manipulation of the hydrologic regime may 

 result in a complete transformation of 

 river and floodplain morphology (Schumn; 

 1969). Clearly, anything that man does to 

 the natural, orderly river channel will 

 induce changes as the river attempts to 

 regain its original efficient configura- 

 tion. Sediment inputs resulting from 

 clearcutting, or sedin^ent starvation from 

 reservoir construction, dredging, shorten- 

 ing and even snagging and dragging are 

 among potential impacts. 



The construction of reservoirs can 

 have both direct and indirect effects, the 

 result of coupling the natural energy of 

 moving water with man's complex of activi- 

 ties on the uplands, l^hile the dissolved 

 solute chemistry of the water is not mark- 

 edly changed, the sediment load settles 

 and is reduced as a result of the stilling 

 effect of the reservoir. Release of water 

 from the reservoir results in scour and 

 resuspension of sediments that move stead- 

 ily downstream as the upper segment of the 

 river lowers its bed (Schumm 1971). In 

 the Red River below Denison Reservoir 

 (OK), scour widened the channel from 1.5 

 to 3.0 m (5 to 10 ft) per year but the 

 river did not regain its pre-reservoir 

 sediment load for 322 km (200 mi) (Ein- 

 stein 1972). The large dams above Augusta, 

 GA, virtually have stopped sediment move- 

 ment from the Piedmont (Meade 1976). Res- 

 ervoirs on the Santee and Savannah trap 

 from 85% to over 90% of incoming sediment. 

 Since as much sediment still is carried in 

 the lower reaches as in pre-dam days, 

 Meade concluded that the immediate source 

 must be the river bed, banks, and flood- 

 plain. Other factors may include extensive 

 shortening and dredging of the Savannah by 

 the Corps of Engineers with corresponding 

 gradient increase and much abnormal bank 

 destruction. Meade stated that since 1910 

 reservoirs on the Savannah River have re- 

 duced the sediment load delivered to the 

 ocean by 50%, thus depriving the Continen- 

 tal Shelf of its former river influx of 

 inorganic nutrients. 



Flood peaks higher than the 1- to 2- 

 year inundations are also impaired by dam- 

 ming. Much riverborne sediment deposits in 

 estuaries are in the "sediment-trap" at 

 the freshwater-saltwater interface. In 

 pre-dam days the locus of these deposits 



moved back and forth seasonally in the 

 estuary, and floods flushed out accumu- 

 lated sediments with a periodicity of 3 to 

 5 years (Meade 1976). This flushing action 

 no longer occurs, and sediments accumulate 

 in the estuary and must be removed by 

 costly dredging. 



Another example of direct downstream 

 effects is flow regulation by huge reser- 

 voirs on the Roanoke River (NC). The 

 Roanoke River (NC) has a "dead zone" 6- 

 12 m (20-40 ft) wide from near the levee 

 top down to the river devoid of all bot- 

 tomland hardwoods except a few willows at 

 the upper edge. This zone, with numerous 

 fallen dead trees, resulted from water 

 levels held artificially high by upstream 

 discharges, well into leafout time. In 

 swales along the Roanoke the lack of 

 former flushing action may have caused 

 several feet of silt to accumulate (Pat 

 White, Williams Lumber Company, Mackeys, 

 NC ; personal communication). 



Indirectly, flow regulation is having 

 an even more pronounced effect on bottom- 

 land hardwood communities. Damming may 

 severely modify or eliminate the seasonal 

 hydroperiod, allowing upland row-crop 

 agriculture and forestry on the flood- 

 plain. On the Savannah River (GA) flood- 

 plain, below a series of large reservoirs 

 in the Piedmont, hundreds of acres of Zone 

 IV bottomland hardwoods are being sheared 

 off, and the floodplain is being planted 

 in soybeans. Even without flow regula- 

 tion, the floodplains of many southern 

 rivers are being cleared of bottomland 

 hardwoods and prepared for conversion to 

 pine plantations, although, ironically, 

 hardwoods often outgrow pines on these 

 sites. Planted loblolly stands on the 

 Altamaha, Oconee, and Ocmulgee Rivers in 

 Georgia have developed a thick understory 

 of wax myrtle and sweetgum, typical Zone 

 IV species, which may indicate the land is 

 still most suitable to the natural commun- 

 ity. Such system-wide changes threaten or 

 eliminate the life support functions of 

 floodplain zones. 



The most serious impacts of reser- 

 voirs on bottomland hardwoods downstream 

 arise from regulating the normal annual 

 rise and fall of the river to which the 

 whole system is keyed. The effects of 



106 



