Estimating the Natural Flow Regime of the Rio Grande

The Rio Grande is the fifth longest river in North America and is a critical water supply for millions of people in the U. S. and Mexico. It is essential that both governements understand the magnitude of transformation to the river's flow regime as the countries engage in bi-national discussion about restoring parts of the river's native ecosystem. Previous studies have focused on estimating changes in the statistical properties of floods, but no studies have estimated the magnitude of reduction of the total annual flow nor changes to the annual hydrograph. Significant reductions in flow on the Rio Grande date to the onset of increased irrigation development in the mid-1800s, and human disturbances precede gaging records. Since the late 1800s, the total annual stream flow has been reduced by 98%, and the snowmelt flood has been virtually eliminated. We used a systems analysis approach to analyze available mean daily discharge data throughout the watershed to estimate the natural unregulated flow regime of the northern branch of the Rio Grande upstream from the Rio Conchos. We specifically estimated what would have been the natural hydrograph of the northern branch if there were no diversions of water for irrigation or municipal water supply in each year between 1958 and 2008, based on evaluation of 34 gaging station records. In each year, we reconstructed hydrographs for successive pairs of gages using estimated unaltered flows as upstream inputs, routed flow as if existing impoundments did not exist, and we made corrections for irrigation diversions; we also evaluated historical flood records. We show that the total natural annual flow for the northern branch was more than 50 times greater than the actual measured flow for this 50-year period. The magnitude of the existing diversions has eliminated the ecological cues in which the native ecosystem evolved. This reduction of flow has shifted the river into fine sediment surplus and initiated widespread channel simplification and floodplain accretion. To date, restoration efforts have proceeded in the absence of a clear sense of the magnitude of change in the natural flow regime. The calculation of this natural hydrograph highlights the daunting, and perhaps, impossible challenge of this effort.