Cascades of sub‐decadal, channel‐floodplain changes in low‐gradient, non‐vegetated reaches near a dryland river terminus: Salar de Uyuni, Bolivia.

The terminus of the ephemeral Río Colorado is located at the margins of Salar de Uyuni, Bolivia, the world's largest salt lake. The low‐gradient (<0.0006 m m‐1), non‐vegetated reaches approaching the terminus provide an excellent natural laboratory for investigating cascades of channel‐floodplain changes that occur in response to quasi‐regular flows (at least once annually) and fine‐grained sediment supply (dominantly silt and clay). High‐resolution satellite imagery (<0.65 m, various dates from 2004 onwards) and field data reveal widespread, pronounced and rapid morphodynamics on sub‐decadal timescales, including channel erosion and chute cutoff formation, and development of crevasse channels and splays, floodouts (unchannelled surfaces at channel termini), and erosion cells (floodplain scour‐transport‐fill features). In particular, following high annual precipitation (>400 mm) in 2004–2005 and two subsequent high magnitude daily precipitation events (~40 mm), all of which led to widespread flooding, numerous crevasse splays formed between 2004 and 2016, avulsions occurred at nearby floodouts, and erosion cells downstream of the splays and floodouts underwent striking morphological changes. High‐precision GPS data reveal two preferential localities for erosion cell development: partially or fully abandoned channels with crevasse splay remnants, and topographic lows between channels. In this overall low‐gradient setting, comparatively high gradients (up to ~0.0006 m m‐1) at the edge of splay deposits and topography created by crevasses and abandoned channels may initiate knickpoint retreat and thereafter erosion cell development. Abandoned channels with splays tend to give rise to narrow, deep erosion cells, while topographic lows promote relatively shallow, wide erosion cells. In both situations, erosion cells may extend upslope and downslope, and eventually connect to form straight channels. The channel‐floodplain morphodynamics near the Río Colorado terminus extend previous analyses of low‐gradient, dryland river systems, particularly because the lack of vegetation and quasi‐regular floods drive cascades of rapid changes on sub‐decadal timescales. © 2018 John Wiley & Sons, Ltd. Highlights: • Rapid cascades of channel‐floodplain changes characterise the ephemeral lower Río Colorado. • Enhanced local slope created by splays and abandoned channels promotes erosion cell development. • Erosion cells may connect up and form straighter channels. Inset: Example of erosion cell development along an abandoned channel. Letters S, T and F represent scour, transport and fill sections in the erosion cells. Pléiades imagery, with general flow direction from lower right to upper left (i.e. oblique to the abandoned channel), and north oriented to the top. Main image: Example of a relatively straight channel that may have resulted from the elongation and connection of erosion cells. The straightness of this channel contrasts with the higher sinuosity evident along older active or abandoned channels. Pléiades imagery, with general flow direction from bottom to top, and north oriented to the top. [ABSTRACT FROM AUTHOR]