More than two years after a precedent-setting move by the federal government cleared a path around environmental laws and legal challenges, the construction of a stretch of border fence across a deep canyon known as Smuggler's Gulch is set to begin next month. The project will require cutting earth from surrounding hills and filling in the canyon with more than 2 million cubic yards of dirt, an operation so large that critics fear disastrous environmental consequences. According to the latest plans from the U.S. Customs and Border Protection, the project will consist of constructing an earthen berm across the canyon...