(1) “River” means a flowing body of water or estuary or a section, portion, or tributary thereof, including rivers, streams, creeks, runs, kills, rills, and small lakes.

(2) “Free-flowing” as applied to any river or section of a river, means existing or flowing in natural condition without impoundment, diversion, straightening, riprapping, or other modification of the waterway except in segments classified as modified recreational rivers. The existence, however, of low dams, diversion works, and other minor structures at the time any river is proposed for inclusion in the Pennsylvania Scenic Rivers System shall not automatically bar its consideration for such inclusion: Provided, That this shall not be construed to authorize, intend, or encourage future construction of such structures within components of the Pennsylvania Scenic Rivers System.