A Vegetation Inventory of the Proposed Eastex Reservoir

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Analysis and Discussion

This summary identifies the major steps in mapping the vegetation cover of the proposed Eastex Reservoir in Cherokee county, Texas, about five miles east of Jacksonville. This summary also reviews the results of the vegetation cover change analysis between 1981 and 1993.

The proposed reservoir will extend northward fifteen miles behind a dam to be built on Mud Creek, and will have a surface area of approximately 10,234 acres (Fig.1) media download(PDF 612.9 KB)

This project had three phases. The first involved mapping the current (1993) vegetation cover in the area of the proposed Eastex reservoir. The second phase consisted of a study to determine the nature of vegetation cover change between 1981 and 1993. The first step in this phase was to complete a vegetation cover map for 1981. Then, the 1981 and 1993 maps were overlaid and the vegetation cover changes assessed. The third phase of this project consisted of the production of a general vegetation cover map of the area along Mud Creek south of the proposed dam to the creek's confluence with the Angelina River, a distance of approximately eighteen miles.

Aerial photography, data digitized from USGS quadrangles, DLG data files, field surveys, and pcARC/INFO were the tools used to complete this project. The staff consisted of five students and myself, working over a five month period. Students had specific tasks but often worked where they were needed. These students worked as biologists, aerial photo interpreters, digitizers and digital data processors, GIS analysts, graphic artists, field workers, and computer system analysts.

The planned pool surface of the Eastex reservoir is to be 315 feet above mean sea level. Color infra-red aerial photography was taken of the area on March 24, 1993. Interpretation was straightforward for the most part. A total of 274 areas of homogeneous vegetation were identified and assigned to seven different vegetation categories. The largest category was Mixed Hardwood Bottomland Forest with 4534 acres or 44% of the total area to be under water. The Water category covered the smallest area, 48 acres, less than 1% of the total (Table 1). The Vegetation Map made from the March, 1993 photography is provided as Figure 2.

The purpose of the 1981 vegetation category map was to enable a vegetation cover type change analysis. The rationale behind the analysis was that the vegetation cover had been adversely affected by the possibility of a dam being built on Mud Creek. The Angelina-Nueces River Authority has been pursuing the building of a reservoir at that site since the late 1970s. The state of Texas has already granted a permit to the river authority for the dam. Currently (1993), the river authority is seeking a federal permit for the dam to complete necessary legal requirements before construction can begin.

As local landowners above the proposed dam site learned of the possibility that a reservoir was being planned, it was hypothesized that they would be stirred into action, such as clearing their land of marketable timber, that they would not have undertaken had the reservoir not been proposed in the area.

To test that hypothesis, vegetation in the reservoir site was mapped once again around the time that the permitting process began at the state level. Thus, aerial photography for 1981 was acquired and the vegetation mapped. For 1981, 294 vegetation polygons were identified. While these were also assigned to seven categories, one category, Mixed Hardwood/Pine Bottomland Forest, did not exist in 1993 (Table 2, Figure 3).

The vegetation polygon maps for the two years were compared and the changes noted. Overall, the maps contained the same categories with two significant differences. In 1981, the largest category was Mixed Hardwood/Pine Bottomland Forest which covered about 22% (2290 acres) of the total area. In 1993, that category did not exist according to aerial photo analysis. The hardwoods were still in the bottomlands, but the pines had been removed. The second change was the emergence of a new category in 1993 that did not exist in 1981: Young Forest.

While the type of bottomland forest environment changed (i.e. selective cutting of pine over the 12 year period) the total acreage in forested bottomland did not change between 1981 and 1993. Some 41% (4217 acres) of the area to be under water consisted of either Mixed Hardwood Bottomland Forest or Mixed Hardwood/Pine Bottomland Forest in 1981. Twelve years later, 44% (4534 acres) was classified as Mixed Hardwood Bottomland Forest. 

On the other hand, the 1993 map had a Young Forest vegetation category that did not exist or at least was not recognizable in the 1981 aerials. These areas showed up on the 1993 aerial photographs as clear-cut areas where new growth was occurring. Some of the new growth was planned, i.e. Christmas tree farms and pine plantations. In other Young Forest areas, the aerials did not indicate any organized regrowth activities. About 10% (1083 acres) of the land within the proposed reservoir was young forest in 1993. A small amount of this new category came from bottomland forest areas (47 acres); most (890 acres) came from what had been classified as upland forest areas in 1981.

The results of the digital overlay operation produced 538 polygons whose classification is summarized in Table 3 and in a series of six maps. Slightly more than half of the land (5,513 acres, 54% of the total acreage) had the same vegetation cover in both years. Nearly half, 46% of the land to be under the surface of the proposed reservoir, had experienced change in cover type. The largest changes involved the expansion of an existing category (Mixed Hardwood Bottomland Forest) and the emergence of a new category, Young Forest. Both categories are related in that their increase over the past dozen years was due to timber harvesting. In the case of the expansion of Mixed Hardwood Bottomland Forest, pine trees had been selectively removed. In the case of the Young Forest category, all trees were removed. The Mixed Hardwood Bottomland Forest category grew by 2,607 acres (from 1927 acres to 4534 acres) in twelve years. The Young Forest category grew from zero acres to 1083 acres (11% of the area) in the same period. It is not clear whether timber cutting in the area over the past twelve years was due to the reservoir or simply reflected inreasing cutting rates that may be occurring across all of East Texas. Timber cutting rates for Cherokee and surrounding counties (Smith, Henderson, Anderson, Houston, Angelina, Nacogdoches, and Rusk) would help answer that question.

Another approach to an answer would be to compare timber cutting at the reservoir site with a similar area somewhere else along Mud Creek. Aerial photography of the Mud Creek area south of the dam was taken at the same time as that taken of the dam site (March 24, 1993).  A map of the bottomlands area was prepared and it showed that the majority of the forested land is currently Mixed Hardwood/Pine Bottomland Forest. It would appear, therefore, that the disturbed bottomland forest north of the dam was due to knowledge of a possible dam.  It should be noted, however, that other factors may have been at work which precluded timber harvesting below the dam.  Perhaps the area below the dam is simply too inaccessible because of frequent flooding.  No inquiry was undertaken into the possible reasons for the existence (as indicated by the aerial photographs) of pristine bottomland forest. 

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