If the National Park Service were to continue to assert that the boundary of Grand Canyon National Park adjacent to the Hualapai Reservation goes up the left bank from the river to the high water line, how might that position be justified? And, which high water line would be used?
First, you would say that you interpret the 1883 executive order for the Reservation, with the words "to" and "along" the river, to mean that coming from landward, the line went to the high water line.
The second step is to say that the 1975 Enlargement Act placing the Park boundary "on south bank" reinforced the high-water-line interpretation. You might add that there were no actions taken between 1883 and 1975 that affected the legal status of the boundary. Given the weaknesses in the departmental solicitor opinions, you would not refer to them, nor get entangled in any of the legal arguments.
Third, you would say that the question of Hualapai concurrence is moot, as is the matter of section 5 of the 1975 Act -- requiring Hualapai consent if any of their Reservation is taken--, since their Reservation boundary always only went to the high water line. You could acknowledge that the Hualapai claim to the middle of the river, but their claim has always been just that --a claim-- and nothing more.
As to which high water line to use, since you are going back to the 1883 order as the basic document, you would acknowledge that no official action determined the high water line at the time or in the official surveys. You could then assert that the only feasible line is one that could be determined by evidences left on the shore.
There would then follow choices: You could compile photographs and descriptive evidence in support of the historic high water levels. You could then state your belief in that line, and take appropriate administrative actions. However, you could acknowledge that for practical purposes, the most feasible administrative line is that reached by the river during usual operations of the Glen Canyon Dam. You could then select a high flow or an average high flow, and state that the level reached constitutes the line to which you will exercise jurisdiction. A more expansive, hard-nosed stance would be to use the high flows reached during the maximum release possible under the current dam operational regime.
Postscript: Bruce Shaw, GCNP Deputy Superintendent in the 1970's, reminds me that at that time the Coast Guard, seeing the Colorado as a navigable river, made a play to regulate river traffic through the Park. However, apparently after taking a look at what was involved, the Coast Guard backed off. Even had the Coast Guard insisted on having a presence, the Park Service, responsible for the conservation of the Canyon's environment, would have been active in attacking the degradation of the rivershore. It may be, then, that any federal agency that dealt in river matters considered the Colorado a navigable river; thus limiting Hualapai jurisdiction to the high water line.