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Experts say finishing US-Mexico border wall as GOP presidential hopefuls want may be daunting

  • In this Sept. 16, 2015, photo, artist Mark Clark, who's studio is located near the border fence, climbs to his rooftop to watch for activity along the border, in Brownsvill, Texas. The staggered fence or “wall,” costing $6.5 million per mile, runs along  54 miles of Texas’ 1,254-mile border with Mexico.  The staggered fence or “wall,” costing $6.5 million per mile, runs along  54 miles of Texas’ 1,254-mile border with Mexico. While 1,254 miles of the U.S.-Mexico border is in Texas, the state has only some 100 miles of wall.  (AP Photo/Eric Gay)

    In this Sept. 16, 2015, photo, artist Mark Clark, who's studio is located near the border fence, climbs to his rooftop to watch for activity along the border, in Brownsvill, Texas. The staggered fence or “wall,” costing $6.5 million per mile, runs along 54 miles of Texas’ 1,254-mile border with Mexico. The staggered fence or “wall,” costing $6.5 million per mile, runs along 54 miles of Texas’ 1,254-mile border with Mexico. While 1,254 miles of the U.S.-Mexico border is in Texas, the state has only some 100 miles of wall. (AP Photo/Eric Gay)  (The Associated Press)

  • In this Thursday, Sept. 17, 2015, photo, Pamela Taylor, whose home is on the south side of the border fence, stands near a sign she erected, in Brownsville, Texas. The staggered fence or “wall,” costing $6.5 million per mile, runs along  54 miles of Texas’ 1,254-mile border with Mexico. She still leaves coolers of water for thirsty migrants, though she wishes more of them would come to the United States legally, the way she emigrated from England.  (AP Photo/Eric Gay)

    In this Thursday, Sept. 17, 2015, photo, Pamela Taylor, whose home is on the south side of the border fence, stands near a sign she erected, in Brownsville, Texas. The staggered fence or “wall,” costing $6.5 million per mile, runs along 54 miles of Texas’ 1,254-mile border with Mexico. She still leaves coolers of water for thirsty migrants, though she wishes more of them would come to the United States legally, the way she emigrated from England. (AP Photo/Eric Gay)  (The Associated Press)

  • In this Wednesday, Sept. 16, 2015, photo, farmer Fausto Salinas stands along the border fence, in McAllen, Texas. The staggered fence or “wall,” costing $6.5 million per mile, runs along  54 miles of Texas’ 1,254-mile border with Mexico. (AP Photo/Eric Gay)

    In this Wednesday, Sept. 16, 2015, photo, farmer Fausto Salinas stands along the border fence, in McAllen, Texas. The staggered fence or “wall,” costing $6.5 million per mile, runs along 54 miles of Texas’ 1,254-mile border with Mexico. (AP Photo/Eric Gay)  (The Associated Press)

Republican presidential candidates insist they'll finish building a border wall along the U.S.-Mexico boundary. But what already stands in Texas is a fragmented series of fencing, with large gaps in between.

While 1,254 miles of border occur in Texas, the state has only some 100 miles of wall.

Adding to the Texas wall may be difficult, courtesy of the border's sheer length, the fact that it sits in the center of the snaking Rio Grande, and because international treaties with Mexico prevent either country from constructing within the river's flood plains. And unlike in other southwestern states, most border land in Texas is privately owned.

Finishing the some 1,300 miles of border fencing would also be expensive, costing billions of dollars to complete.