Water Quality

Regulation of the Colorado by a series of large dams has substantially increased stream salinity by two processes: the evaporation surface of the reservoirs and irrigation return flows (Pontius, 1997). Evaporative losses from the Colorado River reservoirs are especially high because of the arid climate of the region.

Salinity concentration is generally inversely proportional to flow rate, in that it decreases in periods of high flows and increases during periods of drought or otherwise induced low flows. Salinity levels have had significant domestic and

Colorado River Water Use, 1915-2001

Colorado River Water Use, 1915-2001

Year

Figure 3 Trends in Colorado River use in the Upper and Lower Basins, 1915-2001. (Data from the U.S. Department of the Interior, Bureau of Reclamation.)

Year

Figure 3 Trends in Colorado River use in the Upper and Lower Basins, 1915-2001. (Data from the U.S. Department of the Interior, Bureau of Reclamation.)

international impacts in the Colorado River basin. Because of the above-average precipitation in the Colorado watershed in the last several decades, high runoff and flood control releases have helped keep the river within standards set in the U.S.-Mexico treaty. In addition, Congress has taken a series of actions to control salinity. The salinity of the Colorado River water at its headwaters in the Rockies is about 50 mg of TDS (total dissolved solids) per liter. The stream salinity at the Mexican border doubled from 400 mg of TDS per liter in the early 1900s to 800 mg in the 1950s. About 50% of the salt in the river is from natural sources such as saline springs, erosion of saline geologic formations, and runoff, and the remainder comes from irrigation return flows (37%), reservoir evaporation and phreatophyte use (12%), and municipal and industrial effluent (1%) (Lane, 1998).

The 1944 international water treaty left important problems unresolved regarding the quality of water delivered by the United States to Mexico. The domestic impacts, such as pollution and low flow at source regions, resulted in a 1974 agreement in which the United States would assume costs for desalination of Colorado water before it enters Mexico. The agreement also has implications for water availability for the Colorado River delta during exceptionally dry periods.

In recent years, the stability and sustainability of the treaty apportionments have been challenged by three pressures (see Bennett and Herzog, 2000). The first is the demographic transformation underway in the border region. Since the passage of the North American Free Trade Agreement (NAFTA) in 1994, trade between Mexico and the United States has tripled to $261 billion, and with it the number of goods, vehicles, and services crossing the border has increased dramatically (INE, 2003). The second stress is environmental (habitat) considerations, and the third is drought.

Other water quality issues of recent concern along the Colorado include coliform contamination from inadequate waste treatment, limiting certain recreational activities, and perchlorate contamination that has leached into the water supply from an industrial point source near Las Vegas. Neither is directly related to drought, but they may have drought and water supply related implications.

0 0

Post a comment