the Tuscaloosa formation to minor 

 Georgia streams ranging from the rainfall 

 equivalent of 28 cm (11 inches) to as much 



as 102 cm (40 inches) per year from this 

 Cretaceous aquifer. blackwater rivers 

 become visibly clearer in the fall because 

 their flow is derived largely from ground- 

 water base flow. 



It is reasonable to assume that 

 rivers recharge the shallow aquifers at 

 high water in the flat Pleistocene depos- 

 its, but it is yet to be proven how much 

 rivers contribute to the deeper aquifers. 

 The net contribution of alluvial rivers to 

 the principal limestone aquifer is thought 

 to be insignificant (Stringfield and 

 LeGrand 1966). Surface streams and swamps 

 may recharge valley aquifers (Wharton 

 1970; Bedinger 1980). 



ALLUVIAL RIVERS 



Alluvial rivers in the Southeastern 

 United States originate in the mountains 

 and Piedmont and form huge swamps at the 

 junction of the Piedmont and Coastal 

 Plain. Most of these rivers have periods 

 of sustained high flow resulting from the 

 cumulative effect of many tributaries and 

 distant rainfall (Figure 9A). Generally, 

 the annual high winter-spring runoff water 

 overflows the floodplain features. Pat- 

 terns of river discharge vary in different 

 sections of the watershed. For example, 

 discharge peaks are higher in the Apalach- 

 icola River (FL) in the comparatively nar- 

 rower upper section with high levees and 

 steeper gradient, as compared with the 

 flatter stage hydrograph approximately 

 48 km (30 mi) downstream where the water 

 spreads out over a much wider (5 x) and 

 flatter floodplain (Figure 10). Differ- 

 ences in wet (flooded) and dry stages can 

 be dramatic (Figure 11). Discharge volumes 

 may cease to rise and sometimes even fall 

 as the water flows through the floodplain 

 toward its mouth (Figure 12). Evapotrans- 

 piration after March leafout and surficial 

 aquifer recharge may help account for some 

 oi^ this flow reduction (Mulholland 1979; 

 Brown et al. 1979). 



16 



