The United States and Mexico—Sharing the Flow?
The United States and Mexico—Sharing the Flow?
Most people in the U.S. probably don't think much about what water Mexico takes from the Colorado or the Rio Grande Rivers, which originate in the U.S. and flow along the U.S.-Mexico Border for some distance, and, in the case of the Colorado River, flow through Mexico to the sea (Fig. 4). Like the examples above (the Nile and Ganges Rivers), there are treaties that provide for sharing of the flow of these two North American rivers between the U.S. and Mexico. You have already read about the Colorado River Compact of 1922 (Module 8.1). In effect, the flow of the Colorado River is, on average, significantly less than the total amount apportioned to individual states in the watershed. The 1922 Colorado River Compact was vague about the amount of water that was to be supplied to Mexico. This was rectified in a 1944 Treaty that provided for 1.5 million acre-feet of water per year to flow to Mexico (about 10% of the average Colorado River flow).
The Colorado
The Colorado
Until the present, the U.S. has bypassed the requisite amount of water from the Colorado to Mexico every year, regardless of the total flow of the Colorado. Because of recent severe droughts in the southwestern U.S., however, a 5-year Agreement (Minute 319), signed in 2012, was brokered that allows the U.S. to reduce the amount of water shared with Mexico when Colorado River flow was much lower than normal. In that agreement, Mexico, which has little Colorado River storage capacity (only Morelos Dam and reservoir), will be allowed to store some of its surplus water in Lake Mead, behind Hoover Dam. In addition, the U.S. will help finance improvements to Mexico's water infrastructure ($21 million), which was badly damaged by an earthquake in 2010, and pledged to "reconnect" the Colorado River with the Gulf of California. The U.S. and Mexico committed to each supply 5,000 acre-feet of water a year to the delta. Accoring to the LA Times [1] in 2014, because of the Agreement, a "pulse flow" event occurred whereby, in March, nearly 105,000 acre-ft of water was released from Morelos Dam (Mexico) to restore (at least briefly) flow to the Colorado River Delta in the Gulf of California (Fig. 5). The intent was to begin to restore riparian ecosystems along the Colorado River in Mexico. However, in 2022, Mexico’s share of water was cut by 5% and nearly 7% in 2023. These pulse flows continue every spring and vegetation is beginning to thrive.
Of interest is the fact that there was more to the water allocation Treaty of 1944. In that Treaty, both the amount and quality of water allocated to Mexico were stipulated. The TDS of waters released to Mexico had to be below 1000 ppm. Alas, the salinity of Colorado River water behind Morelos Dam was typically greater than that because of evaporation and irrigation return flow (leached salt from arid-region agricultural soils in southern Arizona) So, the U.S. built desalinization plant in 1975 near Yuma to treat water to reduce TDS to maintain the agreed-upon values behind Morelos Dam in Mexico (actually partly in Arizona) according to stipulations made by the International Boundary and Waters Commission (IBWC) in 1973. However, the plant was never put into operation because of a period of high flow and lower salinity on the Colorado River. During the period 1973 to 2006, all the return flow from agricultural operations in the Yuma region (TDS=2500ppm; avg. nearly 125,000 acre-ft) was released to Mexico and flowed to the Ciénega de Santa Clara wetlands in Mexico (Fig. 6). This flow substantially contributed to the significant ecological development of the Ciénega as a wetland. In 2011, however, the desalination plant was tested for a year, and the flow of water to the Ciénega de Santa Clara was substantially reduced, with an associated increase in total dissolved solids (TDS>3200ppm). It remains to be seen whether the relatively low treatment volumes (30,000 acre-ft/y) of the desalination plant as configured are a benefit in light of concerns over the fate of the renewed Ciénega de Santa Clara ecosystem (over 30 yrs of runoff) and its endangered species (Yuma Clapper Rail and desert pupfish).
The Rio Grande
The Rio Grande
The Rio Grande River flows along the U.S. (Texas)-Mexico border for nearly 1,248 miles (2,008 km) including meanders. Although snowmelt from the San Juan Mountains of Colorado (Fig. 7) is a major source of water for the Rio Grande, runoff from northern Mexico also contributes to its flow. As in all arid to semiarid regions, the waters of the Rio Grande River are highly sought after and overallocated. And, as in the case of the Colorado River, the water division between the U.S. and Mexico is regulated by Treaty (see below).
