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4.7 BIOLOGICAL RESOURCES aggressive pioneer weed species such as horseweed (Conyza canadensis)and cocklebur(Xanthium sp.). The latter species is an extremely successful colonizer of open moist soil and shoreline areas during drought years when such seasonal wetland sites dry in the early spring. At the beginning of the study for the DEIR in fall 1991,dense cocklebur stands were found throughoutthe lake basins and broken only where a tractor had disced random wide swaths through the weed complex. The return of a near normal rainfall pattern in Central California during the 1991-92 winter resulted in water impoundment in the lake basins by late December. By mid-January they were filled to near capacity(Figure 4.7-2). Measurements taken by the EIR biologist found depths at the lake centers ranged from an average of 7 to 11 feet. The inshore zones exhibited depths of one to two feet over large horizontal distances, and in many areas stands of last year's annual weed crop extended above the water surface (Figure 4.7-3). Lake water levels remained high throughout mid and late spring after the winter rains ceased. By early June 1992 lake levels had receded only a few feet. Increased evaporation occurred during the warm periods of late June and July, and by mid-summer the surface areas of the lakes were reduced to about one-half of their original extent. At the termination of the field work in late August 1992 the water area was reduced to the middle portion of the cross sectional area of each lake basin and had a surface area of about one-fourth of that present in late winter. However, this remaining aquatic zone still averaged four feet in depth and extended nearly the entire length of South Tracy Lake (Figure 4.7-4). In the north lake it was confined to the southern portion of the lake basin. By late summer a dense annual weed crop had once again occupied the shallow portions of the now dry lake basins (Figure 4.7-5). However,unless the onset of the fall-winter rainy season is exceptionally late this year, it appears that an aquatic zone will persist throughout 1992 and may once again be quite extensive by the new year. Given this natural, unmodified annual cycle for these old Central Valley aquatic habitats, the term "permanent wetland" rather than "seasonal wetland" seems far more appropriate in describing these lakes. Pond/Stream Habitats Two additional permanent wetland habitats, the "Beaver Pond" and the Mokelumne River, are situated on the southern edge of the project site (Figure 4.7-1). The Beaver Pond was formed by damming an arm of South Tracy Lake and is maintained by waste irrigation water from adjacent vineyards (Figure 4.7-6). It holds water throughout the year, even during extended drought periods. This statement is supported by several of the larger fish specimens netted in the fall, 1991,lake survey,which presumably have lived in the lake for a number of years. The lake supports a lush inshore aquatic vascular plant stand which extends out to a depth of approximately 4 feet. Maximum lake depth at several sounding points ranges between 6.5 and 8.5 feet. 4.7-3