Using repeat ground-based topographic surveys and digital elevation model (DEM) differencing to infer reach-scale sediment budgets has become a popular monitoring tool in fluvial geomorphology. However, few studies have used DEM differencing to infer more detailed sub-reach and meso-scale fluvial processes and rates. In part, this might be attributed to the recognized importance of uncertainty in representing individual surfaces and inferring `real' changes from differences. Several recent investigators have suggested applying a minimum level of detection (minLOD) (between 10 and 30 cm), below which `real' changes are indistinguishable from noise. We present a more detailed assessment of uncertainties below typical minLOD thresholds and compare the different morphological interpretations suggested between using or eliminating various portions of the more uncertain data. In addition, we divide the study reaches into distinct morphological units (e.g. submerged bar fronts, bar heads, channel thawlegs, sloughs, banks, etc.) and explore the respective contributions of cut, fill and predicted changes below minLOD thresholds from each. Volumetric and surface-area distributions of elevation changes are presented to illustrate contrasting signals between morphological units. To illustrate these techniques, ground-based survey data from three real-time kinematic Global Positioning System (rtkGPS) surveys of a 801 X 272 m braided gravel reach of the River Feshie in the Scottish Highlands. Annual changes are reported between 2002 and 2003 as well as 2003 and 2004 (21,206, 51,080 and 46,000 points respectively). Although reach-averaged annual sediment transport rates are sometimes inferred from DEM differencing, the time-scales of monitoring (100 years) are so disconnected from the time scale of the process (10-8 years) that the interpretation is difficult. However, meaningful metrics and rates of change at sub-reach and meso-scales beyond the reach-scale sediment budget can be inferred from DEM differencing.
Available at: http://works.bepress.com/joseph_wheaton/123/