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Article Excerpt
Abstract.--Changes in the headwaters of the San Marcos River, with an area of 247 [km.sup.2], have caused major sedimentation and exotic plant invasion problems in its course through the city of San Marcos. Construction of upstream flood control dams, with insufficient flow-through provisions, has reduced the effective unregulated upstream drainage to 47 [km.sup.2] and reduced mean annual flood from 510 [m.sup.3]/sec (18,000 [ft.sup.3]/sec) to 42 [m.sup.3]/sec (1,500 [ft.sup.3]/sec) which is less than the threshold value required for scouring the river channel. Headwaters area construction downstream of the flood control structures, particularly on the Southwest Texas State University campus, has increased sediment production from 160 [m.sup.3] to 920 [m.sup.3] year. Since 1990, the combined effects of these changes have produced up to 0.50 m sedimentation in the main channel and an increase in exotic riparian and aquatic vegetation. Of the remedial actions proposed, the only likely option involves increased efforts to reduce sediment production from construction sites. The October 1998 flood, triggered by a larger than 100-year precipitation event (401 mm/24hr), demonstrated that the flood control structures reduced peak discharge in San Marcos to a discharge that would have been approximately a twenty-five year event. This event did not produce the sediment scour that would have been expected which suggests that sediment increases and not a reduction of flows are the major cause of the sedimentation.
San Marcos, Texas, like many rapidly growing Sunbelt cities, faces the often conflicting goals of protecting aesthetic and recreational resources while providing flood protection. In 1970 the city had a population of 18,900; the 1995 estimate was 37,000 (City of San Marcos 1996) and the 1999 estimate was 41,000 (Greater San Marcos Economic Development Council 2000) even though the "official" 2000 census value was 34,733, which has prompted a city drive for a higher readjusted value (U.S. Bureau of the Census 2001). Southwest Texas State University (SWT), totally within the city limits, has grown from 9,900 students in 1970 to more than 22,000 in 2000 (Southwest Texas State University 2001). The San Marcos River was singled out in the 1996 city comprehensive plan as a unique resource that needs to be protected for the aesthetic benefit of residents and as a basis of the city's tourism industry (City of San Marcos 1996). Each year, the San Marcos River draws more than an estimated five hundred thousand visitors for water based recreation and civic activities adjacent to its banks (Greater San Marcos Economic Development Council 2000). Baseflow for the river flows from springs draining the Edwards Aquifer; floodflow is produced by a 247 [km.sup.2] headwaters with ephemeral flow.
Management and protection of the San Marcos River not only is motivated by concern for its benefit to the city; it is habitat to four federally listed endangered species, two fish, a salamander and Texas wild rice (Texas Parks and
Wildlife Department 1993). In response to a 1992 federal court order to develop a habitat protection plan, the State of Texas has created the Edwards Aquifer Authority that has imposed pumping limits and has developed additional drought management pumpage restrictions (Votteler 1998). Characteristic of its location along the Balcones Escarpment, the San Marcos River has the potential for generating huge flood discharges (Baker 1975). The flood of May 15, 1970, which had an estimated discharge of 2,170 [m.sup.3]/sec (76,600 [ft.sup.3]/sec), resulted in two drownings and the city being declared a federal disaster area (Upper San Marcos Watershed Reclamation and Flood Control District 1991).
The 1970 flood was the stimulus for efforts to create the Upper San Marcos Watershed Reclamation and Flood Control District (USMWRFCD) in 1971. A flood on June 13, 1981 that forced the evacuation of 1,800 people provided the political catalyst for the funding of US Soil Conservation Service flood control dams upstream of San Marcos. The last of five flood control dams on the upper San Marcos watershed was completed in 1991 (USMWRFCD 1991). These dams have a combined capacity of 23 million [m.sup.3] (19,000 acre feet) and reduced the uncontrolled drainage area from 247 [km.sup.2] to 47 [km.sup.2]. (U.S. Soil Conservation Service 1978). Construction in the upstream area, but mostly downstream of the flood control dams, has produced sedimentation in the perennial flow reaches of the river in San Marcos (Miller 1996). Also, the public has increasingly complained about clogging of the channel by vegetation (Wood 1998). The flood control project received a major test in October 1998 when a storm with an official Sa n Marcos 24-hour total of 401 mm (15.78 in) produced runoff amounts that exceeded the 254 mm (10 in) standard project flood design and produced considerable flooding in San Marcos (U.S. Soil Conservation Service 1978; U.s. Natural Resources Conservation Service 1999).
The purpose of this paper is to report on the changes in the headwaters of the San Marcos River and their downstream effects, both planned for and unplanned, since the construction of the five upstream flood control structures that began in 1981. A particular focus of this paper will be the effects of these flood structures on the October 1998 flood. Not included in this analysis are effects of the damming of San Marcos Springs in 1849 to form Spring Lake, which was developed into the popular commercial site known as Aquarena Springs (Mays et al. 1996), nor will this paper analyze the effects of the construction of small irrigation and hydroelectric dams (Rio Vista and Cape's Camp dams) downstream of San Marcos Springs during the late 1 1890s and early 1900s (McGehee 1982; Stovall 1986; Spain 1994). This paper is intended to complement the recent study on water quality characteristics of the San Marcos River by Groeger et al. (1997) and contains updated bibliographic references to augment those in Saunders (1 992).
METHODS AND MATERIALS
After describing the basic hydrology of the upper San Marcos River basin, this paper will analyze the changes in the watershed since 1970 and the efforts to mitigate the flood hazard and also summarize the discharges generated by the October 1998 event in upstream tributaries as well as the main channel in San Marcos. An analysis of the flood frequency of the modern stream will provide for an assessment of the stream's ability to transport the increased sediment supply that construction within the basin has produced. A series of management options for the stream under its new regime of reduced peak discharges and...
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