Historic reconstruction of the ecology of the Rio Grande/Río Bravo channel and floodplain in the Chihuahuan Desert

The Chihuahuan Desert straddles an international border and encompasses some 630,000 square kilometers of the southwestern United States and north-central Mexico. As the only major through-flowing stream in this desert, the Rio Grande represents a critical component of the region’s biodiversity. The desert Rio Grande traverses about 1270 kilometers of the Chihuahuan Desert between the mouths of the Rio Puerco in New Mexico and the Devil’s River in Texas. The Río Conchos, the Pecos River and the Devils River are the only major perennial tributaries that reach the desert Rio Grande, and all meet the river in the lower half of its Chihuahuan Desert course. Thus, the waters of the desert Rio Grande have always been a precious resource for populations—both human and non-human—living in this arid region. As a result of centuries of intensive human settlement, agriculture, and industry within the river’s watershed, the natural environment has been impacted immensely. For the second time in less than a decade, American Rivers has placed the Rio Grande among the 10 most endangered rivers of the United States, and a World Wildlife Fund-sponsored symposium identified the river as a priority site for conservation and restoration. In order to assist policy-makers, land-managers, and property owners with restoration efforts along the Rio Grande through the Chihuahuan Desert, the primary goal of this report is to compile information about what natural environments used to exist along and within the river. In terms of environmental change, the late-nineteenth and early twentieth centuries were among the most significant periods in the river’s history: agricultural development and dam construction impacted river flows and floodplain habitats directly and land-use changes elsewhere in the river’s watershed impacted run-off and sedimentation patterns. For centuries, the waters of the Rio Grande have been used for agricultural pursuits. In the thirteenth and fourteenth centuries, agricultural societies existed along several parts of the desert Rio Grande; they appear to have engaged in flood-water farming that utilized overbank flooding or diverted sheetflow after rainfall for irrigation instead of using ditch irrigation from the main channel of the Rio Grande. Ditch irrigation became more widespread following the establishment of missions and settlements in the sixteenth and seventeenth centuries; although along some portions of the desert Rio Grande (such as in the Mesilla and Presidio valleys) large-scale ditch irrigation did not occur prior to the nineteenth century. Late in the nineteenth century, extensive agricultural development in southern Colorado and northern New Mexico usurped most of the river’s water, resulting in flow disruptions downstream and precipitating the construction of Elephant Butte Reservoir (completed in 1916) to store water for agriculture downstream. Under the natural flow regimes that existed prior to the dam’s construction, the environment along the desert Rio Grande was dynamic and patchy, with organisms adapted to spatial and temporal heterogeneity. This heterogeneity was driven by the river’s flows, which exhibited within- and between-year variability. Above the mouth of the Río Conchos, flows peaked in late spring or early summer when snow melted in the upper reaches of the river’s watershed; below the Río Conchos, peak flows tended to occur in late summer or fall, resulting from summer precipitation in Mexico. In many years, overbank flooding occurred during periods of peak flow. The river exhibited a meandering course through the valley reaches, and because meanders naturally migrate along a channel, small-scale changes in channel location and shape were common. In addition, large-scale channel migration also occurred, sometimes from one side of the river valley to the other. However, in at least one reach of the desert Rio Grande, such large-scale channel shifts appear to have resulted from returns to previously established river channels instead of the creation of a new channel. Meander cut-offs and channel migration also helped to create and maintain numerous backwaters including oxbows, marshes, and sloughs. These non-river aquatic habitats may have been important refugia for aquatic organisms when portions of the river went dry; prior to extensive agricultural development, flow disruptions along limited stretches of the river occurred quite infrequently, and seem to have only occurred during extended droughts