Bank Strength Variability controls the system-scale Morphodynamics of the Solimões River, Brazil

Abstract

Large anabranching lowland river banks comprise sediments of varying strength, resulting from erosional and depositional processes that act over geological timescales. Although bank strength variability is known to affect channel morphodynamics, it often remains unquantified and its effect on the migration of large sand bed rivers is poorly understood. Measurements from a ~100 km long reach of the Solimões River, the upstream part of the Brazilian Amazon River, show that cohesive muds in Pleistocene terraces along its right bank have bank strengths up to three times greater than Holocene floodplain deposits that comprise the left bank. Image analysis reveals that these resistant outcrops control channel-bar dynamics: channel widening and bar deposition are inhibited, which reduces topographic forcing and stabilizes the opposing erodible bank. Analysis of the 1,600 km long Solimões River shows that where the channel is associated with older terraces, lower rates of bank erosion and deposition rates between 1984-2021 are evident. In locations where the channel has migrated away from the resistant terrace, further change analysis finds that channel has a strong tendency to rapidly migrate back. Analysis of water surface slope for the Solimões River finds scant correlation between water gradient and migration. Bank strength heterogeneity is thus the primary control on the large-scale morphodynamics of the world’s largest lowland river.