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Populations of the riparian pioneer species Populus nigra L. which establish on alluvial bars within river channels modulate sediment dynamics and fluvial landforms. Dense cohorts of P. nigra have colonized gravel point bars along the channelized River Garonne, France, during the last 20 years and have enhanced the vertical, lateral and longitudinal development of the bars. For this period, the geomorphic characteristics of two wooded point bars on this laterally stable river are closely linked to the spatial distribution and intensity of establishment and resistance of different cohorts of P. nigra. Furthermore, P. nigra colonization dynamics were controlled by engineer effects of this same species. This relationship is illustrated by a significant correlation between key geomorphic and biological variables measured in situ and characterized with a set of four aerial photographs taken between 2000 and 2010. The development of wooded point bars, which are discrete biogeomorphic units, over the studied period, appear to result from a specific biogeomorphic positive feedback of matter aggregation and vegetation establishment related to sediment trapping and stabilization by pioneer engineer plants. We propose a conceptual model of biogeomorphic unit construction for channelized, lateral stable rivers. We consider the resultant biogeomorphic units as functional from an ecological point of view because P. nigra enhances at the cohort scale (i) its own inherent capacity to resist hydrogeomorphic disturbances, and (ii) its resilience capacity as a result of successful colonization, especially downstream of mature poplar stands. [ABSTRACT FROM AUTHOR]

Rivers have an intricate relationship with the vegetation that colonizes them. Riparian plants, capable of thriving within river corridors, both respond to and influence geomorphology. Yet interactions between river morphodynamics and vegetation tend to be context specific, making it challenging to generalize findings between locations. The current comprehension of vegetation interaction with physical processes, and especially its effects on river morphodynamics, still lacks clarity. This article examines numerous sources of variation in plant responses to, and effects on, river morphodynamics. Vegetation influences on geomorphological parameters vary in terms of intensity and spatial extent along the gradient of river energy and according to the fluvial style. Whilst feedbacks between vegetation and river morphodynamics are readily discernible at a local scale, on larger spatial scales, it can remain difficult to precisely determine cause-and-effect relationships that link hydrogeomorphic and vegetation drivers and the outcomes of their feedbacks. This is especially problematic for those feedbacks that give rise to emergent system landscape behaviour in meandering and island braided rivers. By contrast, in certain river configurations, such as anabranching rivers, the imprint of vegetation on the riverscape can be clearly evident. The imprint of vegetation is also supported by evidence from the ancient alluvial record. Through this review, we highlight key perspectives from a wide range of modern and ancient rivers of varied configuration in order to inform future studies of vegetation responses to, and effects on, river morphodynamics. • Vegetation responses to, and effects on, river morphodynamics are modulated by environmental and biological contexts. • Vegetation-geomorphology interactions are embedded into a complex adaptive system. • Vegetation drives discernible scale-dependent effects on geomorphology at the local scale (10-1 - 102 m). • Large scales: determining cause-and-effect relationships linking hydrogeomorphic and vegetation drivers remains challenging. • Vegetation: Insufficient but Non-redundant parts of Unnecessary but Sufficient (INUS) conditions for various river types. [ABSTRACT FROM AUTHOR]

Abstract: Large tsunamis are major geomorphic crises, since they imply extensive erosion, sediment transport and deposition in a few minutes and over hundreds of kilometres of coast. Nevertheless, little is known about their geomorphologic imprints. The December 26, 2004 tsunami in Sumatra (Indonesia) was one of the largest and deadliest tsunamis in recorded human history. We present a description of the coastal erosion and boulder deposition induced by the 2004 tsunami in the Lhok Nga Bay, located to the West of Banda Aceh (northwest Sumatra). The geomorphological impact of the tsunami is evidenced by: beach erosion (some beaches have almost disappeared); destruction of sand barriers protecting the lagoons or at river mouths; numerous erosion escarpments typically in the order of 0.5–1.5 m when capped by soil and more than 2 m in dunes; bank erosion in the river beds (the retreat along the main river is in the order of 5–15 m, with local retreats exceeding 30 m); large scars typically 20–50 cm deep on slopes; dislodgement of blocks along fractures and structural ramps on cliffs. The upper limit of erosion appears as a continuous trimline at 20–30 m a.s.l., locally reaching 50 m. The erosional imprints of the tsunami extend to 500 m from the shoreline and exceed 2 km along riverbeds. The overall coastal retreat from Lampuuk to Leupung was 60 m (550,000 m2) and locally exceeded 150 m. Over 276,000 m3 of coastal sediments were eroded by the tsunami along the 9.2 km of sandy coast. The mean erosion rate of the beaches was ~30 m3/m of coast and locally exceeded 80 m3/m. The most eroded coasts were tangent to the tsunami wave train, which was coming from the southwest. The fringing reefs were not efficient in reducing the erosional impact of the tsunami. The 220 boulders measured range from 0.3 to 7.2 m large (typically 0.7–1.5 m), with weights from over 50 kg up to 85 t. We found one boulder, less than 1 m large, at 1 km from the coastline, but all the others were transported less than 450 m (<7 m a.s.l.). No fining landward boulder size distribution could be detected. The coincidence of different size modes, from boulders to fine sands, with independent spatial distributions, suggests that all the material was not transported in suspension, but rather by a combination of suspension and bed load transport. Finally, the spatial and size distributions of tsunami boulder deposits mostly depend on the location and characteristics of their source (coral reef, beach rock, platform, dams), together with clast and surface interference during transport. One year after, the coastal environment in northwest Sumatra is still in a post-tsunami dynamic. Thus, the difference between the largest tsunamis (height>30 m) and the moderate tsunamis (height<10 m) could be their long-term impact on coastal environments. [Copyright &y& Elsevier]