In my consideration of the dispute over this boundary, I wrote (Oct 2011) that in 1975 -- after the GCNP Enlargement Act became law -- then-superintendent Stitt started talking about the boundary being on the "historic high water line", and I, --along with the Hualapai and their attorney Marks -- was surprised and displeased because it was our understanding that putting the boundary on the south bank was only intended to give NPS jurisdiction over the water surface in order to control river traffic. Going up to a historic, maximum-ever high water mark would be taking Hualapai land, in a common-sense interpretation of the 1883 reservation establisment, which used "to" the river, and "along" the river.
In writing up the legislative history of the Act, I find that in the NPS documents, Stitt was talking about the "high water" line in May 1973 as NPS was preparing its position for Senate hearings, so indeed it was not a new notion in 1975. However, there was no mention of "historic", and it is clear he was talking about jurisdiction over river traffic.
In the NPS testimony at the Senate hearing on 20 Jun 1973, no mention was made of the boundary. However, the Hualapai testimony given by Vice Chairman Sterling Mahone, noting that the map "establishes the south bank as (the Park) boundary … where the Hualapai Reservation borders on the river", charges that "the effect of this provision is to take land from the Hualapai Reservation, which presently extends to the middle of the Colorado river". He continues, "The (1883) Executive order … contained no language limiting the Hualapai Tribe's interest in the bed of the Colorado River, and therefore, the boundary of the reservation is the middle of the river rather than its south bank." A little further, he points out, "The Hualapai Tribe has gone on record as opposing any cession of its interest in the bed of the Colorado River."
Taken together, these contemporaneous remarks reinforce what Congress intended and did when, by placing the Park boundary on the south bank, it gave jurisdiction and administrative authority over the river surface and its traffic to NPS, while not disturbing the Hualapai claim to the bed of the river out to the middle of the Colorado. The Hualapai never asked for jurisdiction over this river or its traffic; and NPS never asked for any of the river bed or bank that belonged to the Hualapai. As launched and completed in 1973-5, the GCNP Enlargement Act fashioned an acceptable division. That, later on, NPS claimed land by talking about the "historic" high water mark, and that the Hualapai insist they own half the water surface, is mischief-making on the part of both parties.
Stitt opinion is contained in archival material from NPS, as recorded in my notes.
Hualapai statement is in pp 60-2, "Hearing before the Subcommittee on Parks and Recreation of the Committee on Interior and Insular Affairs United States Senate, 93rd Congress, First Session, on S. 1296", June 20, 1973.