Drones and Digital Photogrammetry: From Classifications to Continuums for Monitoring River Habitat and Hydromorphology

Recently, we have gained the opportunity to obtain very high-resolution imagery
and topographic data of rivers using drones and novel digital photogrammetric
processing techniques. The high-resolution outputs from this method are
unprecedented, and provide the opportunity to move beyond river habitat classification
systems, and work directly with spatially explicit continuums of data.
Traditionally, classification systems have formed the backbone of physical river
habitat monitoring for their ease of use, rapidity, cost efficiency, and direct comparability.
Yet such classifications fail to characterize the detailed heterogeneity
of habitat, especially those features which are small or marginal. Drones and
digital photogrammetry now provide an alternative approach for monitoring
river habitat and hydromorphology, which we review here using two case studies.
First, we demonstrate the classification of river habitat using drone imagery
acquired in 2012 of a 120 m section of the San Pedro River in Chile, which was at
the technological limits of what could be achieved at that time. Second, we
review how continuums of data can be acquired, using drone imagery acquired
in 2016 from the River Teme in Herefordshire, England. We investigate the precision
and accuracy of these data continuums, highlight key current challenges,
and review current best practices of data collection, processing, and management.
We encourage further quantitative testing and field applications. If current
difficulties can be overcome, these continuums of geomorphic and hydraulic
information hold great potential for providing new opportunities for understanding
river systems to the benefit of both river science and management.