When EPA Administrator Michael S. Regan last month announced his agency’s intention to change how it defines “Waters of the United States,” my first thought was: “Here we go again!”

He was short on specifics, but farmers and others should keep close watch on where this may be headed. The Obama administration tried the same thing six years ago, when it expanded the definition with language vague enough arguably to include the grass waterways and temporary ponds on my farm.

When it rains so hard on my farm that the ground can’t absorb the downpour, water flows away through a network of channels that we’ve built to protect our soil from erosion. They look like strips of grass, and most of the time...