UC Davis

The Rio Grande-Bravo (RGB) Basin, shared by the United States and Mexico, is a water-scarce basin shaped by climatic, topographic, and ecosystemic contrasts. As part of the natural flow regime, floods support habitat creation by resetting the channel, regulating water quality, and controlling invasive species. However, since 1870 the region has faced considerable socioeconomic growth, which has heavily altered the natural hydrology, resulting in drastic degradation of river health.This research analyzes natural and regulated flood events in the Southern branch of the RGB Basin from 1900 to 2010 at six control points. Flood timing was described with central metrics and measures of spread, which were also used to compare differences between natural and regulated floods. Flood magnitude and frequency were analyzed using flood frequency analysis and magnitude prediction of the 2-, 5-, and 10-year floods. To compare magnitude and frequency of natural and regulated floods, two indexes were developed. The main findings are: (1) natural flood timing reflects the climatic drivers of two main headwaters: flood events during the snowmelt season in the northern branch above the cities Presidio and Ojinaga, and during the monsoon and tropical storm season in the southern branch; (2) in the regulated regime, floods occur earlier in the year, most notably at the basin outlet; (3) natural median floods range from 720 m3/s at the headwaters to 2,250 m3/s at the basin outlet; (4) regulated median flood magnitudes decrease 70% in four stations, with larger floods occurring in stations located upstream the river outlet, opposite to natural hydrology; (5) the 10-year natural flood estimates range from 2,055 m3/s to 6,083 m3/s; (6) natural flood estimates show a pattern of dry and wet periods; and (7) frequency of large floods decrease at all stations, most noticeably after 1960. Changes in flood timing, magnitude, and frequency have significant implications for physical, biogeochemical, and biological ecosystem functions. The life stages of several species are timed with floods, like fish spawning and migration. Physical and biogeochemical functions triggered by predictable, large floods support habitat creation by regulating water quality, preventing channel narrowing, depositing nutrients in the floodplain, and clearing out invasive species, amongst many other functions. Water management alternatives to reproduce functional flows, especially floods, have already been proposed by several authors, showing it is possible to restore ecological functions and satisfy human water management goals.