Ch. 6— Impacts and Mitigation • 121 



will prevent flooding of the road, but will not allow 

 the flow of subsurface water. In some instances, 

 borrow canals adjacent to the highways also have 

 diverted the drainage direcdy into a coastal estuary, 

 permitting saltwater intrusion into the wetland 

 where the normal drainage had been cut off. 



Drainage and Clearing 



Narrow drainage ditches (less than 5-feet wide) 

 may be^xcavated to accelerate and channel sur- 

 face water runoff and to lower ground water levels, 

 increasing the value of the drained land for agri- 

 cultural and forest management. For example, 

 draining and clearing is a major factor associated 

 with wedand conversions in the prairie potholes and 

 in Nebraska, California, the Lower Mississippi 

 River Valley, North and South Carolina, and south 

 Florida; for urban development in south Florida 



and Washington; and for forestry management in 

 North Carolina and the Lower Mississippi River 

 Valley. 



The major ecological impact from draining and 

 clearing wetlands for agricultural purposes is the 

 loss of diverse wildlife habitat. Studies in Missouri 

 where wedand channelization projects were under- 

 taken to reduce flooding problems indicated that 

 78 percent of bottom land hardwood forest pre- 

 viously flooded was converted to crop production 

 after project completion (19). In Louisiana, 51 per- 

 cent of the original 4.5 million hectares of forested 

 wetlands have been converted to agricultural use, 

 mosdy for soybeam and cotton production. The loss 

 of hardwood forests has meant a loss of prime hab- 

 itats for birds and mammals, as well as a loss of 

 critical spawning grounds for aquatic species. 

 Under some circumstances, ditches in agricultural 

 areas also may increase the runoff of pesticides, her- 



Photo credit: Office of Tect^nology Assessment, William Barnard 



The clearing of this pocosin wetland in North Carolina will result in loss of wildlife habitat 



