The habitats of many of the threatened and endangered (T&E) species in the Edwards Aquifer are clustered around two major spring openings, Comal Springs and San Marcos Springs. Comal Springs marks the headwaters of the Comal River and is located in New Braunfels, Texas; San Marcos Springs, in San Marcos, TX supplies stream flow for the San Marcos River. The majority of Edwards Aquifer's discharge drains into Comal and San Marcos Springs. The rate of the springs' stream flow is directly related to water levels in the Edwards aquifer, as discussed on the precipitation and water quality and quantity pages. Although the flow at these springs is considered to be relatively constant, variable demands such as pumping for drinking water and irrigation as well as natural stressors such as drought, can cause the flow in these springs to become low enough to impair the sensitive habitats of several T&E species.
The US Fish and Wildlife Service and the Edwards Aquifer Authority (aquifer governing body) have defined low streamflow levels for the springs which threaten the habitats where T&E species reside. The graphs below depict the stream flow at Comal Springs and San Marcos Springs and the levels required for harm or "take" of the fountain darter
an inhabitant of both springs and their associated river systems. (See "Edwards Aquifer and the ESA" page for discussion of take). The fountain darter relies on native aquatic vegetation in the San Marcos and Comal Rivers. A severe reduction in flow rates may threaten these aquatic plants, thus harming the fountain darter population.
The fountain darter's habitat is one of the most sensitive in the aquifer with regard to stream flow and associated water quality parameters (i.e. temperature, dissolved oxygen, sedimentation), and thus it serves as a criteria species for stream flow monitoring in the aquifer. The Draft Edwards Aquifer Habitat Conservation Plan (EAHCP) defines prime habitat for the fountain darter at flows greater than 150 cubic feet per second (cfs) in Comal Springs and the associated Comal River ecosystem and greater than 100 cfs at San Marcos Springs and the associated San Marcos River ecosystem. The US Fish and Wildlife Service defines "take" for this species as flows of less than 200 cfs for the Comal system and less than 100 cfs for the San Marcos system. According to the EAHCP there is a potential legitimate risk to the fountain darter population at flows of less than 80 and less than 60 cfs in the Comal and San Marcos Rivers, respectively.
The graphs below depict the monthly low flows at Comal and San Marcos Springs versus the flow required to maintain prime fountain darter habitat. Historical average flow at Comal Springs is ~300 cfs and historical average flow at San Marcos Springs is ~175 cfs. Note the effect of the drought of record (1947-1957) on springflow and fountain darter habitat.
Draft Edwards Aquifer Habitat Conservation Plan
San Marcos and Comal Springs Recovery Plan
USGS Edwards Aquifer Resources
|Comal Springs monthly low flow vs flow requirement for maintenance of fountain darter
prime habitat [Figure: Houston Advanced Research Center using USGS data]. Monthly low flows calculated by using the monthly minimum of average daily flow data from USGS station #08169000.
|San Marcos Springs monthly low flows vs flow requirement for maintenance of fountain darter
prime habitat [Figure: Houston Advanced Research Center using USGS data]. Monthly low flows calculated by using the monthly minimum of average daily flow data from USGS station #08170500.