The results of our classification uncertainty analysis show that the
global median bank migration rate (1.52 m/year) is more than an order of
magnitude higher than the median false erosion rate associated with
pixel misclassification error (.06 m/year). The distribution of the
observed riverbank migration rate is approximately log normal, while
false erosion rate is slightly bimodal when binned in log scale (Figure
2b). Of course, it is not true that the bank migration rate is an order
of magnitude higher than the false erosion rate everywhere, in fact many
reaches with low migration rates will have nearly equivalent rates of
false migration. 5.7% of reaches have false erosion rates higher than
their median observed bank erosion rate. Narrow rivers and anabranching
rivers have more bank pixels per unit length than wide single channel
rivers, and as such will have higher average rates of classifcation
errors. The uneven distribution of Landsat images also greatly biases
the false erosion estimates, as areas with fewer images have higher
rates of classifcation errors in the annual watermask products (Figure
S3).
Figure 2 . a) Validation of the riverbank erosion from REAL
(vertical axis) with references from literature (horizontal axis). b)
The distribution of global observed riverbank erosion rate is shown in
blue. The false erosion rate is shown in orange and indicates the
uncertainty from water classification errors.
Our global estimates of riverbank migration are mapped in Figure 3a
after averaging the data to Pfaffstetter level 6 basins. At the largest
scales, these data agree with our expectations: rivers like the Amazon,
Orinoco, ParanĂ¡, Mississippi, Syr Darya, Indus,
Ganges-Brahmaptura-Meghna (GBM), Yangtze, and Niger show very high rates
of erosion. In contrast, rivers in Europe, much of Siberia, and the
Canadian Shield show low average rates of bank migration. European
rivers have been modified and stabilized since at least the
16th century to serve the people that live near them,
and as such show very low rates of riverbank migration (Evette et al.,
2009; Grizzetti et al., 2017). The Canadian Shield region also shows
near-zero riverbank migration rates, but for very different reasons.
Rivers in the Candian Shield region are not heavily modified by humans,
but flow over highly resistive precambrian igneous and metamorphic rock.
Large swaths of the Arctic show low migration rates, with some notable
exceptions in the Russian Far East, Northern Siberia, and parts of the
Yukon River.
At finer scales we can see intrabasin and reach scale patterns. Within
the Amazon basin, the subbasins coming off of the Andes mountains show
the highest rates of erosion, while tributaries from the central trough
are more stable (Figure 3b). Upon closer inspection, and aggregated to
the 10km SWORD reaches, we can see high variability in bank erosion
between reaches, with a distinct drop in erosion downstream of a
confluence with a tributary (Figure 3c). Visual inspection of satellite
images suggests that this tributary (Ituxi River) typically has lower
sediment concentrations than the Purus River. At the finest scale of
REAL, the 200m node spacing, we see meander-scale sediment processes on
the Purus River (Figure 3d). Nodes near the apex of the meander bends
typically show high rates of erosion, while the straight connecting
sections show much lower rates of erosion. We also see a pattern of high
rates of erosion before and after the apex of the bend, indicating the
expected positive bias in erosion on the downstream bank faces.