Water Flow Quantities
The Rio Grande, the second longest river in the United States, is no longer a naturally flowing river. Extensive networks of diversions and dams control flows on both the Rio Grande and the Rio Conchos. Neither river is currently managed to provide an in-stream flow needed to sustain riparian habitat. Persistent drought conditions have existed in northern Mexico and southwest Texas for over six years. The drought withered crops and caused severe malnutrition among the Tarahumara Indians in the highlands of Chihuahua. Livestock died or had to be slaughtered early.
Ironically, reservoirs in New Mexico were full to the point of overflowing because of above average snowfall in New Mexico and Colorado. Yet, these waters were unavailable to the downstream Rio Grande community. The Rio Grande Compact Commission, a three state entity representing Colorado, New Mexico, and Texas, manages water flows of the Rio Grande from its headwaters in Colorado to Fort Quitman, Texas, which lies downstream of El Paso. Established in 1938, the Commission manages private water rights, some of which date to the 1800s, and apportions the Rio Grande’s flow, including a share to Mexico at El Paso.
The bi-national International Boundary and Water Commission (IBWC) manages the water in the Rio Grande from Fort Quitman to the Gulf of Mexico. A 1944 treaty between the United States and Mexico allots all of the water entering the Rio Grande from the major tributaries on the United States’ side and at least one-third of the combined annual flow from the six Mexican tributaries to the United States. The Rio Conchos is the largest of the six Mexican tributaries. Although the treaty also defines a minimum flow based upon a five-year average, the treaty does not establish release schedules for the Mexican rivers.
The tributaries of the Rio Grande carry large amounts of sediment to the river during flash floods. Historically, this sediment would later be carried away by the large flows of the Rio Grande, and the river channel and floodplain would be large enough to handle the average high flows. With the dams, the average peak spring runoff decreased to a third or less of the historical peak flows. With the smaller flows, the river cannot clean out the sediment deposited from the tributary streams. As a result, the river now has no defined channel below Fort Quitman. A relatively small flood in 1987 spread out over the entire floodplain, depositing sediment in fields and irrigation ditches, and greatly reducing farming in the area for several years.
The IBWC monitors the 1944 treaty allocations through a system of gauging stations on the Rio Grande and Rio Conchos. Some of these stations have been monitored since 1889. The Johnson Ranch gauging station near Castolon in Big Bend National Park has measured flows since 1936.
Historically, flows passing through the park have varied considerably. The highest daily flow of the Rio Grande recorded above the Rio Conchos confluence near Presidio/Ojinaga was 13,700 cubic feet per second (cfs) on June 14, 1905; it is now frequently dry. Only 23 cfs flowed on the Rio Conchos near Ojinaga on December 13 and 19, 1973, but 52,619 cfs flowed on October 1, 1978. Since 1896, the greatest flood of record on the Rio Conchos had an estimated momentary flow of 162,094 cfs and occurred on September 11, 1904.
Within the Park, the Johnson’s Ranch gauging station recorded several days in 1953, 1955, 1957, and 1958 when the riverbed was dry with zero cfs being measured. The maximum daily flow of 65,332 cfs occurred on October 1, 1978.