An Introduction to the San Marcos River Basin

Joanna Curran

San Marcos – not considered a large watershedcontained within the Guadalupe Watershed which is a

subwatershed of the Texas Gulf

San Marcos Basin

Texas State University

• Climate type is Humid Subtropical: hot summers and mild winters

• During tropical storm season this area experiences periods of heavy rainfall and short term flooding

• Average annual temperature is 69 F

• Average annual rainfall is 35.74 inches

• Prevailing winds dominate from the Southeast

Background Information

History of Spring Lake

• An earthen dam constructed in 1849 by Gen. Burleson just below the headwaters of the San Marcos River formed Spring Lake.

1901 postcard of Spring Lake

Underwater Mermaid with Ralph, the Swimming Pig

Some springs were only partially capped during operation by the Amusement Park

Glass Bottom Boat Rides

Comal and San Marcos Springs are the major outflow points for the Edwards Aquifer

For legal consideration, the flow at San Marcos Springs is tied to the level of the J-17 well

Fountain Darter Size - Average of one inch long

• Description - Reddish brown; It displays a series of dark, horizontal, stitch-like lines along its sides and three dark spots at the base of the tail.  Dark bars appear below, behind, and in front of the eyes. 

• Habitat - The fountain darter prefers clear, quiet backwaters with a profuse bottom growth of aquatic plants and matted algae.  It is found in the Comal River and San Marcos rivers.

Description - Had dark edges on dorsal and caudal fins, distinctly cross-hatched side, and was the only Gambusia species with lemon yellow median fins.

Range - This species was restricted to a limited portion of the San Marcos River spring run a few kilometers below the headsprings. It always has been rare, and its existence difficult to document.

Remarks - San Marcos gambusia were captured alive in 1979 and 1980. Despite considerable efforts to secure this species since then, none has been taken.

San Marcos Gambusia

Texas Blind Salamander

Size - 3 1/4 to 5 3/8 inches

Description - This salamander is a ghostly white to pink, with an iridescent appearance. The skin is translucent, and the larger organs are visible through the sides and belly. The head is large, with a strongly flattened snout and small black dots representing vestigial eyes under the skin. The body is slender, and the tail is about the same length as the head and body, tapering at the tip.

Range - Found in the Balcones Escarpment of the Edwards Plateau, mostly underneath the city of San Marcos.

Habitat: It lives in the perpetual darkness of underground streams and caves in the Purgatory Creek system. It is seen above ground only when pumping or the natural outflowing of the underground waters brings it to the surface.

Size - 3 to 7 ft. long

Habit - Aquatic perennial, below surface in swift water with only flowering stalk above, or upper stems and leaves above surface in slow water; stems rooting at joints.

Leaves - Linear, elongate, green, to 45 in. long, 1/4 to 1 in. wide.

Texas Wild-Rice

Longitudinal survey of surface sediment into basic size fractions – most of the surface was visible

Longitudinal survey of dominant vegetation in the river – secondary vegetation was also mapped

Surface sediment, vegetation, flow data, water quality data, and endangered species sampling data are combined to identify separate habitat types

Habitat Vegetation 1 Vegetation 2 Vegetation 3 Substrate 1 Substrate 2 Flow Type % Veg. Cover1 Caboamba Microphyllia Elephant Ear Mud None Backwater Pool 1002 Hydrilla Sagitaria Caboamba Mud None Slow Run 1003 None None None Cobbles Large Gravel Pool (Springs) 04 Potamogeton Ludwigia Elephant Ear Large Gravel Med. Gravel Riffle 305 Potamogeton Hydrilla None Gravels Sand Fast Run 506 Potamogeton Wild Rice Elephant Ear Gravels Sand Fast Run 507 Hydrilla None None Mud None Pool 1008 Potamogeton Sagitaria None Gravels Sand Fast Run 609 Potamogeton Hyacinth None Gravels Sand Fast Run 30

