Rio Poco: the once-mighty Rio Grande has slowed to a trickle. (Currents).

For much of the past two years, crossing the U. S.-Mexico border had never been easier. You just strolled down Boca Chica Beach until you noticed that all of the cars parked on the beach had Mexican license plates. You'd just walked straight into Mexico.

There should have been a river in the way: The mouth of the fourth-longest river in the United States, in fact. But the Rio Grande was gone, sapped completely dry before it could reach the Gulf of Mexico by the thirsty farms and cities of northern Mexico and southern Texas. In its place was a wide sandy beach and a short section of orange nylon fencing erected by U.S. Border Patrol agents trying to stop wayward Americans from wandering right out of the country.

When Spanish explorer Alonso Alvarez discovered the Rio Grande in 1519, the river's mouth was more than 30 feet deep, and he could sail his four oceangoing ships 20 miles up the river. As late as 1907, steamships carried passengers and cargo 100 miles inland and river cities such as Brownsville and Matamoros were thriving ports. But last year, all that was left of the 1,885-mile Rio Grande was a shallow lagoon of algae-green water that stopped a quarter mile short of the ocean surf. The river finally broke through to the sea during an October 2002 storm, but remains a shallow, 100-foot wide creek in the sand.

Over the past 30 years, the Lower Rio Grande Valley has experienced explosive growth in just about everything: population, industry, commerce, tourism and agriculture. It's the center of the North American Free Trade Agreement (NAFTA)-driven border economy, home to the largest concentrations of export-oriented assembly plants in Mexico. It's also home to many of the fastest-growing cities in both countries. The total population of the valley has doubled from 1.1 million to more than 2.2 million since 1970. It's expected to double again by 2030.

But amidst the growth, an important fact was overlooked. The valley, geographically speaking, is one step away from being a desert. Groundwater supplies are poor, brackish and unhealthy. The Rio Grande made the lower valley an oasis, providing virtually the only source of water for people, crops, industry and wildlife. But as the border region has grown, the river has not, and people on both banks have a big problem on their hands.

"It's a semi-arid region and the availability of water has always been a constraint on development," says economist Mitchell Mathis, a senior researcher at the Houston Advanced Research Center in Woodlands, Texas, who directed an international study on the lower Rio Grande basin. "They've run up against a wall in terms of having enough water to satisfy everyone."

The immediate problem is the weather. A decade-long drought has reduced the flow of the Rio Grande's tributaries in Mexico, where farmers, irrigators and industrial parks clamor for ever-scarcer river water. As a result, the enormous international reservoirs on the Texas frontier have fallen to some of their lowest levels since they were constructed four decades ago. The water level in the Amistad and Falcon reservoirs--which provide water to the farms and cities of the lower valley--is only about one-third of the normal levels. Water levels at Falcon are so low that Guerrero, a Mexican town submerged when the reservoir was built, is again high and dry.

By the time it reaches Matamoros, the river level is so low it often falls below the intake pipes that provide water to Mexico's second fastest-growing city. Farmers on the Texas side of the lower valley estimate the area has lost $400 million annually due to the scarcity of irrigation water. "With the reduced [water] allocations, growers are having to plant fewer and fewer acres," says soil scientist Bob Wiedenfeld of Texas A&M University. "People are comparing this to the great drought of the 1950s here in Texas, and in Mexico it's even worse than that."

Meanwhile, the ecosystem is paying the price, endangering two of south Texas' most important industries: tourism and commercial fishing. White shrimp, blue crabs, sea trout and other fish require brackish water to reproduce, and the mouth of the Rio Grande is one of the few places they could find it. "When there's a sand bar, the small shrimp can't get out and the [breeding] shrimp can't get in," says Walter Zimmerman, who owns 23 shrimp trawlers in Port Isabel, Texas. "If you don't have these nursery areas then you won't have fish and shrimp, so why would people come down here?"

South Texas is also on both major migratory bird flyways, attracting tens of thousands of bird watchers every year. But the birds--and the $100 million ecotourism industry--rely on tiny enclaves of surviving wetland and brush forest habitat that are suffering from a lack of water. "We're putting in less water and more pollutants, which creates a big problem for aquatic life," says biologist Salvador Contreras of Monterrey's Universidad Autonoma de Nuevo Leon.

But even if the drought ends, the lower Rio Grande Valley faces a serious water supply challenge. The total population of the region is expected to double to 4.9 million by 2030 as people move from the interior to work in export-oriented industries. "There will be more and more demands for water," Mathis says.

When NAFTA was negotiated, there was attention given to improving sewage treatment in the region, but water use and supply issues were largely ignored. "The public interest community raised concerns about how unparalleled growth would effect an arid region, but they were just shoved under the rug at the NAFTA negotiations," says Mary E. Kelly, director of the Texas Center for Policy Studies in Austin.

Any long-term solution must focus on agriculture, which uses more than 80 percent of the lower valley's water. Mathis' study showed that just a 10 to 20 percent reduction in irrigation demand would allow the region to meet all its water needs in 2030. That shouldn't be hard as huge quantities of water are wasted in open irrigation ditches. New piping, sprinkler and drip systems would save tons of water, but they're very expensive.

Water managers also need to leave more water in the river, and not just for the sake of fishermen and bird watchers. It's essential to keeping water purification costs down. Since the Rio Grande is the main wastewater canal for the region's growing cities, reducing its flow has increased pollution concentrations, requiring increasingly expensive water treatment before it can be reused. The water, Mathis says, may prove more valuable to the region for dilution within its banks than irrigating arid fields beyond them. CONTACT: Houston Advanced Research Center, [phone], www.harc.edu; Water in the West, www.waterinthewest.org/riverbasins/ riogrande/riogrande.htm.