Behind them is a borrow pit where dirt was removed to support the nearby levees.
From the stltoday.com
The top soil and vegetation is removed from the borrow pit and stockpiled for subsequent rehabilitation of the extraction area.
From the en.wikipedia.org
For now, permission to create a borrow pit is granted through a site permit, which is usually given within a week to month of the contractor's application.
From the toledoblade.com
First, the facility must be turned from a mile-long borrow pit into a destination point with a 1.25-mile long racecourse that meets international rowing standards.