An Evaluation of Spring Flows to Support the Upper San Marcos River Spring Ecosystem, Hays County, Texas Kenneth S. Saunders Kevin B. Mayes Tim A. Jurgensen Joseph F. Trungale Leroy J. Kleinsasser Karim Aziz Jacqueline Renée Fields Randall E. Moss River Studies Report No. 16 Resource Protection Division Texas Parks and Wildlife Department Austin, Texas August 2001 Table of Contents An Evaluation of Spring Flows to Support the Upper San Marcos River Spring Ecosystem, Hays County, Texas Introduction..........................................................................................................................................1 Methods................................................................................................................................................6 Results................................................................................................................................................12 Segment Description........................................................................................................................12 Mesohabitat Description....................................................................................................................13 Hydraulic Models ..............................................................................................................................15 Macrophyte Data Summaries .............................................................................................................15 Fish Data Summary ...........................................................................................................................16 Suitability Criteria ..............................................................................................................................16 Instream Habitat Model......................................................................................................................17 Habitat Time Series...........................................................................................................................18 Critical Depths for Z. texana ...............................................................................................................20 Water Quality....................................................................................................................................20 Discussion..........................................................................................................................................23 Historical Hydrology ..........................................................................................................................24 Rich Biodiversity...............................................................................................................................24 Dominance of Run-type Mesohabitats................................................................................................25 Water Quality....................................................................................................................................25 Recreation .......................................................................................................................................28 Spring Flow and Ecosystem Characteristics........................................................................................28 Acknowledgments.............................................................................................................................30 References.........................................................................................................................................30 i List of Figures FIGURE 1.—Daily mean flow based on spring flows recorded at USGS Gage #08170000 (San Marcos Springs at San Marcos, TX) for the period of record 26 May 1956–30 September 1998. Monthly average naturalized flows were developed for the period from January 1934–December 1989 (HDR Engineering, Inc. 1993). Daily median equal to 157 ft3/s based on USGS record. ....................................3 FIGURE 2.—Flow frequencies are based on daily spring flows recorded at USGS Gage #08170000 (San Marcos Springs at San Marcos, TX) for the period of record 26 May 1956–30 September 1998. Bars represent frequency in bins (5 ft3/s) and solid line represents cumulative frequency percentage. There are 15,468 daily records for this period. ......................................................................................3 FIGURE 3.—Watershed of the upper San Marcos River................................................................................4 FIGURE 4.—Study area of the upper San Marcos River................................................................................7 FIGURE 5.—Bio-grid sampling sites used for habitat availability and utilization data collection on the upper San Marcos River................................................................................................................................9 FIGURE 6.—Normalized wetted width in pool, riffle, and run mesohabitats in relation to discharge in the upper San Marcos River. Data based on all modeled cross-sections in the main channel (upper) and in the natural channel (lower). Main channel wetted width normalized to long-term median spring flow (1956-1998). Natural channel wetted width normalized to 140 ft3/s.......................................................15 FIGURE 7.—Zizania texana percent weighted usable area (% WUA) in relation to spring flow in the upper San Marcos River main channel segments..........................................................................................18 FIGURE 8.—Heteranthera liebmannii percent weighted usable area (% WUA) in relation to spring flow in the upper San Marcos River main channel segments...........................................................................18 FIGURE 9.—Vallisneria americana percent weighted usable area (%WUA) in relation to spring flow in the upper San Marcos River main channel segments. ...............................................................................18 FIGURE 10.—Sagittaria platyphylla percent weighted usable area (% WUA) in relation to spring flow in the upper San Marcos River main channel segments. ...............................................................................18 FIGURE 11.—Potamogeton illinoensis percent weighted usable area (%WUA) in relation to spring flow in the upper San Marcos River main channel segments...........................................................................18 FIGURE 12.—Spring flow ranges that contribute to above average habitat conditions based on 75th percentile %WUA frequencies calculated from habitat time series. Vertical lines represent 25th and 75th percentile spring flows...............................................................................................................19 FIGURE 13.—Spring flow ranges that contribute to average habitat conditions based on 25th-75th percentile %WUA frequencies calculated from habitat time series. Vertical lines represent 25th and 75th percentile spring flows...............................................................................................................19 FIGURE 14.—Spring flow ranges that contribute to below average habitat conditions based on 25th percentile %WUA frequencies calculated from habitat time series. Vertical lines represent 25th and 75th percentile spring flows...............................................................................................................19 FIGURE 15.—San Marcos River, temperature by station, measured hourly, Nov-1994 through May-1997. Horizontal line in box: sample median. Box top and bottom: 75th and 25th percentiles. Upper and lower fences: limits of data.................................................................................................................21 FIGURE 16.—San Marcos River, summer conditions, temperature by station, measured hourly, May-1996 through Jul-1996. Horizontal line in box: sample median. Box top and bottom: 75th and 25th percentiles. Upper and lower fences: limits of data. .............................................................................21 FIGURE 17.—San Marcos River, winter conditions, temperature by station, measured hourly, Dec-1995 through Feb-1996. Horizontal line in box: sample median. Box top and bottom: 75th and 25th percentiles. Upper and lower fences: limits of data. .............................................................................21 ii FIGURE 18.—San Marcos River, dissolved oxygen (mg/L) by station, measured hourly, Nov-1994 through May-1997. Horizontal line in box: sample median. Box top and bottom: 75th and 25th percentiles. Upper and lower fences: limits of data.................................................................................................22 FIGURE 19.—San Marcos River, summer conditions, dissolved oxygen by station, measured hourly, May- 1996 through Jul-1996. Horizontal line in box: sample median. Box top and bottom: 75th and 25th percentiles. Upper and lower fences: limits of data. .............................................................................22 FIGURE 20.—San Marcos River, winter conditions, dissolved oxygen by station, measured hourly, Dec- 1995 through Feb-1996. Horizontal line in box: sample median. Box top and bottom: 75th and 25th percentiles. Upper and lower fences: limits of data. .............................................................................22