Federal Register / Vol. 56. No. 157 / Wednesday. August 14. 1991 / Proposed Rules 



40453 



Permanent or periodic inundation, or 

 soil saturation at the surface, at least 

 seasonally, are the driving forces behind 

 weUand formation. The presence of 

 water in the soil due to inundation for 15 

 or more consecutive days or saturation 

 from surface water or from groundwater 

 to the surface for 21 or more consecutive 

 days diuing the growing season in most 

 years typically creates anaerobic 

 conditions, which affect the types of 

 plants that can grow and the types of 

 soils that develop. These conditions 

 hold true for most weUands, especially 

 those at the upper end of the soil 

 moisture gradient Anaerobiosis does 

 not necessarily occur in all wetlands 

 and those where it may not occur 

 include vegetated sand beirs, seepage 

 areas, springs, and the upper edges of 

 salt marshes. Wetlands have at least a 

 seasonal or periodic abundance of 

 water. For example, this water may 

 come from direct precipitation, 

 overbank flooding, surface water nmoff 

 due to precipitation or snow melt, 

 ground water discharge, tidal flooding, 

 irrigation, or other humein-induced 

 activities. The frequency and duration of 

 inimdation and soil saturation vary 

 widely from permanent flooding or 

 saturation to irregular flooding or 

 saturation. Of the three technical 

 criteria for wetiand identification. 

 weUand hydrology is often the least 

 exact and most diiflcult to establish in 

 the field, due largely to eumual. 

 seasonal, and daily fluctuations. 



Numerous factors influence the 

 wetness of an area, including 

 precipitation, stratigraphy, topography, 

 soil permeability, and plant cover. The 

 frequency and duration of inundation or 

 soil saturation are important in 

 separating wetlands from nonwetiands. 

 Areas of lower elevation in a floodplain 

 or marsh usually have longer duration of 

 inundation and saturation and often 

 more frequent periods of these 

 conditions than most areas at higher 

 levels. Floodplain configuration may 

 significantly affect the duration of 

 inundation by facilitating rapid runoff or 

 by causing poor drainage. 



Soil permeability related to the 

 texture of the soil also influences the 

 duration of inundation or soil satiu-ation. 

 For example, clayey soils absorb water 

 more slowly than sandy or loamy soils, 

 and therefore have slower permeability 

 and remain saturated much longer. 



Type and amount of plant cover affect 

 both the degree of inundation and the 

 duration of satiu'ated soil conditions. 

 Excess water drains more slowly in 

 areas of abundant plant cover, thereby 

 increasing duration of inundation or soil 

 satiu-aUon. On the other hand. 



transpiration rates are higher in areas of 

 abundant plant cover, which may 

 reduce the duration of soil satxu-ation. 

 To determine whether the weUand 

 hydrology criterion is met. one should 

 consider recorded data, aerial 

 photographs, and observed field 

 conditions that provide direct or indirect 

 evidence of inundation or soil 

 saturation. Prolonged saturation often 

 leaves evidence of such wetness in the 

 soil (e.g., sulfur odor) and these 

 properties are useful for verifying 

 wetland hydrology provided the area's 

 hydrology has not been significanUy 

 modified on-site or upstream in the 

 watershed. If the hydrology has been 

 significantly disturbed, particular care 

 must be taken in assessing the wetland 

 hydrology criterion; refer to distiu'bed 

 area procedures to determine whether 

 weUand hydrology still exists. 



Measuring Wetland Hydrology 



In certain instances, especially 

 disturbed situations, it may be 

 necessary to determine an area's 

 hydrology by actively collecting on-site 

 hydrologic data from direct 

 measurements or observations. The 

 duration and frequency of inundation by 

 flooding may be established by 

 evaluating long-term stream or tide 

 gauge data or by examining aerial 

 photos covering at least a 5-year period 

 and comparing results with the weUand 

 hydrology criterion. Saturation at the 

 siuface may be determined by making 

 observations in an unlined borehole and 

 establishing whether or not the soil is 

 saturated to the surface for 21 or more 

 consecutive days during the growing 

 season in most years. A procedure for 

 this is presented in the Distiu'bed Areas 

 section of the manual. In general, if soil 

 saturation is observed at the surface for 

 21 or more consecutive days diuing the 

 grov/ing season (or the area is inundated 

 for 15 or more consecutive days). 

 weUand hydrology probably exists. 

 Interpretation of the above 

 observations, however, must always be 

 done with consideration of recent 

 rainfall conditions (e.g., within the past 

 few weeks) as well as the long-term 

 rainfall patterns (e.g.. abnormally wet or 

 dry periods) preceding and during the 

 time the hydrologic data were recorded. 



Historical Recorded Hydrologic Data 



Historical recorded hydrologic data 

 usually provide both short- and long- 

 term information on the frequency and 

 duration of flooding, but littie or no 

 information on soil satiiration periods. 

 Historical recorded data include stream 

 gauge data, lake gauge data, tide gauge 

 data, flood predictions, and historical 

 flood records. Use of these data is 



commonly limited to areas adjacent to 

 streams and other similar areas. 

 Recorded data may be available from 

 the following sources: (1) CE district 

 offices (data for major waterhodies and 

 for site-specific areas from planning and 

 design documents), (2) U.S. Geological 

 Survey (stream and tidal gauge data), (3) 

 National Oceanic and Atmospheric 

 Administration (tidal gauge data), (4) 

 State, county and local agencies (flood 

 data), (5) SCS state offices (small 

 watershed projects and water table 

 study data), and (6) private developers 

 or landowners (site-specific hydrologic 

 data, which may include water table or 

 groundwater well data). 



Aerial Photographs 



Aerial photographs may provide 

 direct evidence of inundation or soil 

 saturation at the surface in an area. 

 Inundation (flooding or ponding) is best 

 observed during the early spring in 

 temperate and boreal regions when 

 snow and ice are gone and leaves of 

 deciduous trees and shrubs are not yet 

 fully developed. This allows detection of 

 wet soil conditions that would be 

 obscm-ed by the tree or shrub canopy at 

 full leaf-out For marshes, this season of 

 photography is also desirable, except in 

 regions characterized by distinct dry 

 and rainy seasons, such as southern 

 Florida and California. WeUand 

 hydrology would be best observed 

 during the wet season in these latter 

 areas. 



It is most desfrable to examine several 

 consecutive years of early spring or wet 

 season aerial photographs to document 

 evidence of wetiand inundation or soil 

 saturation. In this way. the effects of 

 abnormally dry or wet springs, for 

 example, may be minimized. In 

 interpreting aerial photographs, it is 

 important to know the antecedent 

 weather conditions. This will help 

 eliminate potential misinterpretations 

 caused by abnormally wet or dry 

 periods. Contact the U.S. WeaUier 

 Service for historical weather records or 

 the U.S. Geological Survey for 

 hydrologic records. Aerial photographs 

 for agricidtural regions of the country 

 are often available at county offices of 

 the Agricultural Stabilization and 

 Conservation Service. 



Field Observations 



Direct Evidence of Water 



At certain times of the year in 

 weUands. and in certain types of 

 wetiands at most times, weUand 

 hydrology is quite evident since surface 

 water or saturated soils (e.g.. soggy or 

 wetter underfoot) may be observed. The 



