Oct 262010

The Colorado River flows almost entirely in the USA, though its extensive delta is in Baja California. The USA and Mexico have negotiated usage rights designed to guarantee a minimum flow reaching Mexico and to safeguard the water available for numerous cities in the USA that depend on water from the Colorado, and for the rapidly-growing urban centers of Mexicali, Tijuana, Tecate and Rosarito in Baja California.

Even so, the amount of water reaching Mexico has declined dramatically as a result of the Hoover and Glen Canyon dams and other diversions of Colorado River water in the USA. The Colorado River delta has been almost totally dry for most of the last decade.

A new study led by a NASA scientist, links increased human activity in the USA over the past 150 years to earlier annual snowmelt in the Rocky Mountains and reduced flow rates in the Colorado River.

According to the study:

  • lake sediments reveal that between 5 and 6 times as much dust now falls on the Rocky Mountains as 150 years ago – an increase attributed to soil disturbances caused by agriculture and grazing
  • dust settling on snow makes the surface darker, enabling it to absorb more incoming solar energy (ie. the dust changes the albedo of the surface)
  • this increased absorption of solar energy causes earlier snowmelt and more evaporation to enter the atmosphere – peak spring runoff now comes three weeks earlier than 150 years ago
  • this earlier snowmelt exposes vegetation, causing plants to lose more water to the atmosphere than previously – this loss is estimated at almost 1 billion cubic meters (35 billion cubic feet) of water each year
  • this loss of water causes river discharges to be lower than previously; the annual runoff averages less than 95% of the levels found prior to extensive human settlement
  • earlier snowmelt leads to earlier peak runoff into rivers. This complicates water management, especially during the summer.

How much is 1 billion cubic meters of water?

  • sufficient to meet the demands of the entire city of Los Angeles for 18 months
  • more than 50% of the amount of water guaranteed to enter Mexico each year via the Colorado by a 1944 USA-Mexico treaty
  • about 14% of the current volume of Lake Chapala, Mexico’s largest natural lake

How can the situation be reversed/ameliorated?

One successful strategy, mentioned in the article, has been the Taylor Grazing Act (1934). This has improved conditions on public grazing lands, decreasing the amount of dustfall in the Rockies by about 25%. Other strategies are needed since climate change may well exacerbate the effects the additional dust has had on river flows.

Related posts:

Rivers, reservoirs and water-related issues are discussed in chapters 6 and 7 of Geo-Mexico: the geography and dynamics of modern Mexico. Ask your library to buy a copy of this handy reference guide to all aspects of Mexico’s geography today! Better yet, order your own copy…


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