Going with the flow: a Texas river winds through town and country

Natural History, Feb, 2002 by Robert H. Mohlenbrock

Texas's San Marcos River has its source in the city of San Marcos, halfway between San Antonio and Austin, where clear water emerges from a series of limestone springs. In the mid-nineteenth century the headwaters of the river were dammed, forming Spring Lake, which remains a popular attraction. From there the river flows sixty-three miles to the Guadalupe River, which feeds into the San Antonio River near its outlet in the Gulf of Mexico. Along the San Marcos's course are numerous pools interspersed with small rapids.

A plant found only in Spring Lake and the uppermost reaches of the San Marcos River is Texas wild-rice, an aquatic grass whose clustered grains and upper leaves ride above the water's surface. This relative of commercially marketed wild rice was first recognized as a separate species in 1932. At that time it was abundant, but in 1967 botanist William Emery reported that only one plant still grew in Spring Lake and the remaining ones were confined to a stretch of river between half a mile and two miles below the dam. Among the causes of the species' decline were pollution and the use of an underwater mowing machine to reduce vegetation in the lake. The debris that floated downstream after each mowing apparently interfered with the plant's pollination. Although conditions have been improved, the grass remains on the U.S. Fish and Wildlife Service's endangered species list.

About ten miles before it enters the Guadalupe River, the San Marcos River passes through Palmetto State Park, an ideal place to observe floodplain and adjacent upland habitats. The most surprising species I encountered there was anaqua (Ehretia anacua), a twenty- to forty-foot-tall tree that generally ranges farther south, from along the southern border of Texas into northern Mexico. It is one of about fifteen species in its genus; all the others grow in tropical regions of the Old World. Some of these species are valuable timber trees, while others, such as China's E. microphylla (known as Philippine or Fukien tea), are popular with growers of bonsai. The Texas plant, too, is a frequent choice for bonsai, and its white flowers also make it an attractive ornamental in a desertlike landscape. The leaves are among the roughest to the touch of any plant's; to me they feel like Velcro.

Bottomland forest. Hardwood trees common to northern and eastern Texas grow above flood level on a moist river terrace. Cedar elm, sugarberry (a type of hackberry), and red ash predominate in creating a fairly open canopy about sixty feet above the forest floor. Other tall trees are bur oak and Shumard oak. Abundant box elders make up a somewhat shorter layer. Beneath these grows red buckeye, whose clusters of bright red flowers enliven the forest in early spring. This is the habitat for anaqua. Shrubs: chittamwood (a bumelia also called gum-elastic), yaupon, spatulate-leaved hawthorn. Wildflowers: baby blue-eyes, large-flowered buttercup, Missouri violet, white avens, branched foldwing (Dicliptera brachiata).

Palmetto swamp. A lush habitat appears where surface water accumulates in shallow depressions to form seasonal or fairly permanent ponds. These zones harbor dwarf palmetto, with its fan-shaped leaves, and buttonbush, with its spheres of tiny white flowers. Mosquito fern, a diminutive floating fern, grows in some pools. Because the canopy is relatively open here, the vegetation is often overgrown with rattan vine (supplejack), greenbrier, wild grape, pepper vine, trumpet creeper, Virginia creeper, and poison ivy. Wildflowers: wild blue iris, creeping buttercup, brookweed, water pennywort.

Upland forest. Higher and drier terrain supports a scrubby growth of post oak and blackjack oak. Also fairly common are bluejack oak, netleaf hackberry, honey mesquite, and prickly ash. Prickly pear cactus grows here too--evidence of the arid conditions. Shrubs: American beautyberry, wafer ash. Wildflowers: Drummond's phlox, hairy phacelia, Indian paintbrush, smooth spiderwort, Quaker-ladies, wild indigo, yellow wingstem.

For visitor information, contact:

Park Ranger
Palmetto State Park
78 Park Road 11 South
Gonzales, TX 78629
(830) 672-3266
www.tpwd.state.tx.us/park/palmetto/

Robert H. Mohlenbrock, professor emeritus of plant biology at Southern Illinois University, Carbondale, explores the biological and geological highlights of U. S. national forests and other parklands.