The two Big Bend NFCAs represent two contiguous, but very diverse segments (above and below Mariscal Canyon) with distinct differences in base flow, sediment movement and water quality. These differences are primarily due to reduced base flow and water quality in the upper segment and considerable spring flow inputs and improved water quality in the lower segment (Bennett et al. 2014). As a result, the lower segment remains largely intact and still supports a high diversity of native aquatic species (Bennett and Urbanczyk 2014). However, more than half of the 42 native species in the two Big Bend NFCAs are imperiled and of those, almost 50% are already extirpated or extinct. In addition, 29% of the species currently in these two NFCAs are non-native.

Bennett et al. (2014) noted the numerous threats to the aquatic natural resources of the river corridor that have been documented, including channel narrowing and sediment accumulation (Dean and Schmidt 2011; Dean et al. 2011), deteriorating aquatic habitat, invasive and exotic species (Everitt 1998), increasing mercury concentrations in fish (Heard et al. 2012), continued water-quality deterioration (Sandoval-Solis et al. 2010; Bennett et al. 2012), groundwater extraction (Donnelly 2007) and climate change (Ingol-Blanco 2011). The combination of regional water management and invasive, non-native riparian species has changed stream flow, sediment dynamics and riparian vegetation cover (Everitt 1998; Schmidt et al. 2003; Dean and Schmidt 2011). The once wide and shallow channel is now filled with sediment and has become narrow and deep. Non-native riparian plants (Giant Reed and Saltcedar) affect channel sediment retention, aquatic habitat and riverside communities by covering up and eliminating backwaters and side channels, diminishing channel conveyance capacity and increasing flooding frequency (Dean and Schmidt 2011; Garrett and Edwards 2014). Non-native feral livestock are also negatively impacting natural resources. Feral pigs Sus scrofa, burros Equus africanus asinus, horses Equus ferus caballus, and cattle Bos spp. occur throughout the river corridor.

In addition to the focal fish species, other aquatic species of concern include Salina Mucket Potamilus metnecktayi, Tampico Pearlymussel Cyrtonaias tampicoensis, Texas Hornshell Popenaias popeii, Big Bend Rough-footed Mud Turtle Kinosternon hirtipes murrayi, Big Bend Slider Trachemys gaigeae, American Beaver Castor canadensis, and Pecos River Muskrat Ondrata zibethicus ripensis. Areas being actively managed for conservation include Big Bend National Park, Rio Grande Wild and Scenic River, Big Bend Ranch State Park, Black Gap Wildlife Management Area, Chinati Mountains State Natural Area and Alamito Creek watershed.

Sources

Bennett, J., B. Brauch, and K. Urbanczyk. 2012. Estimating ground water contribution from the Edwards-Trinity Plateau Aquifer to the Big Bend reach of the Rio Grande, Texas. Geological Society of America Abstracts with Programs 44:2.

Bennett, J., M. Briggs, and S. Sandoval Soliz. 2014. Rio Grande – Río Bravo river corridor. Pages 21-23 in M. D. Wesson, C. Hallmich, J. Bennett, C. Sifuentes Lugo, A. Garcia, A. M. Roberson, J. Karges, and G. P. Garrett, editors. Conservation Assessment for the Big Bend-Río Bravo Region: A Binational Collaborative Approach to Conservation. Montreal QC: Commission for Environmental Cooperation.

Bennett, J., and K. Urbanczyk. 2014. Springs of the Lower Canyons. Pages 23-24 in M. D. Wesson, C. Hallmich, J. Bennett, C. Sifuentes Lugo, A. Garcia, A. M. Roberson, J. Karges, and G. P. Garrett, editors. Conservation Assessment for the Big Bend-Río Bravo Region: A Binational Collaborative Approach to Conservation. Montreal QC: Commission for Environmental Cooperation.

Dean, D. J., and J. C. Schmidt. 2011. The role of feedback mechanisms in historic channel changes of the lower Rio Grande in the Big Bend region. Geomorphology 126:333–349.

Dean, D. J., M. L. Scott, P. B. Shafroth, and J. C. Schmidt. 2011. Stratigraphic, sedimentologic, and dendrogeomorphic analyses of rapid floodplain formation along the Rio Grande in Big Bend National Park, Texas. Geological Society of America Bulletin 123:1908–1925.

Donnelly, A. C. A. 2007. Groundwater availability run. Austin (TX); Texas Water Development Board Report No. 06-16.

Everitt, B. L. 1998. Chronology of the spread of tamarisk in the central Rio Grande. Wetlands 18:658–668.

Garrett, G. P., and R. J. Edwards. 2014. Changes in fish populations in the Lower Canyons of the Rio Grande. Pages 396–408 in Proceedings of the sixth symposium on the natural resources of the Chihuahuan Desert region (C.A. Hoyt and J. Karges, editors). Chihuahuan Desert Research Institute, Fort Davis, TX. http://hdl.handle.net/2152/62996

Heard, T. C., J. S. Perkin, and T. H. Bonner. 2012. Intra-annual variation in fish communities and habitat associations in a Chihuahuan Desert reach of the Rio Grande/Río Bravo del Norte. Western North American Naturalist 72:1-15.

Ingol-Blanco, E. 2011. Modeling climate change impacts on hydrology and water resources: Case study Río Conchos basin. Ph.D. Dissertation. University of Texas at Austin, Austin, Texas. www.crwr.utexas.edu/reports/2011/rpt11-3.shtml.

Sandoval-Solis, S., B. Reith, and D. C. McKinney. 2010. Hydrologic analysis before and after reservoir alteration at the Big Bend reach, Rio Grande/Río Bravo. Center for Research in Water Resources Online Report 10-06, University of Texas at Austin.

Schmidt, J. C., B. L. Everitt, and G. A. Richard. 2003. Hydrology and geomorphology of the Rio Grande and implications for river rehabilitation. Pages 25-45 in G. P. Garrett and N. L. Allan, editors. Aquatic Fauna of the Northern Chihuahuan Desert. Museum of Texas Tech University, Special Publications 46.