Fine-grained sediment is a natural and essential component of river systems and plays a major role in the hydrological, geomorphological and ecological functioning of rivers. In many areas of the world, the level of anthropogenic activity is such that fine-grained sediment fluxes have been, or are being, modified at a magnitude and rate that cause profound, and sometimes irreversible, changes in the way that river systems function. This paper examines how anthropogenic activity has caused significant changes in the quantity and quality of fine-grained sediment within river systems, using examples of: land use change in New Zealand; the effects of reservoir construction and management in different countries; the interaction between sediment dynamics and fish habitats in British Columbia, Canada; and the management of contaminated sediment in USA rivers. The paper also evaluates present programmes and initiatives for the management of fine sediment in river systems and suggests changes that are needed if management strategies are to be effective and sustainable.
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.
Abstract. Downstream changes in particle size that occur in the Waipaoa River, a 104-km-long gravel bed river in which rapid aggradation in the historic (post-1800) period was triggered by the conversion of native forest to pasture, are summarized in this paper. The textural data presented are unique for a field situation, not only because of the spatial resolution and extent of the sampling program but also because they provide information about the pattern of fining at different points in time. They are supported by equally comprehensive topographic survey data from which local rates of aggradation can be derived. Despite variability induced by lateral sediment inputs, there is an essentially continuous pattern of fining along the entire length of the river. Fining occurs in both the fine and coarse size fractions of the bed material. The highest rates of fining occur in the larger percentiles of the subsurface bed material and in the surface bed material. Downstream fining in the Waipaoa River appears to be a response to changes in flow hydraulics that are regulated by the concave configuration of the long profile. The fining gradient developed rapidly (in <45 years). It does not appear to be influenced by the rate of aggradation (nor the overall rate of sediment supply to the channel system), because, in the short term, aggradation has a negligible impact on the inherited form of the long profile.
[1] We derive a sediment budget for Te Weraroa Stream, New Zealand, the principal drainage in a small (29 km 2 ) steepland catchment where gully erosion, triggered by conversion to pasture early in the twentieth century, was ameliorated by reforestation that commenced in 1962. Estimates of sediment production were made using the change in gully area observed in sequential aerial photographs. Channel storage was assessed from stream cross-section surveys. At its peak, gully erosion affected $6% of the total catchment area. The amount of sediment contributed from gullies declined by 62% as the forest became established, but of the 28.7 Mt of sediment generated by gully erosion between 1950 and 1988, 48% was stored in the channel along the lower 8 km of Te Weraroa Stream. Even if the amount of sediment generated by gully erosion continues to decline, it likely will be many decades before the gravel is released from storage.
Rapid vertical accretion on the Waipaoa River floodplain is conditioned by the river's high suspended sediment load (30 000-40 000 mg 1-1 at flood stage). Cumulative sediment accumulation curves derived from three cores suggest an average (post-1850) rate of vertical accretion of c. 60 mm a -1, though a 15 year lacuna in flood activity has depressed the post-1948 rate to c. 40 mma -1. Rates of aggradation during floods are several orders of magnitude larger than the time-averaged rate. Within a 44 km long reach, cross-section surveys indicate that 0.2-0.8 m of sediment was deposited between 1979 and 1990. Over this period floodplain storage accounted for 5% of the total suspended sediment load, and 16% of the suspended sediment load transported during events that exceeded bankfull stage. The Waipaoa River floodplain may be representative of floodplains bordering rivers with high suspended sediment loads, produced by rapid, episodic vertical accretion, on which overbank deposition occurs across the entire floodplain, and is complemented by channel aggradation. Such rivers are able to construct high banks. Thus channel capacities are greater and the incidence of overbank flows is less than in rivers where overbank deposition is slow relative to the rate of floodplain destruction by lateral migration. The difference between our time-averaged estimate for sequestration on the Waipaoa River floodplain and comparable estimates for actively meandering rivers, and meandering rivers with low sediment loads, reinforces the notion that there is a link between the sediment transport regime of a fiver and its sedimentary record. To elucidate this link it is necessary to view vertical accretion in the context of the flood events that generated it, rather than in the context of a time-averaged sediment budget.
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