The government is on track to catch nearly 100,000 illegal immigrants at the border this month, Homeland Security Secretary Kirstjen Nielsen said Monday, renewing the administration’s plea for Congress to do something. That number would be the worst in more than a decade, and it’s more troubling than the worst years because the migrants are children and families who despite being unauthorized, are almost impossible to deport. “The situation at our Southern Border has gone from a crisis, to a national emergency, to a near systemwide meltdown,” Ms. Nielsen said in delivering an update on the state of her department 16 years after it was established. Ms. Nielsen said the border wall is part of the solution, but also said Congress must change the laws to allow for detention and faster deportations of illegal immigrant children and families. According to the latest numbers, 98 percent of those caught at the border in 2017 is still in the U.S. today, underscoring how difficult it is to remove them in the current framework. March’s nearly 100,000 migrants — which includes those encountered at ports of entry demanding admittance without permission, and those nabbed by agents after they jumped the line between the ports — compares to about 76,000 in February. At the beginning of this decade, the rates were less than 30,000 a month. Ms. Nielsen said her department’s challenges go well beyond the border.