10 Wild Rice None None Gravels Sand Run 8011 Wild Rice Ludwigia Sagitaria Mud None Very Slow Run 8012 Hydrilla Wild Rice Elephant Ear Mud Small Gravel Slow Run 10013 Hydrilla Wild Rice Elephant Ear Gravels Sand Slow Run 5014 None None None Cobbles Large Gravel Plunge Pool 015 Hydrilla Wild Rice None Large Gravel Med. Gravel Deep Pool 10016 Hydrilla Wild Rice None Gravels Sand Run 1517 Hydrilla Potamogeton Elephant Ear Mud None Slow Run 10018 Hydrilla Wild Rice Elephant Ear Med. Gravel Sand Fast Run 3019 Hydrilla Elephant Ear None Small Gravel Sand Run 2020 Hydrilla Potamogeton Sagitaria Gravels Sand Run 8021 Hydrilla Potamogeton Sagitaria Gravels Sand Slow Run 9022 None None None Mud None Slow Run 023 None None None Mud Small Gravel Fast Run 024 Hydrilla Wild Rice Potamogeton Gravels Sand Run 8025 Hydrilla Wild Rice Elephant Ear Large Gravels Med. Gravel Run 8026 None None None Mud Small Gravel Slow Run 027 Elephant Ear None None Mud Small Gravel Slow Run 5

Moving forward from 1993 to present

Instead of looking at only the river, a watershed approach is used to study the San Marcos

Modeling tools are available

USGS Stream Gaging Stations

922 km2

922 km2

1071 km2

1071 km2

290 km2

290 km2

125 km2

125 km2

2178 km2

2178 km2

An Evaluation of Spring Flows to Support the Upper San Marcos River Spring EcosystemTexas Parks and Wildlife

Report published in 2001Used RHABSIM: Riverine Habitat Simulation Model to identify the percentage of the San Marcos River that would be suitable habitat for the endangered species under different flow scenario

Measured at cross-sections along the channel length for input to the hydraulic model

Three sediment types were selected :1) a moderately fine sediment from Sewell

Park in the San Marcos River2) a coarse sediment from the confluence of

Sessom Creek and the San Marcos River3) a very fine soil adjacent to the San Marcos

Riverhigh root to shoot ratio of plants grown in clay which had intermediate nutrient concentrations; and, low productivity by plants grown in gravel and clay suggest that soil texture as well as nutrient concentration play an important role in Texas wildrice growth.

Paula Power: Journal of Aquatic Plant Management, 1996

1996 study grew Texas Wild Rice in controlled conditions with the goal of finding the preferred substrate for growth

http://www.riverrats.net/smrr/ranger.htm

The San Marcos River Rangers - Measure Temperature, Dissolved Oxygen, Conductivity, pH, Nitrate, Phosphates in the San Marcos River

Texas Watch - Measures Temperature, Dissolved Oxygen, Conductivity, pH, Nitrate, Phosphates in the San Marcos River as well as throughout the watershed and elsewhere in Texas

http://www.texaswatch.geo.txstate.edu

San Marcos River Rangers Monitoring Sites:

1 – Aquareana Springs

2 – Springlake Dam East

2 – Springlake Dam West

3 – Sessom Creek

4 – Purgatory Creek

5 – Rio Vista

6 – IH35

7 – Thompson Island

8 – Fish Hatchery

9 – Cummings Dam

10 – Westerfield Crossing

Current Issue: Effects of NRCS dams built between 1981-1991 upstream of Spring Lake

•NRCS dams were built to reduce flooding in San Marcos

•In the meantime, construction (esp. at Texas State) has increased the supply of sediment to the channel

•construction sediments, and naturally eroded sediments, enter the system downstream of the dams

•flows that would normally move the sediments are reduced by the NRCS dams

•A large gravel and sand bar/delta has developed just downstream of Spring Lake dam

Current Master’s Thesis work: Fluvial Geomorphology and Texas Wild Rice Habitat in the San Marcos River

1) have floods changed in magnitude and frequency since the creation of the NRCS flood control dams?

2) how has the geomorphology of the upper San Marcos changed since the NRCS dams were completed and do these changes correlate to Texas wild-rice habitat?

3) does a flood enhance or degrade Texas wild-rice habitat?

 The findings of this study-in-progress suggest that the ecosystem is dependent on a flow regime composed of consistent springflows along with the full range of peak flows necessary for flushing, scouring, sediment transport, and channel maintenance.

Transect B5a Sewell Park 1995 Study

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Transect B13 Sewell Park 1995 Study

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Transect B20 Sewell Park 1995 Study

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Transect B5b Sewell Park 1995 Study

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An Example of the SWAT (Soil and Water Assessment Tool) Model from the Blanco Watershed

Elevation + Land Use + Soils +

Weather Data

Weather = SWAT

SWAT takes the user input data of land use, weather, soils, etc along with adjustable parameters and hydraulic equations to make predictions over each sub-basin and define HRU’s