Land does not have to be wet to be considered a waterway or wetland by the federal government; the definitions include even dry washes and certain vegetated lowlands.
The Law
Section 404 of the Clean Water Act, enforced by the Army Corps of Engineers, governs activities affecting waterways and wetlands.
Risks
The Corps can mandate complete restoration of illegally filled property to its original condition.
Recommendation
Conduct thorough due diligence before acquiring land that may be classified as waterways or wetlands.