Bexar Watersheds

All of the land within the borders of Bexar County is inside one of six large watersheds. Skirting the southern edge of the County is the Atascosa River watershed (white). The largest watershed coming through the County is associated with the Medina River (blue). The San Antonio River watershed (cyan) is the principal water carrier through the County. Both the Leon Creek watershed (yellow) and the Salado Creek watershed (orange) join the San Antonio River inside the County borders. The Cibolo Creek watershed (green) joins the San Antonio River much further downstream, 1 mile east of Panna Maria in Karnes County. The ragged border of Bexar County traces the waterway of Cibolo Creek and one of its tributaries, Balcones Creek. Across the ragged Bexar County border is Kendall County, Comal County and Guadalupe County. Cibolo Creek also passes through Wilson County and Karnes County to its confluence with the San Antonio River.


Surrounding Watersheds
The path length along the waterway is about 150 miles.
Nearby Cities
The straight-line distance between Cibolo Creek's headwaters
northwest of Boerne and it confluence with the
San Antonio River near Panna Maria is about 87 miles.






Where the width of Cibolo Creek's waterway is restricted, its waters run fast, especially during storms. The forces from the rapid waterflow breaks down rocks and carry away the resulting fine sediments. In locations where the waterway is wider, the stormwaters slow and sediment settles in and around the creekbed. The tan shades on the map to the right are areas where sediment has been deposited over the millenia. The waterway through the areas of deposition tend to wander around, creating meanders.




Geologic Atlas


Edwards Cross-Section

The lands in this area were beneath the sea a very long time ago. During that aquatic time, the death of sea creatures deposited material that has become our limestone. These lands rose from the sea such that the limestone would begin to disolve away to create the porosity of our aquifers. Tectonic forces played on the western lands creating the Edwards Plateau, the Hill Country, and the Balcones Escarpment. Upstream erosion has warn away some of the sea deposits in the Hill Country and carried them downstream into the Blackland Prairies. The more recent ancient geologic layers that can be found underneath the Blackland prairies can no longer be located above the Balcones Escarpment.


Edwards Faults
The differential rise that elevated the Hill Country created
many geologic faults along the Balcones Escarpment.
Edwards Flows
Subsurface water flow moves to the east, to feed the
Comal and San Marcos Springs.


















Many of the long waterways in this part of Texas contain a somewhat sharp bend as they pass through the Balcones Escarpment. These bends were a byproduct of tectonic forces and the resulting uplift. In general, the bends are cases where an old waterway heading to the east changed its path to the southeast by taking over other waterways. This hijacking of waterways is called stream piracy.














Stream Piracy


Stream Slope

When one follows the lowest part of the waterway (called the thalweg) from its origin to its eventuality, a curious thing is observed. Although we recognize that the lands around the waterway clearly show an abrupt elevation change through the Balcones Escarpment, that feature disapears along the thalweg. The waterway erodes and deposits in such a way as to adjust its slope to become more constant. The graph on the left is from the headwaters of the Cibolo Creek at about 2000 feet above sea level to the confluence at about 200 feet above sea level. Recognize that the Creek's passage through the Balcones Escarpment is between the 70 mile marker and the 80 mile marker. The slope in the thalweg is managed by deeper cuts and a lengthened pathway, in this case near Bracken Bat Cave. This is the area where the Cibolo Creek watershed narrows considerably.


Length
flat-map = 87 miles, water path = 153 miles

  • Headwaters Turkey Knob
  • Location Kendall County, Boerne, Texas
  • Elevation 2,044 ft
  • Coordinates 29.83833 -98.88611
  • Confluence San Antonio River
  • Location Karnes County, Panna Maria, Texas
  • Elevation 205 ft
  • Coordinates 28.95306 -97.87333
COUNTIES
  • Kendall
  • Bexar
  • Comal
  • Guadalupe
  • Wilson
  • Karnes






PARKS
  • Boerne City Lake Park
  • Boerne River Road Park
  • Boerne City Park
  • Boerne Cibolo Nature Center
  • Fair Oaks North Park
  • Bexar County Cibolo Preserve
  • Schertz Pickrell Park
  • Schertz Cutoff Park
  • Schertz Crescent Bend Nature Park
  • Cibolo Niemietz Park
  • Wilson County Jackson Nature Center
CITIES
  • Boerne
  • Fair Oaks Ranch
  • Timberwood Park
  • San Antonio
  • Bulverde
  • Bracken
  • Luxello
  • Selma
  • Schertz
  • Universal City
  • Cibolo
  • Lower Valley
  • Zuehl
  • Saint Hedwig
  • New Berlin
  • La Vernia
  • Cestohowa
  • Sutherland Springs
  • Panna Maria




According to Texas Parks and
Wildlife, the following fish have
been caught in Cibolo Creek:
  • Largemouth bass
  • Bluegill
  • Channel catfish
  • Rio Grande cichlid
  • Longnose gar
  • Green sunfish
  • Sunfish hybrid
  • Redbreast sunfish
  • Redear sunfish


FLOOD FREQUENCY DETERMINATION AT SELMA'S USGS GAGE
-Flood Standard- -24 Hour Rainfall- -Selma USGS Gage- -Year-
- - 24,201 cfs 11/22/2004
10-yr 6.0 inches 36,700 cfs -
- - 39,000 cfs 07/02/2002
25-yr 7.5 inches - -
- - 69,600 cfs 06/22/1997
50-yr 9.0 inches 78,450 cfs -
100-yr 10.0 inches 93,940 cfs -
- - 98,100 cfs 10/17/1998
500-yr 13.7 inches 147,380 cfs -


LIKELIHOOD OF FLOODING (if all else stays the same)
There is a 1% chance that there will be a 100-year flood within the next 1 year
9.5% 10 years
22% 25 years
39% 50 years
64% 100 years
0.2% 500-year 1 year
2% 10 years
5% 25 years
9.5% 50 years
18% 100 years
(ie. There is a 22% chance that there will be a 100-year flood within the next 25 years)

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