Extensive channel narrowing in southeastern France provides an illustration of geomorphic response to land-use changes. The study region comprises a range of environments, from large piedmont and intramountain gravel-bed rivers, to small mountain streams. Field measurements and analysis of historical data demonstrate two distinct periods of channel change. From 1850 to 1950, channel narrowing is interpreted to be the result of a recovery process in response to widespread channel destabilization induced by major floods during the second half of the 19th century. At the time, the largely deforested basins were highly responsive to flooding, whereas the recovery process was accelerated by floodplain and basin-scale land use changes (afforestation) and torrential control works, which in turn reduced sediment delivery and enabled vegetation development in channels. From 1950 to 1970, channel narrowing accelerated in most of the studied rivers. This recent phenomenon is considered as a human-induced fluvial adjustment, directly related to forest development on river margins and human abandonment of intensive floodplain land uses. At the same time, long-profile degradation occurred as a result of long-term bedload supply decrease. In small mountain streams, channel narrowing is mainly explained by channel incision which seems to progress from upstream where sediment sources are progressively stabilized by afforestation.
Human-induced modifications of the vegetation cover in river basins may cause strong geomorphic responses by disturbing sediment supply, transport and deposition regimes. The response is particularly noticeable in upland regions, where sensitivity to change is enhanced by strong coupling between river channels and hillslopes, as is exemplified by studies undertaken in the East Coast region, North Island, New Zealand, and in the Southern French Prealps. Both regions have been affected by land-use change during the past 150 years (deforestation and reforestation) that can be chronologically linked to geomorphic change on hillslopes and on valley floors. In this paper we use these studies to draw attention to: (1) the magnitude of the change in sediment production associated with a modification to the vegetation cover; (2) the impact that reforestation has on the sediment production and the channel system; and (3) the relative influence of anthropogenic and climatic forcing on the channel response. Finally, we consider the manner in which land use has been used as a tool to manage sediment production in France and New Zealand. The results obtained in both regions demonstrate the strong effect that the vegetation cover has on hillslope erosion processes, through its impact on the landsliding threshold (in New Zealand) and the total sediment yield (from paired forested and non-forested catchments in France). Consideration of channel response serves to emphasize the sensitivity of upland regions to land-use change, and suggests that the successful discrimination between the respective influences of climatic and land-use change depends, in large part, upon the ability to detect spatial and chronological links (or, conversely, gaps) between causes and effects.
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