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.