The original portions of the fence were built (in 1994) as a part of Operation Gatekeeper & intended to force immigrants away from the urban areas & into more dangerous desert & mountainous terrain. The greater physical risks (read: death) were supposed to act as a deterrent, ignoring the economic realities fueling the migrant flow. At the time, there was no push to physically fence off all 2000 miles of the border, which yes, has serious economic costs & geographical challenges.

According to the ACLU in 1998:

The Border Patrol's blueprint for Gatekeeper itself says that the environment to which the migrant traffic was to be channeled — places where "the days are blazing hot and the nights freezing cold" — posed a "mortal danger" to illegal entrants. Three hundred and sixty migrants, including women and children, have died at the California border since the start of Operation Gatekeeper in 1994, most of them from exposure in the Tecate mountains and dehydration in the Imperial desert. Increasingly, migrants are drowning in the swift currents of the All American canal that cuts across Calexico and Mexicali, in attempts to avoid long treks across the desert.

The Border Patrol acknowledged as much in 1995:

Through Operation Gatekeeper the border below San Diego became more difficult to cross than at any time before in Border Patrol history. By concentrating resources in the more congested western area of the border, the Border Patrol deterred illegal entry there and drove much of the traffic toward the more isolated eastern area near Otay Mesa where border crossers are more easily detected and caught.

According to the California Rural Legal Assitance Foundation:                                                              

Gatekeeper was developed with help from the U.S. Department of Defense's Center for Low Intensity Conflicts. The strategic plan recognizes that "illegal entrants crossing through remote, uninhabited expanses...can find themselves in mortal danger" and assumed that the "influx will adjust to Border Patrol changing tactics." (12)  Gatekeeper has been implemented in three phases. Each raised the risks of migrants dying.

The objective of Phase I, launched in October, 1994, was to seal the westernmost 14 miles of the border. As a result, migrants began using more desolate and dangerous routes (mainly the Otay Mountains), and began dying of exposure and exertion. In a report on Gatekeeper by the U.S. Justice Department's Inspector General, the Otay Mountains are described as being "extremely rugged, and includ[ing] steep, often precipitous, canyon walls and hills reaching 4,000 feet."(13)

Phase II began in the spring of 1996. Gatekeeper was extended to the entire San Diego sector. This effort to reroute the migrant foot traffic was stepped up in response to an outcry by East San Diego County residents about the massive illegal border crossings which had materialized there. As the next step in this phase, the migrants were, in the words of Alan Bersin, the Clinton Administration's border czar, "forced to enter into a much more inhospitable terrain," i.e., the Tecate Mountains. The peaks rise over 6,000 feet and the snow can fall at altitudes as low as 800 feet. From mid-October to mid-April, there is a greater than 50% probability of below-freezing temperatures. The winter after Phase II was introduced, 16 migrants froze to death in just one month.

Phase III began in fall, 1997. As the Immigration Commissioner explained, "the next real step in moving east gets you into the desert and [like the mountains, it is] very formidable territory."(14)  The shortest route that migrants hike in the Imperial desert is ten miles. Of course, migrants going the desert route will have already been hiking through the Baja California side of the desert when they arrive at the border. In the summer of 1997, a total of 27 migrants died of dehydration.  The figure last year was 68.  Now that Gatekeeper has been extended to Arizona, the desert deaths have soared there, too.  The effects are, of course, also being felt in Texas.  On the eve of a presidential summit In Guanajuato this February, the University of Houston's Center for Immigration Research said its researchers had found a "clear correlation and pattern" between the new enforcement strategy and the ever-mounting death toll at the border.

Notwithstanding, the Bush Administration did not heed a recommendation by the Carnegie Endowment that the U.S. government "freeze" the building of additional fences, etc., with an eye towards "recasting the U.S.-Mexico migration relationship."(15)  Instead of reviewing the existing policy, President Bush proposed an additional $100 million for "border management."  Up to now [2001-A], Operations Gatekeeper, Safeguard and Rio Grande are estimated to have cost between six to nine billion dollars.   link