Amache National Historic Site Act This bill establishes the Amache National Historic Site in Colorado as a unit of the National Park System to preserve, protect, and interpret for the benefit of present and future generations resources associated with the incarceration of civilians of Japanese ancestry during World War II at Amache, also known as the Granada Relocation Center, and the military service of incarcerees at the Granada Relocation Center. The National Historic Site shall not be established until the date on which the Department of the Interior determines that a sufficient quantity of land or interests in land has been acquired to constitute a manageable park unit. After Interior makes such a determination, it shall publish in the Federal Register notice of the establishment of the historic site. Interior may acquire only by donation any land or interests in land located within the boundary of the Camp Amache National Historic Landmark. Any acquired lands or interests in land shall be included within the boundary of the historic site. Interior must prepare a general management plan for the historic site. Interior may enter into agreements with public or private entities to establish and operate facilities outside of the exterior boundary of the historic site for administration, visitor services, and curation of personal property; and other public or private entities to carry out this bill. Interior shall provide the town of Granada, Colorado, with access to those areas of the historic site determined necessary for the operation and maintenance of water infrastructure and appurtenances. Additionally, Interior may permit the town of Granada to construct or install new water infrastructure, systems, or appurtenances limited to the areas determined to be necessary. Finally, Interior may accept, for addition to and administration as part of the historic site, the donation of water infrastructure, systems, or appurtenances within the boundary of the site (including associated water rights) if the water infrastructure, systems, or appurtenances are no longer used by the town of Granada.