Chute cutoff-driven abandonment and sedimentation of meander bends along a fine-grained, non-vegetated, ephemeral river on the Bolivian Altiplano

Abstract

The origin and development of meandering river planforms has long been a focus of research in the geosciences. Most attention has focused on perennial meandering rivers with well developed riparian vegetation assemblages, and while increasing interest is turning towards the dynamics and sedimentology of ephemeral meandering rivers with sparse to no vegetation, additional field data are required. As a contribution, this study presents a remote-sensing and field-based analysis of chute cutoff-driven abandonment and sedimentation of meander bends along the fine-grained, non-vegetated, ephemeral Río Colorado on the Bolivia Altiplano. Along the 25km long sinuous study reach, located between 25 and 50km upstream of the present-day margins of Salar de Uyuni, quasi-regular flood events (typically at least one per year) drive bend cutoffs and wider channel-floodplain dynamics, despite low specific stream power (<10W/m2) and cohesive (dominantly silt and clay) bed and bank sediments. Along the reach, twenty-two cutoffs are evident, with chute cutoff the dominant mechanism. Three chute cutoffs (CC1, CC2, CC3) occurred between 1996 and 2016; for one bend (CC3), high-resolution (<0.65m) satellite imagery and field investigations reveal the details of pre-, mid- and post-chute cutoff processes and sedimentary products. Together, the findings suggest that for any given bend, an increase in bend amplitude (mean ratio of meander bend length to chute channel length ∼4) combines with a typically high diversion angle between the channel-belt axis and the upstream limb of the meander bend (mean ∼98°) to result in declining flow efficiencies. This enhances overbank flooding and promotes deposition along the upstream limb of the bend. Overbank flooding promotes the development of chute channels, which commonly initiate as shallow (typically <1m) headward eroding channels, while sediment derived from overbank flow and headward erosion tends to be deposited in the downstream limb of the bend. By obstructing flow, such deposition reduces sediment transport capacity within the meander bend as a whole, thereby further inducing deposition within the upstream limb. During subsequent floods, deepening and widening of a dominant chute channel and sedimentation and shallowing of the meander bend continues. Along the reach, episodic La Niña-driven flood events drive phases of more rapid bend migration and clusters of chute cutoffs. Our findings contribute to a more comprehensive understanding of meandering river dynamics across the full range of Earth’s conditions, and also may help to improve interpretations of Earth’s pre-vegetation rivers and meandering fluvial forms on other planetary bodies.