Upstream Changes and Downstream Effects of the San Marcos River of Central Texas‪.‬

Abstract. -- Changes in the headwaters of the San Marcos River, with an area of 247 k[m.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 k[m.sup.2] and reduced mean annual flood from 510 [m.sup.3]/sec (18,000 f[t.sup.3]/sec) to 42 [m.sup.3]/sec (1,500 f[t.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 (401mm/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. **********