This photo, released by U.S. Customs and Border Protection, shows why U.S. border barriers won't stop the families from reaching U.S. soil and exercising their legal right to seek asylum.

The Rio Grande spans about two-thirds of the U.S.-Mexico border, and nearly all of the new border wall the president wants to build is along the river. Border walls are linear and need a solid base, but the river banks are unstable and follow a circuitous course. So the U.S. Army Corps of Engineers is building new barriers along the river levee. The walls create a strip of U.S. territory between the river and the fencing.

The "no-man's land" on the U.S. side of the river is what you see here.

Migrants are shown lined up outside the wall in El Paso on March 7. This group of 127 Guatemalan migrants had waded through the river to turn themselves in to U.S. agents, the first step in initiating the asylum process. They crossed the border illegally, but they have the legal right to seek asylum because they reached U.S. soil. Border Patrol agents must take them into custody and begin processing their claims.

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The barrier here is about 10 years old. Though it is not the steel bollard design that Customs and Border Protection is adding along the river elsewhere, this is considered a relatively modern span of border fencing.

Like the new sections the government is building, there is a gate here to give U.S. agents and others access to both sides of the wall -- all on U.S. land.

U.S. Border Patrol agents open a gate in the fence as they check migrants' documents and bring them through. Vans and buses are arriving to take the migrants to a nearby border patrol station for additional processing. They will be transferred to U.S. Immigration and Customs Enforcement (ICE), which in most cases releases families from custody after a few days.

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They then wait inside the United States to see an immigration judge -- which could take several months or longer.

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