Whenever application is filed for a project hereunder involving navigable waters of the United States, and the commission shall find upon investigation that the needs of navigation require the construction of a lock or locks or other navigation structures, and that such structures cannot, consistent with a reasonable investment cost to the applicant, be provided in the manner specified in subsection (a) of section 804 of this title, the commission may grant the application with the provision to be expressed in the license that the licensee will install the necessary navigation structures if the Government fails to make provision therefor within a time to be fixed in the license and cause a report upon such project to be prepared, with estimates