You may begin in the bottoms of the upper Sabine and proceed down river in wet habitat to the gulf shore. Then go southwestward hopping bayous, swamps and sloughs to the mouths of the Trinity, San Jacinto, Brazos, Colorado, San Bernard, etcetera, etcetera, etcetera. Take a right turn on any and go upstream as far as possible. You have just dispersed like so many wetland species have always done. Also, when the climate has fluctuated from wet to dry in the past mesic species have probably expanded and contracted with it.
Sand adapted species exist on a narrow archipelago.
As the Sparta and other Eocene sands outcrop from the Rio Grande to the Louisiana border they have been chopped into pieces by rivers and their wide floodplains. The sandy segments have many species with low tolerance for other soil types and consequently high incidence of endemism. There are examples of species that are endemic to one or a few sandy segments.