The San Marcos is not a “natural” river, unless one is generous enough to consider the meddling attentions that humans have paid it over the years to be a natural process. Although much of the river is choked with plant life, there is no telling how many of the species that inhabit it are indigenous.

No, the San Marcos River is a naturally occurring spring-fed river, although its headwaters have been modified by human intervention. The river originates from the San Marcos Springs, which emerge from the Edwards Aquifer. Historically, these springs bubbled up into a natural marshland, but in the mid-19th century, a dam was built to harness water power, creating Spring Lake. While the lake itself is a man-made reservoir, the water within it comes from over 200 artesian springs that have flowed for thousands of years. The river is one of the oldest continuously inhabited sites in North America due to its reliable water source. In 2026, it is celebrated as a biological treasure, home to several endangered species like the Texas Blind Salamander and Texas Wild Rice, which exist nowhere else on Earth. The river maintains a constant temperature of 72°F (22°C) year-round, making it a popular natural destination for tubing and swimming, even though the "start" of the recreational area is defined by the man-made structures of the Meadows Center.