CONGAREE RIVER, SC 



«.,";SruVE'.°Tr.?-H G F B E E BD C B A 



ROANOKE RIVER, NC 



^•^0MmmJ^M}K ~ 



E D E D E F G 



V V II IV IV V 



CANOOCHEE RIVER. GA 



A B E C E D E 



^ 



muijMMi 



IV- V 



Figure 40. Cross-sectional transects (aspect is looking downstream) of nine southeast- 

 ern rivers and floodplains, indicating zones (I-V) and major vegetational and natural 

 features (A-J). This figure is cross-referenced to Table 15, which provides an expla- 

 nation for each vegetational or natural feature category (A-J) of each floodplain. See 

 Table 15 also for cross references to dominance types (Tables 11-14) found on these 

 transects. 



Flooding 



The most common natural disturbances 

 in bottomland ecosystems are associated 

 with floods. The biota are to a variable 

 degree adapted to flood forces. Annual 

 inundations adapt the bottomland hardwoods 

 for the larger and more catastrophic flood 

 events that occur with low frequencies 

 (100- to 1000-year floods). The wide, 

 shallow root crowns and trunk buttresses, 

 which are adaptations to the moist, anae- 

 robic conditions, serve to counter exces- 

 sive scour and toppling by flood or wind. 

 Deleterious effects may occur, however, 

 depending on flood timing, frequency, 

 depth, and velocity. The categories of 

 flood disturbances include (1) anaerobic 

 conditions, (2) mechanical abrasion and 

 breakage of plant tissues, (3) siltation, 



(4) propagule and seedling washout, and 



(5) erosion. 



Flooding may retard or speed succes- 

 sional trends. Severe erosive flooding 



inhibits point bar succession (see Chapter 

 1), slows natural levee development and 

 community establishment on levees, and can 

 forestall the filling in of scour chan- 

 nels, swales, and depressions between hum- 

 mocks. Moderate flooding enhances mature 

 community development through less damag- 

 ing effects on plant survival and growth 

 and through reduced erosion and net depo- 

 sition of floodwater sediments. 



Fire 



Fire is not important as a natural 

 disturbance in bottomlands because of the 

 prevalence of water and the lack of a sub- 

 stantial litter layer. This is especially 

 true of the wetter portions of alluvial 

 floodplains. However, Putnam (1951) states 

 that a serious fire season occurs on an 

 average of about every 5 to 8 years in 

 bottomland hardwood forests (in the Mis- 

 sissippi Valley). It is conjectured that 

 the Indians maintained canebrakes in Zone 

 V understory by deliberate fall burning. 



77 



