At a cost of over $1 million a mile, fence between southern San Diego county and Tijuana, Mexico, will soon become a nearly impenetrable double fence. The stated goal is to stop undocumented workers, but the real effect may be irrevocable damage to a natural area that has thrived with cross-border cooperation. PNS commentator Rasa Gustaitis is the editor of California Coast & Ocean magazine.

The rusty fence that runs between San Diego County and Tijuana, extending into the Pacific surf as a line of high steel bars, stands for all the ironies and contradictions of United States border relations with Mexico.

Built about 15 years ago with military landing mats, and easily scaled, the old fence is now being reinforced by a stronger and higher second fence, with a road between the two for the Border Patrol. The aim is to stop all foot traffic across the final reach of the Tijuana River.

The Army Corps of Engineers is in charge of the Border Barrier Project, working with the Border Patrol and the California National Guard. The work is mandated under a 1996 bill by San Diego Congressman Duncan Hunter, which provides $12 million for the job, and waives requirements to comply with the Endangered Species Act and the National Environmental Policy Act "to the extent the Attorney General determines necessary to ensure expeditious construction of the barriers and roads."

Supporters hope the fence, which is to stretch 14 miles eastward to Otay Mountain, will reduce crime and allow them to hike and ride horses in border parklands without fear of meeting unwelcome strangers. Opponents are outraged and dismayed. The fence is not only a Berlin Wall, they say, an insult to our neighbor, but an environmental disaster.

If built as now planned, this project will undermine years of patient work by numerous agencies and individuals to protect and restore one of California's major coastal estuaries. It will destroy habitat set aside for rare and endangered species, cut into steep eroding slopes, fill canyons, bury the site of a 4000-year- old village under concrete and cut across Border Field State Park (aka Friendship Park).

And it will put behind bars the monument erected in 1851 on the border line, overlooking the ocean near Mexico's northernmost lighthouse, to commemorate the 1848 Treaty of Guadalupe Hidalgo.

"I am really disturbed that the federal government would undertake a design approach that desecrates the landscape and shows so little sensitivity to the cultural heritage of this unique place," said Jim King of the Coastal Conservancy, a state agency that has worked to improve the Tijuana River Valley's environmental quality for some 25 years.

More than $400 million in public funds have been invested over the last 25 years to improve water resources and habitat in the Valley, King said. (Of that, the bulk went to an international sewage treatment plant.) Prompted by individual citizen initiatives, government agencies brought together scientists, technical experts, educators and others to work across the border on watershed issues.

The 1848 boundary ignores local geography. The upper two-thirds of the Tijuana River drainage basin is in Mexico, so whatever happens there affects what's below. On the Mexico side of the Border Highlands, steep unstable slopes of north-facing canyons are crowded with flimsy unsewered houses. Winter rains bring flash floods, sending sewage and vast loads of sediment down into the estuary. A chronic complaint in San Diego is beach water pollution.

As it flows through Tijuana, the river is squeezed into a concrete ditch and its former floodplain is densely developed. There were plans to continue channeling and urban development north of the border, but conservation advocates fought and in 1982 won the creation of the 2,500-acre Tijuana River National Estuarine Research Reserve, which encompasses most of the valley's tidal and brackish marshlands and some uplands.

Each year thousands of school children come to the reserve to learn about wetlands and the shared watershed. A binational citizens group, "pro esteros," is busy trying to protect estuaries in Baja California and diminish erosion by revegetating canyon sides in Mexico.

All this has required long, patient effort especially as these borderlands are thought of mainly as a no man's zone, where nightly the Border Patrol chases desperate people coming northward in search of work and a better life.

Since 1994, when Operation Gatekeeper more than doubled the Border Patrol's manpower and provided advanced technological equipment, including sensors and high-intensity lighting, the number of people trying to cross here has dropped dramatically. The traffic has shifted inland to the mountains and Arizona.

Evidently unsatisfied with progress in controlling the border, Rep. Hunter introduced legislation for further fortifications and Congress passed it.

Until April 1998, neither local governments on both sides of the border nor any of the 11 agencies that constitute the Management Authority for the National Reserve had seen any designs for what the Army Corps was preparing. When they finally managed to look at some engineering drawings, an uproar arose. If border reinforcement is needed, can't a single strong fence do the job, many asked, without such damage to the landscape and parks?

Since then, at several meetings with public agency and citizen group representatives, the Border Patrol has explained that the proposed fence will allow them to concentrate surveillance in a way that would actually help the habitat because it would cut down on human traffic across the reserve. The Corps has decided to conduct and Environmental Impact study.

California Resources Secretary Mary Nichols is now preparing to send comments from state agencies to the Corps, suggesting that more sensitive approaches to the illegal crossing problem be considered.