The waterfalls meant to evoke memories of the 9/11 dead stand three stories high in a field at the Brooklyn Navy Yard, spilling down into a pool. Four large American flags poke through the grass to mark off each corner of a World Trade Center tower footprint.

For more than three weeks, builders of the 9/11 memorial have been tinkering with details like water flow with a mock-up of the cascades, part of Michael Arad’s “Reflecting Absence” memorial at Ground Zero.

The architect saw the 30-foot waterfalls — which will empty into huge reflecting pools set above the spots where the towers once stood — tested for the first time yesterday.

“These voids, even though water falls into them . . . they never fill up, they always remain empty, and that was very important to me,” Arad said.