Wetland Hydrology 



2.8. Permanent or periodic inundation, or soil sat- 

 uration to the surface, at least seasonally, are the 

 driving forces behind wedand formation. The pres- 

 ence of water for a week or more during the grow- 

 ing season typically creates anaerobic conditions in 

 the soil, which affect the types of plants that can 

 grow and the types of soils that develop. Numer- 

 ous factors influence the wetness of an area, in- 

 cluding precipitation, stratigraphy, topography, 

 soil permeability, and plant cover. All wetlands 

 usually have at least a seasonal abundance of wa- 

 ter. This water may come from direct precipitation, 

 overbank flooding, surface water runoff due to 

 precipitation or snow melt, ground water dis- 

 charge, or tidal flooding. The frequency and dura- 

 tion of inundation and soil saturation vary widely 

 from permanent flooding or saturation to irregular 

 flooding or saturation. Of the three technical criteria 

 for wedand identification, wedand hydrology is of- 

 ten the least exact and most difficult to establish in 

 the field, due largely to annual, seasonal, and daily 

 fluctuations. 



Wetland Hydrology Criterion 



2.9. An area has wetland hydrology when 

 saturated to the surface or inundated at 

 some point in time during an average rain- 

 fall year, as defined below: 



1 . Saturation to the surface normally 

 occurs when soils in the following 

 natural drainage classes meet the 

 following conditions: 



A. In somewhat poorly drained 

 mineral soils, the water table is 

 less than 0.5 feet from the sur- 

 face for usually one week or 

 more during the growing season; 

 or 



B . In low permeability (<6.0 inch- 

 es/hour), poorly drained or very 

 poorly drained mineral soils, the 

 water table is less than 1.5 feet 

 from the surface for usually one 

 week or more during the grow- 

 ing season; or 



C. In more permeable (> 6.0 inch- 

 es/hour), poorly drained or very 

 poorly drained mineral soils, the 

 water table is less than 1.0 feet 

 from the surface for usually one 

 week or more during the grow- 

 ing season; or 



D. In poorly drained or very poorly 

 drained organic soils, the water 

 table is usually at a depth where 

 saturation to the surface occurs 

 more than rarely. (Note: Organic 

 soils that are cropped are often 

 drained, yet the water table is 

 closely managed to minimize ox- 

 idation of organic matter; these 

 soils often retain their hydric 

 characteristics and if so, meet 

 the wetland hydrology 

 criterion.) 



2. An area is inundated at some time if 

 ponded or frequently flooded with 

 surface water for one week or more 

 during the growing season. 



(Note: An area saturated for a week during the 

 growing season, especially early in the growing 

 season, is not necessarily a wetiand. However, in 

 the vast majority of cases, an area that meets the 

 NTCHS criteria for hydric soil is a wedand.) 



Summary 



2.10. The technical criteria are mandatory and 

 must be satisfied in making a wetiand determina- 

 tion. Areas that meet the NTCHS hydric soil crite- 

 ria and under normal circumstances support hydro- 

 phytic vegetation are wetlands. Field indicators and 

 other information provide direct and indirect evi- 

 dence for determining whether or not each of the 

 three criteria are met. Sound professional judge- 

 ment should be used in interpreting these data to 

 make a wetland determination. It must be kept in 

 mind that exceptional and rare cases are possibili- 

 ties that may call any generally sound principle into 

 question. 



