THE POWER OF PERSPECTIVE: A HUNDRED YEARS OF OBSERVATIONS FROM A GLEN CANYON BENCH MARK

The life history of a survey mark includes its birth, measuring relative position in relation to its neighbors, measuring its position on the planet, and finally combining the measurements so all reference a common datum. As a mechanism of geomorphological or geophysical monitoring, each mark provides a reference frame from which horizontal and vertical distances can be observed and repeated. While a station’s life history cannot be discerned from a simple coordinate, the collection and cataloging of observations from a mark’s perspective can substantially increase its monitoring value.

As a case study we look at a Bench Mark in Glen Canyon, AZ (Point Designation GP0239) along the Colorado River and only 4 kilometers downstream of Glen Canyon Dam. By studying the life history of this mark along with data from three of its neighbors, we unlock secrets learned from nearly a hundred years of improvements in global positioning and technological advances in spatial measurements. Prior to impoundment within Lake Powell, a First-order leveling survey was performed from Flagstaff to Lees Ferry, and upstream along the Colorado River through Glen Canyon to a line North of Crossing of the Fathers, Utah. By observing four of these Bench Marks from 4 to 20 kilometers downstream from Glen Canyon Dam with the Global Position System, we have measured and modeled their current orthometric height differences. When comparing these results with predam measurements, and considering the accuracies of each, the data indicate a lowering of the regional equipotential surface since the more than 4-trillion gallons of water have amassed within Lake Powell. We suggest that changes in regional equipotential, not physical movement or compression of the earth’s crust, are responsible for a 30 cm lowering of orthometric heights at the GP0239 Bench Mark.