33 U.S.C. § 585
Reserve component training at water resources development projects
In carrying out military training activities or otherwise fulfilling military training requirements, units or members of a reserve component of the Armed Forces may perform services and furnish supplies in support of a water resources development project or program of the Corps of Engineers without reimbursement.
This section shall not apply to any member of a reserve component of the Armed Forces who is employed by the Corps of Engineers on a full-time basis.