GEOMORPHIC CHANGE DETECTION IN GRAND CANYON: COMPARISON OF 2000 LIDAR AND 1923 SURVEY DATA

Debris flows in Grand Canyon constrict the Colorado River, increase the water-surface fall through rapids, and raise the elevation in the upper pool, potentially decreasing the fall in upstream rapids. Long-term monitoring of changes created by debris-fan reworking help assess the stability of pool-rapid controls of the river's hydraulics. In 1923, Col. Birdseye led a U.S. Geological Survey team in measuring the longitudinal water-surface profile through Grand Canyon using simple theodolite and stadia survey techniques. In 2000, a LIght Detection And Ranging (Lidar) over-flight collected topographic data centered on the river corridor and water surface, representing the first comprehensive measurement of water-surface profile since 1923. Though absolute topographic accuracy of the Birdseye Survey is poor by today's standard, comparison of the two water-surface profiles is possible owing to the known stability of certain pool-rapid sequences as documented with repeat photography. Using unchanged rapids as "anchors" (i.e. those rapids not historically effected by debris flows), the Birdseye data were tied to the Lidar data at specific points along the river. The Birdseye data between anchors were then adjusted horizontally and vertically, aligning the two data sets and allowing direct comparison. By juxtaposing the Birdseye and Lidar profiles, the effect of 77 years of Grand Canyon debris-flow activity and river reworking on the river water surface was quantified. Comparison of the 1923 and 2000 data can identify rapids with elevated water surfaces, thus marking those tributaries that produced historical debris flows. For example, Crystal Rapid, greatly constricted and elevated by a large debris flow in 1966, saw a net change in water-surface elevation of 1.9 m; moreover, the increased elevation of its upstream pool reduced the overall drop though Boucher Rapid 2.7 km upstream. The study finds Lidar data of great use in combination with historical surveys, despite some limitations in accuracy for both.