decreased lateral stability of active channels, or allows water to spread 

 over a large area, is not desirable. Decreased water depth and velocity 

 increases sedimentation rates, alters water temperature, and alters dis- 

 solved oxygen levels. These chan9,es in aquatic habitat usually affect the 

 local distribution and community structure of benthos and fish. 



The effects of scraping in vegetated areas of inactive floodplains and 

 terraces can be similar to those described for pits. However, long-term 

 changes typically are minimal because the lack of standing water in the 

 closed site will facilitate re-establi shment of pre-mi ning vegetation con- 

 d i t i ons. 



In-channel locations that are dredged have the potential for causing 

 the least change to channel hydraulics, terrestrial biota, and aesthetics; 

 however, they can have the greatest effect on water quality and aquatic 

 biota. Gravel replenishment rates are highest in this location. Mining 

 exposed gravel bars in active floodplains potentially has the least effect 

 on terrestrial systems. Sites in inactive floodplains and terraces affect 

 the terrestrial biota and scenic quality most, but potentially have no 

 affect on the aquatic system. In general, the farther a material site is 

 located from a channel the greater the potential effect on the terrestrial 

 biota and scenic quality and the smaller the effect on the channel 

 hydrology-hydraulics, aquatic biota, and water quality. This relationship 

 constitutes the major tradeoff consideration in locating material sites in 

 f I oodp I a i ns. 



If material sites are located and operated to prevent or greatly mini- 

 mize effects on channel hydraulics, and to utilize only exposed gravel bars, 

 the probability of major localized changes to a floodplain is generally 

 greatly reduced. Where exposed gravel bars are not available or are inade- 

 quate, a tradeoff decision between sites must be made that weighs the poten- 

 tial effects of aquatic disturbances against terrestrial disturbances. In 

 these cases, minimization of hydraulic change to active channels should be 

 important in the decision — major hydraulic changes can have a greater 

 long-term effect on terrestrial systems than the controlled disturbances 



38 1 



