What prompts various agencies to conduct wild and scenic river studies?

Under Section 5(a), Congress directs that a study be conducted on identified river segments (usually within three years).  The designated federal agency conducts a study and subsequently reports its findings through the appropriate Secretary.  As a general rule, where joint agency jurisdictions are involved, the cooperating agencies coordinate their efforts prior to making recommendations or submitting reports.

Under Section 5(d)(1), federal agencies are directed to identify and evaluate potential additions to the National Wild & Scenic Rivers System through agency land or resource planning processes.

Source
Interagency Wild & Scenic Rivers Council