not occur, the effects of the depressions are localized in nature. 

 The vegetation in the depressions may change to more water-tolerant 

 grasses and sedges, such as Eleocharis spp. and Scirpus spp. Con- 

 sumers that depend upon the existing vegetation for food and/or cover 

 may ultimately increase or decrease. 



The depressions allow a faster runoff of surface water in localized 

 areas. The long-term effects on sediment loss and nutrients are 

 slight. If vehicle tracks completely breach the levee, major changes 

 in water flows will occur. The effects may be serious - potentially 

 resulting in conversion of ecosystems. However, each system is 

 unique, and predictions must be based on the types of ecosystems 

 present on either side of the levee/spoil bank, ecosystem size, levee 

 alignment relative to critical circulation patterns, and other site- 

 specific information. 



The degree and significance of wildlife displacement resulting from 

 seismic preexploration activities are not long lasting since the 

 activity is short term (two weeks or less). The activity is 

 characterized by considerable human movement and noise so that 

 tolerances of the most sensitive species should be considered. The 

 effects are felt considerably beyond the project boundaries. 



4. Key attribute alterations 



The alteration which leads to the most significant impacts on the 

 ecosystem is the creation of depressions in the land surface. 

 Effects which are large in areal extent and long term may result 

 from induced changes in waterflow regimes of adjacent systems (if 

 levees are breached). As discussed earlier, the total area affected, 

 and thus, the kinds and number of consumers, is site specific. 



Gravity preexploration . 



1. Activity sequence 



Gravity surveys involve the placement of a relatively small piece of 

 equipment on the levee surface. This unit, the gravity meter, is carried 

 to stations by truck, marsh buggy, boat, or foot. Survey crews are not 

 required since data need not come from predetermined locations. It is 

 necessary only to plot the location of the data-collection stations on a 

 map. A small amount of vegetation is trampled during the placement of 

 the gravity meter. Several minutes are required for data collection; the 

 unit is then packed and carried to the next station. The number of 

 stations required for a gravitymeter survey is less than the number of 

 shot holes necessary for a seismic survey. Two men with adequate tran- 

 sportation can efficiently conduct a gravity meter survey. 



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