San Marcos Mercury | Local News from San Marcos and Hays County, Texas

MOLLY CUMMINGS, an University of Texas biology professor, writes that the San Marcos City Council should not rely on a Texas State University biology professor’s findings that removing historic Cape’s Dam will not harm the San Marcos River’s delicate aquatic ecosystem. 


As a professor at the University of Texas who works with a group of fish species that live in the San Marcos River, I am concerned about the proposed destruction of Cape’s Dam reported over the last few weeks.

Cape’s dam is actually a weir (a barrier that alters flow without completely stopping it) that promotes vegetation growth along river banks providing an important habitat for adult and juvenile fish. Weir removal will have consequences that do not bode well for fish or Texans alike: Lower water levels and greatly increased flow above the dam will reduce refuge for small fish that live along its banks, and thus reduce biodiversity in the river.

A river’s biodiversity potential is an aggregate of all its microhabitat parts. Weir removal means that the fast-moving parts of a river expand at the expense of the slow-moving water areas where vegetation and fish nurseries reside. Species that will be adversely affected by the weir’s removal include many small fish that use the slow-water regions as a nursery for their juveniles (e.g. the endangered fountain darter) as well as others that serve as prey fish for larger fish in the food chain such as mosquitofish (named for their assistance in controlling the mosquito population by eating their larvae). These fish depend upon the patchy regions of slow flow that are created when weirs are present and banks along the side with heavy vegetation.

If the weir is destroyed it will greatly reduce the number of plants and small fish species that can’t withstand high flow rates, and this will have dire consequences for the entire fish population here, including those that Texas’ sport fishers rely on.

Destruction of Cape’s Dam weir is a strike against maintaining Biodiversity in Texas’ diminishing aquatic habitats. It is certain to cause irreparable harm to a huge and more than century-old habitat above it and below Rio Vista Falls. This section of the river above the dam is critical for habitat protection, as the downstream sections are unsuitable due to pollution from a superfund site and fish hatchery.

As the San Marcos City Council deliberates the fate of Cape’s Dam, I hope they will depend not only on analyses by hydro-geologists, but also biologists like me who study local aquatic fauna and their specific habitat needs.


MOLLY CUMMINGS is a biology associate professor in the University of Texas’ College of Natural Sciences. The San Marcos Mercury welcomes original letters to the editor about issues of public interest. Send letters through our contact page or email them to Editor & Publisher Brad Rollins.

COVER: Wrecked by floods in October 2013 and again in May, Cape’s Dam on the San Marcos is the subject of an escalating debate about whether to rebuild the historical structor or scrap it completely. MERCURY FILE PHOTO by BRAD ROLLINS

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