Riverscape mapping with helicopter-based Structure-from-Motion photogrammetry

• Structure-from-Motion provides a low-cost option for fluvial remote sensing at spatial scales of tens of kilometers
• Research examines downstream patterns in morphology at high spatial resolutions on the Middle Fork John Day River, Oregon
• Provides guidance for future SfM photogrammetric surveys

Recent developments in the remote sensing of fluvial systems have provided researchers with unprecedented views on the complexity of rivers. An aerial perspective is key to mapping and understanding the river at a variety of spatial scales. I employed a helicopter-mounted digital SLR camera and Structure-from-Motion (SfM) photogrammetry to bridge the gap between smaller scale aerial surveys from platforms like small unmanned aerial systems and larger scale commercial aerial photography or airborne LiDAR collections. This low-cost solution produced high spatial resolution aerial photography and digital elevation models for a 32-km segment of the Middle Fork John Day River in east central Oregon. Using these data, I extracted channel morphology data at 3-m intervals downstream and took an inductive approach to evaluating the controls on channel morphology and the human influences on the river using a combination of segment-scale and hyperscale analyses. The SfM process produced 10 cm/pixel orthophotographs and DEMs with submeter horizontal accuracy, but the DEMs suffered from a systematic distortion that resulted from the parallel camera geometry of the flight plan. The riverscape has been affected by human actions such as mining, cattle grazing, and restoration; however, differentiating a human signal from the natural patterns of channel morphology was difficult. The hyperscale analysis provided insight into several interesting downstream patterns in channel morphology that, with further analysis, could provide explanations on the physical controls of channel morphology. Overall, SfM has the potential to be a powerful, low-cost addition to the fluvial remote sensing toolkit.