Gary Strieker, CNN's global environmental correspondent, traveled down the Rio Grande, or Rio Bravo, to observe nature and people along the river that forms part of the U.S.-Mexico border.

Question: What is the most striking contrast you observed in life along the river?

Strieker: The river forms the boundary between the United States and Mexico for more than 1,000 miles, a long journey that reveals many dramatic contrasts. The most obvious is the stark difference between the river's highly managed course through El Paso and Ciudad Juarez, where most of its flow is diverted into concrete canals for municipal use and irrigation, and its later transformation downstream, below Big Bend National Park, into a wild and churning river twisting through spectacular canyons. It's a striking difference that shows how much impact our civilization has had along most of the river's course.

Other contrasts include stark changes in cultures from one side of the river to the other, especially in rural areas where highly mechanized agriculture on the U.S. side changes abruptly to labor-intensive methods in Mexico.

Q: What is the natural environment like and how does that affect the people who live there?

Strieker: Most of the border region is arid desert with rainfall of less than 8 inches a year. The river is usually the only source of surface water for 100 miles in any direction. That's why, throughout history, people have settled close to the river and relied on its water to sustain themselves, their crops and livestock. But because most of the river's flow has now been diverted for agricultural use, and is often virtually dry during the winter months, it is no longer a reliable source of water for major population centers.

However, in prehistoric times the river fed into huge lakes that seeped deep underground in areas like the El Paso/Ciudad Juarez region, creating vast sub-surface aquifers that still exist long after the lakes disappeared. These aquifers are now the main source of drinking water for many cities and towns along the river's route.

But today even these aquifers are starting to run dry because of skyrocketing population growth and unceasing demands for more water. There's now increasing concern that economic development along the border could be restricted by future water shortages -- a reminder to everyone in the region that the natural environment cannot be ignored.

Q: What is the condition of the river and what is the biggest threat to the health of the river?

Strieker: There are two main threats to the river: Lack of water and pollution.

Obviously a river can't exist without water, but this is in fact what is happening to the Rio Grande/Rio Bravo. Most of the river is impounded in big dams upstream in New Mexico, where much of the water is diverted for cities and farmers.

Economic growth in Mexico is drawing more water from its rivers, and in the years ahead there will be less water from those sources reaching the Rio-Grande/Rio Bravo. All the way to the Gulf of Mexico, cities and farmers are using more water from the river, and its levels are going down. This year there isn't even enough flow to break through a small sandbar at the river's mouth near Brownsville, Texas.

Pollution is another problem that can eventually be solved although it is now very serious. On the Mexican side, pollution is mostly raw sewage from urban areas, pouring directly into the river. Recently Ciudad Juarez became the first Mexican city along the river to open a wastewater treatment plant.

On the U.S. side there's industrial and agricultural pollution, aggravated by increased economic activity on the border. On long stretches of the river through populated areas the water is contaminated by coliform bacteria, chemicals and industrial wastes. This is obviously having a negative impact on aquatic life in the river, and some of this damage may be irreversible.

Q: How do people from the region regard the river?

Strieker: The river is still a stark dividing line between a highly industrialized land of ample opportunity and a developing nation struggling to provide a better standard of living for its people. Everywhere along the river, U.S. Border Patrol agents work day and night to stop the flow of illegal immigrants from Mexico.

From the U.S. side in most populated areas, one can always see dozens of Mexicans on the opposite bank, waiting for a chance to swim across. It's a perpetual cat-and-mouse game, the agents and the illegals watching each other warily. For the hopeful immigrants, the river is a means of transport to a better life.

Because there's not much water and the flow is slow, it's not much of a barrier to them. In some places, like Laredo, Texas, the river actually offers protection to the immigrants; thick stands of cane on the banks provide ideal hiding places for those who swim across. It's almost impossible for the Border Patrol agents to monitor these areas, especially at night.

For miles along the river, plastic bags litter the banks. Immigrants carry their clothes in the bags when they cross, discarding them when they move on into the United States. In some areas like El Paso/Ciudad Juarez, where job opportunities in new Mexican assembly plants have relieved the pressure somewhat, the flood of illegal immigration may be slowing down, but the powerful attraction of the American economy has not changed.

Watch Gary Strieker's video report Saturday, June 9 on CNN.