An Introduction to the San
Marcos River Basin
Joanna Curran
San Marcos – not considered a large watershed
contained within the Guadalupe Watershed which is a
subwatershed of the Texas Gulf
San Marcos Basin
Texas State
University
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
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
Size - Average of one inch
Fountain Darter
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.
San Marcos Gambusia
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.
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.
Texas Wild-Rice
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.
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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
Vegetation 1
Caboamba
Hydrilla
None
Potamogeton
Potamogeton
Potamogeton
Hydrilla
Potamogeton
Potamogeton
Wild Rice
Wild Rice
Hydrilla
Hydrilla
None
Hydrilla
Hydrilla
Hydrilla
Hydrilla
Hydrilla
Hydrilla
Hydrilla
None
None
Hydrilla
Hydrilla
None
Elephant Ear
Vegetation 2
Microphyllia
Sagitaria
None
Ludwigia
Hydrilla
Wild Rice
None
Sagitaria
Hyacinth
None
Ludwigia
Wild Rice
Wild Rice
None
Wild Rice
Wild Rice
Potamogeton
Wild Rice
Elephant Ear
Potamogeton
Potamogeton
None
None
Wild Rice
Wild Rice
None
None
Vegetation 3
Elephant Ear
Caboamba
None
Elephant Ear
None
Elephant Ear
None
None
None
None
Sagitaria
Elephant Ear
Elephant Ear
None
None
None
Elephant Ear
Elephant Ear
None
Sagitaria
Sagitaria
None
None
Potamogeton
Elephant Ear
None
None
Substrate 1
Mud
Mud
Cobbles
Large Gravel
Gravels
Gravels
Mud
Gravels
Gravels
Gravels
Mud
Mud
Gravels
Cobbles
Large Gravel
Gravels
Mud
Med. Gravel
Small Gravel
Gravels
Gravels
Mud
Mud
Gravels
Large Gravels
Mud
Mud
Substrate 2
None
None
Large Gravel
Med. Gravel
Sand
Sand
None
Sand
Sand
Sand
None
Small Gravel
Sand
Large Gravel
Med. Gravel
Sand
None
Sand
Sand
Sand
Sand
None
Small Gravel
Sand
Med. Gravel
Small Gravel
Small Gravel
Flow Type
Backwater Pool
Slow Run
Pool (Springs)
Riffle
Fast Run
Fast Run
Pool
Fast Run
Fast Run
Run
Very Slow Run
Slow Run
Slow Run
Plunge Pool
Deep Pool
Run
Slow Run
Fast Run
Run
Run
Slow Run
Slow Run
Fast Run
Run
Run
Slow Run
Slow Run
% Veg. Cover
100
100
0
30
50
50
100
60
30
80
80
100
50
0
100
15
100
30
20
80
90
0
0
80
80
0
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 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
1996 study grew Texas Wild Rice in controlled
conditions with the goal of finding the preferred
substrate for growth
Three sediment types were selected :
1) a moderately fine sediment from Sewell Park in
the San Marcos River
2) a coarse sediment from the confluence of
Sessom Creek and the San Marcos River
3) 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
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.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 wildrice 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.
An Example of the SWAT (Soil and Water Assessment
Tool) Model from the Blanco Watershed
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
Weather Data
Elevation + Land Use + Soils + Weather = SWAT