Channel breakdown and avulsion in arroyos feeding the Little Colorado River, Arizona, USA

Improved knowledge regarding dryland river character and behaviour is important as the increasing threats of climate change and anthropogenic activities exacerbate water security issues in channels prone to discontinuity and termination. Dryland river character and behaviour is determined by a complex and dynamic interplay between extrinsic controls and intrinsic processes. Polacca and Oraibi Wash are two discontinuous dryland rivers, tributaries of the Little Colorado River in semi-arid, north-eastern Arizona, USA. Valley-scale morphometrics and downstream longitudinal trends were derived from remote sensing and geospatial analyses, hydraulic modelling, and field measurements, to assess the key controls, patterns, and processes driving channel diminution and discontinuity. Both rivers are entrenched along most of their valleys and experience downstream hydro-geomorphic changes as valley confinement eases in their low gradient floodplains. The rivers exhibit non-equilibrium responses to a concomitant decline in calculated discharge (from ~82 to <4 m 3 s −1 ), stream power (from ~334 to <5 W m −2 ), and channel cross-sectional area (from ~82 to <3 m 2 ), which are the principal factors contributing to channel breakdown and the development of broad floodouts. Small distributary channels and avulsions are evident in the lowland reaches where channel discontinuity occurs, with a dominance of knickpoints in distal floodout reaches as they attempt to re-establish channel continuity. Using historical aerial imagery we show that knickpoint headcut retreat rates have ranged from ~1.7 to 53 m a −1 . Channel breakdown patterns and processes in the Polacca and Oraibi Washes are similar to other dryland rivers around the world, and reveal common underlying factors that are central to river termination and reformation. This study reveals a combination of intrinsic erosion and sedimentation processes and avulsion threshold responses that trigger channel adjustment resulting from downstream fluvial decline. However, key extrinsic controls, such as valley confinement and hydroclimate, influence and limit the rate and nature of these adjustments. Early recognition of these geomorphic changes can inform river, land, and water management strategies across the American southwest. • Two dryland rivers in southwestern USA have downstream declines in channel size and hydrology leading to channel breakdown and floodout formation. • Twentieth century channel adjustments have led to splays, avulsions, and terminations in the widest part of each valley. • Distributary formation and avulsive river behaviour are influenced by cycles of erosion and deposition.