CRS-59 



can upset functions in adjacent wetland areas by altering flow velocities 

 and patterns of water elevation. 



Impoundment 



Enclosing an area in order to flood it is the opposite of draining, but 

 the effect on wetland functions and values is similar. Farm ponds may be the 

 most common type of impoundment. Also, low dams are often installed in wild- 

 life refuges. Large impoundments, such as those created by the nation's major 

 dams, drown wetland areas adjacent to former stream channels. In some cases 

 these have been replaced by new wetland areas along the banks of the resulting 

 reservoir, but the new wetlands are usually of little functional value because 

 reservoir water elevations may be seasonally altered, often substantially, to 

 meet other needs. The comparative magnitude of lost wetlands and created wet- 

 lands associated with reservoir development has not been measured. 



Impoundments generally reduce the overall functions of wetlands. Some 

 impoundments, such as for the salt ponds in San Francisco Bay, destroy all the 

 natural functions of the area converted to the pond. Of the four classes of 

 activities directly affecting wetlands, impoundments may be both the least 

 common and the least understood at this time. 



Indirect Alterations 



Activities occurring at some distance from the wetlands can also affect 

 wetland functions. The key to most wetland functions is water, so any actions 

 that alter flow or quality characteristics are important. Common activities 

 altering water that have affected wetland areas range from damming and with- 

 drawal of water for other uses to polluted discharges. Some of these changes 



