Large alluvial rivers transport water and sediment across continents and
shape lowland landscapes. Repeated glacial cycles have dominated Earth's
recent climate, but it is unclear whether these rivers are sensitive to
such rapid changes. The Amazon River system, the largest and
highest-discharge in the world, features extensive young terraces that
demonstrate geologically rapid change temporally correlated with changes
in runoff from Quaternary climate cycles. To test the plausibility of a
causal relationship, we use a simple model to estimate from empirical
measurements how quickly a river profile responds to changes in
discharge or sediment supply. Applying this model to data from 30
gauging stations along alluvial rivers throughout the Brazilian Amazon,
we find that many rivers of the Amazon basin can respond faster than
glacially induced changes in runoff or sediment flux. The Amazon basin
is unusually responsive compared to other large river systems due to its
high discharge and sediment flux, narrow floodplains, and low slopes. As
a result, we predict that the Amazon basin has been highly dynamic
during Quaternary glacial cycles, with cyclical aggradation and incision
of lowland rivers driving repeated habitat and environmental change
throughout the region. This dynamic landscape may have contributed to
the exceptional biodiversity of the region and patterns of ancient human
settlement.