Canyons, rivers and shifting sand - plus a multibillion-dollar price tag, more violence and less business - make sealing the border all but impossible It stretches between two seas and slithers across four states. It climbs mountains, traverses canyons and slices through cities. Most of it, including long stretches in Arizona, California and New Mexico, is vast and open, marked by wooden posts linked with sagging barbed wire, cement obelisks or nothing at all. In some cities, 10- to 15-foot-tall fences protect it. For 1,254 miles in Texas, it becomes a plodding, chocolate-colored river called the Rio Grande. The line...