Aquarena Center
- Nonprofit nature center at San Marcos Springs. Includes information on exhibits, ecology of the springs and their endangered species, educational programs, scientific diving, and facilities rental for special events.
- http://www.aquarena.txstate.edu/

San Marcos River Rangers
- Collect water quality data from the headwaters of the San Marcos River at Aquarena Springs to Prairie Lea. Features mission statement, river profile, and monitoring schedule and sites.
- http://www.riverrats.net/smrr/ranger.htm

Birding in San Marcos
- Birding locations, descriptions, maps, plus pictures and descriptions of the birds of central Texas.
- http://www.centuryinter.net/birding/