Is congressional approval required to exceed the 320 or 640 acre average?

Yes, per Sections 3(b) and 15(1) for rivers designated by Congress under Section 3(a), but not for rivers designated by the Secretary of the Interior under Section 2(a)(ii). For 2(a)(ii) rivers, states and/or local government set the boundaries, if any, for rivers in their systems; these rivers are not subject to Section 3(b) or the 320/640-acre limitation.

Interagency Wild & Scenic Rivers Council