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

Abstract

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.

Identifying threshold responses of Australian dryland rivers to future hydroclimatic change

“…Alternatively, indirect approaches to discharge estimation in dryland rivers 52 could be used to generate supporting hydrological or hydraulic information to help establish modern climate-hydrology-geomorphology relations across hydroclimatic gradients, as has been done successfully for different dryland rivers worldwide (e.g. 10,22,53 ). By applying ergodic reasoning, these relations could then be used as the basis for predicting river response under altered hydroclimates.…”

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

“…The low energy, fine-grained Río Colorado provides a rare example of an ephemeral river that meanders in the absence of vegetative influences, with active bend development occurring on yearly to decadal timescales. Satellite The Río Colorado is unusual but not unique, and similar morphodynamics may characterize other ephemeral meandering rivers worldwide, especially those that terminate on the margins of salt lakes (Li et al, 2019). Along with further studies of these other rivers, the findings from the Río Colorado will contribute to a more comprehensive understanding of contemporary meandering river dynamics, and may also help to improve interpretations of Earth's pre-vegetation rivers and meandering fluvial forms on other planetary bodies (cf.…”

“…1B). Previous studies have focused mainly on the fan-shaped lower reaches approaching the terminus (Donselaar et al, 2013;Li et al, 2014aLi et al, ,b, 2015bLi et al, , 2018Li et al, , 2019Li and Bristow, 2015;van Toorenenburg et al, 2018) and also have investigated the anabranching morphodynamics on the nearby Río Capilla (Li et al, 2015a). In this study, greater attention is focused on a 25 km long reach of the Río Colorado located between 25 and 50 km upstream of the margins of Salar de Uyuni (Fig.…”

“…The lower Río Colorado is dominated by rapid-rise flood events that have been shown to be a significant control on channel morphology and cascades of wider channel-floodplain changes (Li et al, 2014a(Li et al, ,b, 2018(Li et al, , 2019. Discharge (Fig.…”

“…Previous studies of the middle and lower reaches of the Río Colorado and neighbouring rivers have revealed the widespread, pronounced J o u r n a l P r e -p r o o f channel-floodplain dynamics that result from quasi-regular (typically at least once a year) within-channel and overbank flooding ( Li, 2014;Li et al, 2014a;Li et al, 2018) and have drawn preliminary comparisons with the morphodynamics of non-vegetated rivers in similar physiographic contexts worldwide, including in North America and Asia (e.g., Li et al, 2019). While some of the previous studies on the Río Colorado have noted chute cutoffs that occur as part of meander-belt development and wider cascades of channel-floodplain changes (e.g., Li et al, 2019), the geomorphological, hydraulic or sedimentary processes of cutoffs have not been investigated in any detail. Our study aims to: (1) use satellite imagery, hydraulic modelling and field investigations to document the processes of chute cutoff-driven abandonment and sedimentation of meander bends;…”