I found the NPS designations.
Current designation;
National rivers and wild and scenic riverways preserve freeflowing streams and their immediate environment with at least one outstandingly remarkable natural, cultural, or recreational value. They must flow naturally without major alteration of the waterway by dams, diversion, or otherwise alteration. Besides protecting and enhancing rivers, these areas provide opportunities for outdoor activities like hiking, canoeing, and hunting.
National scenic trails are generally long distance footpaths winding through areas of natural beauty.
Possible Preserve designation;
In 1974, Big Cypress and Big Thicket were authorized as the first national preserves. This category is established primarily for the protection of certain resources. Activities like hunting and fishing or the extraction of minerals and fuels may be permitted if they do not jeopardize the natural values. National reserves are similar to the preserves. Management may be transferred to local or state authorities. The first reserve, City of Rocks, was established in 1988.
Pretty ambiguous. Obviously reserves can be transferred, but does that include the "similar" preserves? I searched the CFR several ways with nothing appropriate popping up, so it appears to be discretionary policy, leaving ample wiggle room for whomever is in charge at the moment.