Rio Grande River water is in demand because of the intense agriculture in New Mexico-Texas (Fig. 8) as well as in northern Mexico. This water supply deficit has been exacerbated by prolonged drought in the southwest. Figure 9 is a long-term record of flow of the Rio Grande River (at Otowi Bridge) reconstructed by tree ring records calibrated to more modern flows (see TreeFlow [2]). Note the frequent cycles of surfeit and drought, and the most recent steadily decreasing flow trend beginning about 1990.
In all, there are 15 dams on the Rio Grande River, many of them in New Mexico. Flows are significant until Elephant Butte Reservoir in New Mexico. El Paso, TX is 125 river miles downstream of Elephant Butte Reservoir and just upstream of the American Dam. Releases from Elephant Butte Reservoir control streamflow to El Paso. At American Dam, much of the flow in the Rio Grande is diverted for irrigation and municipal uses in Texas and Mexico. From the American Dam, the Rio Grande has little or no flow until joined by the Río Conchos about 300 miles downriver, which originates in the Sierra Madre Occidental in Mexico (see below). The Pecos River, a major U.S. tributary, joins the Rio Grande another 300 miles or so downriver near Langtry, TX (Fig. 7); the Pecos flow is also controlled by a dam upstream from its confluence with the Rio Grande. Further downriver, the flows in the Rio Grande River decrease significantly as the result of withdrawal for agricultural and municipal use in southwest Texas as well as the relatively low influx of water from tributaries. In Some years, the Rio Grande flow does not even make it to the sea near Brownsville, TX.
About 75% of water withdrawals from the Rio Grande River are in support of agriculture. Population growth has also been a factor, particularly in Mexico, where the population has nearly doubled since 2005, and nearly 6 million people depend on the Rio Grande River and related groundwater basins for drinking water. The U.S.-Mexico Treaty of 1848 established the international boundary, modified slightly by later "Conventions." The Treaty of 1944 between the two countries partitioned water from the Rio Grande River along the Texas-Mexico Border (as well as stipulating Colorado River flows to Mexico, see above), modified slightly by a 1970 Treaty, and authorized both countries to construct, operate, and maintain dams on the main channel of the Rio Grande. The International Boundary and Water Commission (IBWC) was assigned the task of dealing with water quality issues along the international border.
According to the Treaty of 1944, the U.S. is entitled to about one-third of the flow of the Rio Conchos from Mexico, which amounts to about 350,000 acre-ft/y on average. By the Treaty, Mexico is obligated to release 1.5 million acre-ft over a five-year period. During times of drought, it is difficult to meet the annual expectation, and, typically, Mexico releases more water in good rainfall-runoff years and conserves during drought periods, although at one point Mexico did not meet their obligation for nearly ten years. This pattern makes it difficult for agriculture in southwest Texas because water resources cannot be adequately predicted, and, in 2013, a controversy erupted between Texas and Mexico because of long-term drought that peaked in 2011 (Texas Observer: On the Border, a Struggle over Water [5]) --another example of the difficulties of sharing even major rivers.
Systems Thinking: Controls on Rio Grande River Flow to the Sea
Systems Thinking: Controls on Rio Grande River Flow to the Sea
Consider the water supplied by the Rio Grande River. In many years there is a trickle of water, or less, that reaches the sea. Why? Obviously, the water inputs are less than or equal to the outputs.
The Rio GrandActivate Your Learning
Construct a simple system diagram that represents the interplay between the "forces" that influence the flow of the Rio Grande River. Think about aspects of climate, population growth, and water demand as they influence Rio Grande River flow to the sea. Treat the Rio Grande flow/storage as a "reservoir" (total annual water availability in that system) and consider the most important inputs and outputs and the factors that drive them (refer to Module 1 for a background on systems thinking and systems diagrams). When you complete your system diagram on paper, click on the link to see what we expected you to include.
Click for answer.
Once you have studied the diagram, construct the "equations" for Annual Runoff and Annual Water Demand. Do the units match? How do you think this system would behave if the changes in inputs and outputs were large on a yearly basis?
Click for answer.