Wildlife Habitat Appraisal for the Proposed Allens Creek Reservoir Site
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Table 8 displays the compensation requirements given that the entire proposed reservoir area, and the area along Allens Creek below FM 1458 downstream to the Brazos River will be severely impacted by the second year after flooding is initiated. The second column from the left shows the annualized Habitat Units (HU) lost for each habitat cover type. HUs are computed by multiplying the Habitat Quality (a.k.a. WHAP score) by the size of the area in acres. The largest HU losses will occur for the grass and the bottomland forest cover types, at 1,967 and 1,960 HUs, respectively. Table 8 assumes that, if a piece of land outside of the reservoir is to be designated as a mitigation area, then its habitat quality (i.e., WHAP score) can be raised. Table 8 shows WHAP score improvements in increments of 0.10 as column headings, referred to as management potential. The body of the table contains the acreage required to replace the HUs that will be lost due to flooding, given a specific improvement in WHAP score. For example, if the WHAP score for bottomland forest in the mitigation area is to be raised 0. 10 (e.g., from 0. 70 to 0.80), then 19,602 acres of mitigated bottomland forest will be required to replace the 1,960 lost HUs. However, if the WHAP score is raised 0.50 (e.g., from 0.30 to 0.80), then 3,920 acres would be required to replace the 1,960 lost HUs.
The bottom row of the table shows the mitigation acreage for all cover types. These vary from 47,065 to 5,229 acres.
Table 9(PDF 5.4 KB) views the subject of compensation and mitigation acreage from a different perspective. It assumes that a mitigation area identical to the destroyed area will be improved. For each habitat cover type and for all combined, three scenarios are displayed. The first scenario assumes that the mitigation property is improved 25% of the remaining amount that is possible for improvement. For example, bottomland forest scored 0.74 and the maximum possible score is 0.95, a difference of 0.21. An improvement of 25% of the remaining amount possible for improvement would be 0.052 (i.e., 0.25 x 0.21). Thus, if the WHAP score for the bottomland forests in this identical piece of land were improved 0.052 (i.e., from 0.74 to 0.792), then 37,787 acres would be required to replace the lost 1,960 HUs. However, if the bottomland forest could be improved 100% of the amount remaining for improvement, then 9,447 acres would be required.
Table 10(PDF 4.6 KB) is a summary of the appraisal procedure results. It shows the habitat quality scores (i.e., WHAP scores) and the percent that each WHAP score is of the possible maximum for that habitat type. The bluff forest habitat type scored the highest and the crops habitat type scored the lowest. Also displayed in Table 10 are the total acres and the HUs that will be lost for each habitat type and the total for all habitat types.
Figure 4 is a map of the potential jurisdictional wetlands on the proposed reservoir (Jacob 1995). The total area of the wetlands was computed to be 1,733 acres. The proposed reservoir area is overlain almost exclusively by the Brazoria clay soil series and its analogues as mapped by the U.S. Soil Conservation Service (Greenwade et al. 1984).
Hydrologic indicators of flooding were present across the floodplain (Jacob 1995). According to local residents, the Brazos River flooded the proposed reservoir in October 1994 and during the winter of 1991/92. Flooding indicators consisted of sediment drift lines and water marks on trees. While important, these indicators say nothing about the frequency and duration of the hydrologic events. It appears that present-day flooding from either the Brazos River or Allens Creek is insufficient to support wetland hydrology. Hydrology to sustain wetlands on the Brazos bottomland must thus be from precipitation, with landscape position within the bottomland having primary control over where wetlands will occur.
The majority of the potential wetlands (Jacob 1995) areas are mapped as Brazoria depressional soils (Bs) by the U.S. Soil Conservation Service (Greenwade et al. 1984). The most notable of these areas is referred to as Alligator Hole. The deepest depressions have a meander-like pattern, and are probably remnants of former cutoff channels and oxbow lakes, the relief of which have been subdued by infilling with clayey overbank sediments. These depressions receive more water than the surrounding Brazoria clay soil, but hydrologic indicators are only slightly more pronounced. Accurate delineation of wetlands based on the available hydrologic indicators is confounded because of the presence of flooding from the Brazos River. The indicators in the depressions are not sufficiently pronounced to overcome the masking effect of overbank flooding. The areas mapped as Brazoria depressional soils are significantly wetter than the surrounding soils.
The Brazos bottomland has been highly disturbed by human activity since settlement times (Jacob 1995). It is probably safe to assume that none of the proposed reservoir retains pristine vegetation, including the uncultivated depressions, which are predominantly in bottomland forests. The dominant tree in the depressions is the weedy hackberry (Celtis laevigata), a facultative tree (Reed et al. 1988) with little ecological preference with respect to wetlands. The best indicator tree species in the wetter areas is the green ash (Fraxinus pennsylvanica), a facultative wetland plant. Though herbaceous vegetation is limited under the trees, the best indicator species is probably the raven-foot sedge (Carex crus-corvi), an obligate wetland plant that appears to be densest in the wettest areas.
The soils on the Brazos River bottomland present some particularly difficult problems with respect to hydric status (Jacob 1995). The Brazos River sediments are in great part derived from the Permian Red beds of the Rolling Red Plains region of Texas. In addition, the sediments on the floodplain are quite young (less than 500 years old) and thus have had little time to become modified by the factors of soils formation. These red soils are similar to the red soils of the Red River Valley cited in Corps of Engineers Wetlands Delineation Manual (1987) as problem soils. No prominent indicators of contemporary redox conditions could be identified in the Brazos River bottomland soils of the project. There were no iron pore or ped coats typically found in the more mature prairie depressions of Pleistocene uplands. The only indicator of wetness in the depressions was a slightly gleyed color. Depressional soils were typically 5YR 4/2 versus 4/3 and 4/4 in the nondepressional soils. This one-chip difference in the color chroma on a Munsell Color Chart was the only hydric soil indicator. The wetter the soil, the deeper the 2-chroma colors in the soil profile.