Cooperative Fish and Wildlife Research Units Program:
Education, Research and Technical Assistance for Managing Our Natural Resources

The flow regimes and associate ecosystem function of many rivers in western North America have been significantly altered from their historic state due to drought, anthropogenic alteration and urban use. These changes have had even more critical impacts on rivers in the arid regions of the Southwest; the Rio Grande in particular is characterized as having a substantially altered flow regime since the late 1800s. In addition, the construction of dams, human use and consumption, and declining yearly precipitation has significantly decreased flows, degraded water quality, and changed the frequency and magnitude of large flood events throughout the river. This reduction in stream flow in conjunction with the invasion of non-native vegetation has dramatically altered the physical template of the river and have led to likely changes in ecosystem function, food web structure, and a decrease in the habitat complexity of the river. Consequently, many native fishes, including the federally endangered (ESA) Rio Grande silvery minnow, have been extirpated from this region. However, while the geomorphic history is relatively well documented, potential alteration in biological ecosystem function and changes to food web structure, as a result of this altered physical template, are not well understood. The three primary and linked goals of our study are to: 1) better understand if there have been fundamental changes to ecosystem function and food web structure relative to the historical state, 2) determine the most critical limiting factors to native fish maintenance and recovery, and 3) link 1&2 to changes (past and future) to the hydrologic regime. We hypothesize: (1) altered hydrology and turbidity has created a mismatch between growth and food resources, (2) altered hydrology and an increase in non-native vegetation has altered fish habitat and allochthonous inputs, and/or (3) reduced magnitude flood events stimulates accumulation and decomposition of organic matter, leading to more frequent hypoxia events. We will test these hypotheses using a multi-faceted approach including 1) Examining the fish community and structure 2) comparing current algal samples with any historic data available and tracking available algal food resources over time 3) gathering allochthonous inputs (e.g., leaf litter and terrestrial invertebrates) and compare to fish isotope composition, and 4) examine discharge patterns around known hypoxic events. The results of this study have important international implications, as we move towards developing a new treaty with Mexico aimed at increasing the amount of water that stays in and moves through the Rio Grande River.