Federal Register / Vol. 56. No. 157 / Wednesday, Aug^lst 14. 1991 / Proposed Rules 



40473 



considered wetland, a hydric soil area 

 usually must have a mean prevalence 

 index (PIM) of less than 3.0. A minimum 

 of three transects are required in each 

 delineated area of hydric soil, but 

 enough transects are required so that 

 the standard error for PIM does not 

 exceed 0.20 percent. 



Compute the mean prevalence index 

 for the three transects by using the 

 following formula: 



PIM = PIT 



N 



where 

 PIM = mean prevalence index for 



transects; 

 PIT = sum of prevalence index values for 

 transects: 

 aU 

 N = total number of transects. 



After computing the mean prevalence 

 index for the three transects, proceed to 



Step 13. 



Step 13. Calculate the standard 

 deviation(3) for the prevalence index 

 using the following formula: 



(PIl-HM) ♦ 



.a. 



(PI2-PIM) 



♦ (PIl-PW) 



(Note: See formulas in Steps 8 and 10 for 

 symbol definitions.) 



After performing this calculation, 

 proceed to Step 14. 



Step 14. Calculate the standard error 

 (sx) of the mean prevalence index using 

 the following formula: 



sx = 



nTn 



where 

 S = standard deviation for the Prevalence 



Index 

 N = total number of transects 

 (Note: The sx cannot exceed 0.20. If sx 

 exceeds 0.2O, one or more additional 

 transects are required. Repeat Steps 6 

 through 14, as necessary, for each additional 

 transect) 



When sx for all transects does not 

 exceed 0.20. proceed to Step 15. 



Step 15. Record fmal mean prevalence 

 index value for each hydric soil map 

 unit and make a wetland determination. 

 All areas having a mean prevalence 

 index of less than 3.0 meet the 

 hydrophytic vegetation criterion. If the 

 community has a prevalence index 

 equal to or greater than 3.0. it is usually 

 not hydrophytic vegetation except under 

 certain circumstances; consult the 

 section on exceptions. Proceed to Step 



16. , , 



Step 18. Determine whether the 

 wetland hydrology criterion is met. 

 Record observations and complete the 

 hydrology section on the appropriate 

 data form. If the wetland hydrology 

 criterion is met. then the area is 

 considered a wetland. If the area has 

 been hydrologically disturbed, one must 

 determine whether the area is 

 effectively drained before making a 

 wetland determination; this type of area 

 should have been identified in Step 2 

 (see disturbed areas discussion). If the 

 area is effectively drained, it is 

 considered nonweUand: if it is not, the 

 wetland hydrology criterion is met and 

 the area is considered a wetiand. 



(Caution: Seasonally saturated wetland 

 may not appear to meet the hydrology 

 criterion at certain times of the growing 

 season; see discussion of exceptions.) 



Step 17. Delineate the wetland 

 boundary. After identifying the wetland, 

 delineate the boundary by refining the 

 limits of the area that meets all three 

 criteria (including any problem area 

 weUands). Mark the boimdaries with 

 flagging tape, if necessary. 



Appendix 5. Descriptions of wetlands 

 that are exceptions to the three criteria 



Prairie Potholes 



Potholes are glacially-formed 

 depressions that are capable of storing 

 water (Eisenlohr 1972). They are 

 generally located in the north central 

 United States and southern Canada. 

 Although potholes may occur in forested 

 areas, the majority occur in the prairie 

 region where they are subject to arid or 

 semi-arid climatic conditions. Most 

 potholes are small, generally less than 

 an acre in size. 



Pothole soils are generally pooriy 

 drained, slowly permeable soils capable 

 of ponding water. Precipitation is the 

 basic source of water in potholes. 

 Runoff from the drainage area is highly 

 variable, but it is the key in determimng 

 if and how long ponding will occur. 

 Precipitation in the pothole region vanes 

 appreciably from year to year. Average 

 precipitation is far too small to meet the 

 demand of evaporation and as a result 

 most potholes are dry for a significant 



portion of the year, containing water for 

 only a short period generally early in the 

 growing season. In years of drought, 

 potholes may not pond water at all. 

 However in most years, seasqnal 

 replenishment can be expected 

 (Eisenlohr 1972). 



In certain areas, the vast majority of 

 potholes are farmed, either occasionally 

 or every year, depending upon the 

 duration of ponding. Many potholes 

 have been either partially or totally 

 drained to enhance agriculhiral 

 production. The drastically fluctuating 

 climate and alteration for fanning have 

 resulted in highly disturbed conditions 

 that make wetiand identification 

 difficult. Aerial photographs, ASCS 

 compliance slides, and other offsite 

 information that depict long-term 

 conditions are often better indicators of 

 wetland conditions than onsite 

 indicators reflecting only a single point 

 in time. 



Plant communities in potholes are 

 usually disturbed, eitiier naturally or 

 due to farming, and many do not exhibit 

 vegetation typical of more stable 

 weUands. The process of annual drying 

 (drawdown) in potholes enables the 

 invasion of FAC. FACU, or UPL plant 

 species during dry periods which may 

 persist into the wet seasons. Stewart 

 and Kantrud (1971) have recognized this 

 condition in describing vegetation 

 phases in tiieir classification of 

 wetiands for the Prairie Pothole Region. 

 The phases are as follows: 



For Noncropland Areas 



Drawdown Bare Soil Phase 



As surface water in the open water 

 phase graduaUy recedes and disappears, 

 expanses of bare mud flats, which often 

 become dry. are exposed. Ordinarily, 

 this phase is of short duration, but in 

 intermittent-alkali zones and 

 occasionally in the more saline deep 

 marsh zones, it may persist for 

 considerable periods. 



Natural Drawdown Emergent Phase 



Undisturbed areas vrith emergent 

 drawdown vegetation are considered to 

 be in this phase. This growth is 

 composed mostly of annual plants, 

 including many forbs, that genninate on 

 the exposed mud or bare soil of the 

 drawdown bare soil phase. After the 

 drawdown emergents become 

 established, surface water is 

 occasionaUy restored by heavy summer 

 rains. Characteristic plant species of tms 

 phase include: Eleocharis aciculans 

 (tenestrial fonn). Ruwex mantimus. 

 Kochia scoparia. Xanthium itahcum. 



