Untangling Geomorphic Processes in the Grand Canyon with Topographic Time Series from Hybrid Surveys

The explosion of different methods for collecting high-resolution repeat topographic datasets in rivers has revolutionized our ability to monitor rivers and track their evolution through time. Increasingly available ground-based (e.g. total stations, rtkGPS, terrestrial laser scanners), boat-based (e.g. single and multi-beam echo-sounding), and airborne (e.g. LiDaR, spectral-based depth extraction and photogrammetry) datasets are at the disposal of researchers to build topographic spatiotemporal time-series of river reaches. Unfortunately, no single method works everywhere. As such, many researchers combine these various methods to build a representation of a river. This has been the case for over a decade now in the Grand Canyon, where over 30 miles of river and over 60 discrete sandbar sites along the Colorado River below Glen Canyon Dam have been surveyed repeatedly with a combination of multi-beam SONAR, single-beam SONAR, photogrammetry, LiDaR and total station surveys. Survey data is analyzed to explore how experimental flood releases, timed with natural sediment inputs, alter landforms downstream of the dam. We present results of analyses using a new version of the Geomorphic Change Detection software that was extended to explicitly deal with this hybrid dataset problem. The software is used to come up with independent spatially variable estimates of DEM error, which vary both according to space and survey method, and then propagate those errors into change detection analyses based on DEM differencing. Uncertainty analysis is used to threshold out changes that are indistinguishable from noise and estimate uncertainty in volumetric sediment budget estimates of net changes in storage. A budget segregation feature is exploited to decompose the budget by specific geomorphic mechanisms of change. By utilizing a hybrid of available datasets for estimating sediment budgets, we gain insight into how different eddy-bar-fan complexes are evolving through time in response to a highly regulated flow regime and experimental flood releases.