A few weeks ago, Tom Cotton and David Perdue, Republican senators from Arkansas and Georgia, introduced an immigration bill that would cut the number of legal immigrants to this country each year in half, from about a million to about 500,000.

In a press conference, Cotton offered a rationale for his bill. “There’s no denying this generation-long surge in low-skilled immigration has hurt blue-collar wages,” he said. If we can reduce the number of low-skill immigrants coming into the country, that will reduce the pool of labor, put upward pressure on wages and bring more Americans back into the labor force.

It seems like a plausible argument. That is, until you actually get out in the real world.

Cotton and Perdue’s position, which is now the mainstream Republican position, is based on the unconscious supposition that American society is like a lake, with a relatively fixed boundary. If you cut the supply of fish coming from outside, there will be more food for the ones born here.

The problem is that American society is actually more like a river. Sometimes the river is running high, with a lot of volume and flow, with lots of good stuff for everybody, and sometimes it’s running low.