An Introduction to the San Marcos River Basin

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An Introduction to the San Marcos River Basin

• Joanna Curran

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San Marcos not considered a large
watershedcontained within the Guadalupe
Watershed which is a subwatershed of the Texas
Gulf
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San Marcos Basin
Texas State University
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Background Information

• 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

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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.

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1901 postcard of Spring Lake
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Underwater Mermaid with Ralph, the Swimming Pig
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Some springs were only partially capped during
operation by the Amusement Park
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Glass Bottom Boat Rides
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Comal and San Marcos Springs are the major
outflow points for the Edwards Aquifer
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For legal consideration, the flow at San Marcos
Springs is tied to the level of the J-17 well
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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.

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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
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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.
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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
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Longitudinal survey of surface sediment into
basic size fractions most of the surface was
visible
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Longitudinal survey of dominant vegetation in the
river secondary vegetation was also mapped
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Surface sediment, vegetation, flow data, water
quality data, and endangered species sampling
data are combined to identify separate habitat
types
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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
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USGS Stream Gaging Stations
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922 km2
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922 km2
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1071 km2
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1071 km2
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290 km2
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290 km2
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125 km2
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125 km2
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2178 km2
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2178 km2
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An Evaluation of Spring Flows to Support the
Upper San Marcos River Spring Ecosystem Texas
Parks and Wildlife Report published in
2001 Used 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
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1996 study grew Texas Wild Rice in controlled
conditions with the goal of finding the preferred
substrate for growth

• Three sediment types were selected
• a moderately fine sediment from Sewell Park in
  the San Marcos River
• a coarse sediment from the confluence of Sessom
  Creek and the San Marcos River
• a very fine soil adjacent to the San Marcos River

high 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
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The San Marcos River Rangers - Measure
Temperature, Dissolved Oxygen, Conductivity, pH,
Nitrate, Phosphates in the San Marcos River
http//www.riverrats.net/smrr/ranger.htm
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.texas
watch.geo.txstate.edu
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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
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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

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Current Masters Thesis work Fluvial
Geomorphology and Texas Wild Rice Habitat in the
San Marcos River

• have floods changed in magnitude and frequency
  since the creation of the NRCS flood control
  dams?
• 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?
• 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.
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An Example of the SWAT (Soil and Water Assessment
Tool) Model from the Blanco Watershed
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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 HRUs
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