Historical archives of grey-scale river channel imagery are extensive.
Here, we present and test a methodology to extract detailed
quantitative topographic data from such imagery of sand-bed rivers.
Extracting elevation information from rivers is difficult as they are
characterized by a low relative relief (<4 m); the area of interest may
be spatially extensive (e.g. active channel widths >500 m in large
braided rivers); the rate of change of surface elevation is generally
low except in the vicinity of individual channel banks where the rate
of change is very high; there is the complication that comes from
inundation; and there may be an added complication caused by blockage
of the field of view by vegetation. Here, we couple archival
photogrammetric techniques with image processing methods and test these
for quantification of sand-bed braided river dynamics, illustrated for
a 500 m wide, 3 km long reach of the South Saskatchewan River, Canada.
Digital photogrammetry was used to quantify dry areas and water edge
elevations. A methodology was then used to calibrate the spectral
signature of inundated areas by combining established two media digital
photogrammetric methods and image matching. This allowed determination
of detailed depth maps for inundated areas and, when combined with dry
area data, creation of complete digital elevation models. Error
propagation methods were used to determine the erosion and deposition
depths detectable from sequential digital elevation models. The result
was a series of elevation models that demonstrate the potential for
acquiring detailed and precise elevation data from any historical
aerial imagery of rivers without needing associated calibration data,
provided that imagery is of the necessary scale to capture the features
of interest. We use these data to highlight several aspects of channel
change on the South Saskatchewan River, including bar movement, bank
erosion and channel mulling.