Revealing the pace of river landscape evolution during the Quaternary: recent developments in numerical dating methods

During the last twenty years, several technical developments have considerably intensified the use of numerical dating methods for the Quaternary. The study of fluvial archives has greatly benefited from these enhancements, opening new dating horizons for a range of archives at distinct time scales and thereby providing new insights into previously unanswered questions. In this contribution, we separately present the state of the art of five numerical dating methods that are frequently used in the fluvial context: radiocarbon, Luminescence, Electron Spin Resonance (ESR), 230Th/U and terrestrial cosmogenic nuclides (TCN) dating. We focus on the major recent developments for each technique that are most relevant for new dating applications in diverse fluvial environments and on explaining these for non-specialists. Therefore, essential information and precautions about sampling strategies in the field and/or laboratory procedures are provided. For each method, new and important implications for chronological reconstructions of Quaternary fluvial landscapes are discussed and, where necessary, exemplified by key case studies. A clear statement of the current technical limitations of these methods is included and forthcoming developments, which might possibly open new horizons for dating fluvial archives in the near future, are summarised

Example of Holocene alpine torrent response to environmental change: contribution to assessment of forcing factors [ Un exemple de réponse d'un torrent alpin aux changements environnementaux à l'Holocène contribution à l'analyse des facteurs de forçage]

Although the major features of Holocene torrent evolution in the Southern Alps have been described during the last twenty years, it is now admitted that both the intrinsic characters of each watershed and the terms of their anthropisation strongly influence the geomorphological response to environmental change Located in the middle Ubaye valley, the Abéous is to our knowledge the only torrent in the French Alps which has provided sufficient data for it to be possible to reconstruct its history throughout all the Holocene The authors put forth the recurrence of debris flow faciès in the stratigraphy of this torrent's alluvial fan, including dunng the Atlantic, and note an increase of its torrential characteristics as from the Bronze Age They also focus on the conditions prevailing at the time of sedimentation of the thick silt levels which are part of the «main Holocene fill» In the light of this long evolution, this article proposes an analysis of the contemporary spontaneous reforestation and active channel narrowing trendSi les grandes lignes de l'évolution de la torrentialité Holocène des Alpes du Sud sont désormais connues, on sait que les caractéristiques intrinsèques de chaque bassin versant et les modalités de leur anthropisation influent sur la réponse géomorphologique fournie aux changements environnementaux. Situé dans la moyenne vallée de l'Ubaye, l'Abéous est à notre connaissance le seul torrent des Alpes françaises à avoir livré suffisamment d'informations pour que l'on puisse en reconstituer l'histoire tout au long de l'Holocène. Les auteurs mettent en évidence le caractère récurrent des faciès torrentiels dans la stratigraphie du cône de déjection de ce torrent, y compris à l'Atlantique et constatent une accentuation du caractère torrentiel depuis l'Age du Bronze. Ils s'interrogent par ailleurs sur les conditions de formations des épais niveaux limoneux qui composent pour partie le «remplissage holocène principal». A la lumière de cette longue évolution, l'article propose une analyse de la dynamique contemporaine de reboisement spontané et de rétraction de la bande active du torrentFlez Christophe, Lahousse Philippe. Example of Holocene alpine torrent response to environmental change: contribution to assessment of forcing factors [ Un exemple de réponse d'un torrent alpin aux changements environnementaux à l'Holocène contribution à l'analyse des facteurs de forçage]. In: Quaternaire, vol. 15, n°1-2, 2004. Fluvial Archives Group. Clermond-Ferrant 2002. pp. 167-176

The Riou Bourdoux torrent (Alpes-de-Haute-Provence, France) - a "monster" under close surveillance

Parmi tous les torrents de la vallée de l'Ubaye, le Riou Bourdoux est sans conteste le plus célèbre. Ses débordements redoutés lui ont valu de recevoir le qualificatif de "monstre". Le Service de Restauration des Terrains en Montagne a donc entrepris sa correction à partir de 1866. Durant la "période héroïque" (1866-1914), Prosper Demontzey et ses successeurs ont ainsi mis en application un ambitieux plan d'intervention qui aboutira à la délimitation d'un périmètre de reboisement et de réengazonnement, et à la construction d'environ 2 000 seuils et barrages. Délaissé durant la parenthèse des deux guerres mondiales, le dispositif a malheureusement subi de nombreux dommages. Depuis 1950, il fait l'objet d'un incessant travail de rénovation et de renforcement, afin de maintenir un niveau de sécurité acceptable. Néanmoins, l'instabilité qui règne encore sur de nombreux versants entretient toujours un risque de déclenchement d'une importante lave torrentielle. L'attention portée au Riou Bourdoux n'est donc pas prête à retomber, d'autant que la vulnérabilité du site a été sensiblement accrue par une reconquête récente du cône de déjection (aérodrome, zone artisanale et industrielle)

Impact of the magnitude and frequency of debris‐flow events on the evolution of an alpine alluvial fan during the last two centuries: responses to natural and anthropogenic controls.

The dynamics and the surface evolution of a post‐LGM debris‐flow‐dominated alluvial fan (Tartano alluvial fan), which lies on the floor of an alpine valley (Valtellina, Northern Italy), have been investigated by means of an integrated study comprising geomorphological field work, a sedimentological study, photointerpretation, quantitative geomorphology, analysis of ancient to modern cartography and consultation of historical documents and records. The fan catchment meteoclimatic, geological and geomorphological characteristics result in fast rates of geomorphic reorganization of the fan surface (2 km2). The dynamics of the fan are determined by the alternation of low‐return period catastrophic alluvial events dominated by non‐cohesive debris flows triggered by extreme rainstorms which caused aggradation and steepening of the fan and avulsion of its main channel, with periods of low to moderate streamflow discharge punctuated by low‐ to intermediate‐magnitude flood events, causing slower but steady topographic reworking. The most ancient parts of the fan surface date back at least to the first half of the 19th century, but most of the fan surface has been restructured after 1911, mainly during the debris‐flow‐dominated events of 1911 and 1987. Phases of rapid fan toe incision and fan degradation have been recognized; since the 1930s or 1940s, the Tartano fan has been subjected to a state of deep entrenchment and narrowing of the main trunk channel and distributary area. Post‐Little Ice Age climate change and present‐day surface uplift rates have been considered as possible explanations for the observed geomorphic evolution, but tectonic or climatic controls cannot account for the order of magnitude of the erosional pace. Anthropogenic controls plausibly override the natural ones: in particular, the building of a dam in the late 1920s, about 2 km upstream of the fan, seems to have triggered fan dissection, having altered the sediment discharge through sediment retention