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.

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[1] We compute the particulate organic carbon (POC) yield of the Waipaoa River, New Zealand, using sediment rating curves in conjunction with measurements of the carbon content of the suspended sediment. To ascertain the source of the carbon and the extent to which the POC flux is tied to different erosion processes, we determined the stable isotopic carbon composition (d 13 C) and carbon to nitrogen (C/N) ratio of weathered bedrock, soil, and regolith. Most POC is derived from suspended sediment generated by gully erosion (incision into weathered bedrock), supplemented by landsliding during extreme events. The specific yield of POC from the headwaters (drainage area 1580 km 2 ) is 55 g m À2 yr À1 , which is very high by global standards and by comparison with other turbid steep-land rivers. The annual loss to floodplain storage is $4% (3.6 Kt) of the mean annual POC yield (86.7 Kt). Thus the Waipaoa River is a very effective conduit for transporting POC to the ocean.

High-resolution digital elevation models (DEMs) were derived from sequential aerial photography of an active fluvio-mass movement (gully) complex in New Zealand's North Island East Coast region, to measure geomorphic changes over approximately one year. The gully showed a complex behaviour, combining fluvial and mass movement erosion, deposition, and reworking of materials stored in an active debris fan. During the measurement period 5200 ± 1700 m 3 of material were eroded from the 8·7 ha gully complex and 670 ± 180 m 3 from the 0·8 ha depositional fan, giving a total of 5870 ± 1710 m 3 for the entire gully complex-fan system. The results provide a high-resolution description of gully behaviour over a short time period, and also demonstrate that mass movement (slumping and debris flows) accounted for almost 90 per cent (4660 ± 200 m 3 ) of the sediment generated. This erosional response is described in terms of gully evolution by comparing the gully complex to other systems in the region in various stages of development. The effect of gully evolution on geomorphic coupling between the gully complex and channel system is described, and coupling is also shown to vary with the magnitude and frequency of rainfall events.From a land management perspective the success of strategies, such as tree planting, to mitigate against gully erosion depends on the stage of gully development -particularly on whether or not mass movement erosion has begun. In contrast to gully rehabilitation efforts elsewhere, basin-wide afforestation in the early stages of gully incision is favoured over riparian planting, given that mass movement assisted by excessive groundwater pressure is the main process leading to uncontrollable gully expansion.To protect land effectively against continuing gully erosion of headwater catchments and resulting downstream aggradation, it is necessary for land managers to understand the spatial and temporal variability of gully development fully so that mitigation efforts can be targeted appropriately.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The University of Chicago Press is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Geology. A B S T R A C TSteepland rivers draining small, coastal watersheds often have very high suspended sediment loads, but the event characteristics of suspended sediment concentration and yield in this class of river is not well documented. Continuous monitoring at four sites in the Waipaoa River basin, New Zealand, demonstrates that during individual and composite events, suspended sediment concentration versus water discharge relations typically show clockwise hysteresis and that event maximum concentration is poorly related to event peak discharge. The signature of different erosion processes is also imprinted on the event yield magnitude frequency distributions. Gully-dominated tributary basins produce relatively high yields at all frequencies, reflecting greater sediment availability, whereas in tributary basins, where shallow landsliding is the dominant erosion process, there is a steep increase in yields in relation to return period. We estimate that flood discharges from the Waipaoa River approach or exceed the critical suspended sediment concentration (∼40,000 mg L Ϫ1 ) for hyperpycnal plume generation (because of negative buoyancy) at the river mouth once every ∼40 yr, but in the neighboring Waiapu and Uawa Rivers, the threshold concentration may be exceeded once a year and two to three times a year, respectively.