Channel Sediment Storage Research Articles

Sediment and wood accumulations in humid tropical headwater streams: Effects of logging and riparian buffers

The amount of sediment, wood, and detritus (leaves and small branches) accumulations were examined in four headwater tributaries subjected to different treatments (unlogged, logged with riparian buffer, and logged with no buffer) 16 and 40 months after timber harvesting in the Bukit Tarek Experimental Watershed, Peninsular Malaysia. Sediment transport from hillslope logging roads and skid trails resulted in large sediment storage in the stream channels. Riparian reserves appear to mitigate logging impacts to streams; however, more sediment was found in a tributary channel with a 20-m buffer than an unlogged channel because some of the nearby roads and skid trails were connected to the stream. Sediment storage in channels declined within 40 months after logging in catchments with and without riparian buffers. Vegetation recovery on skid trails and logging roads reduced both sediment supply and delivery. The volume of large wood was 1.5 to 2-times greater in the stream without riparian reserves than the stream of an unlogged catchment or in a stream with a riparian buffer. No consistent change in detritus accumulations was found among streams in 2001 and 2003, although more detritus was present in 2003 in all streams. The effects of logging and the effectiveness of riparian buffers appear to depend on the hydrologic connections between hillslopes and headwater streams.

Observations of Bed Load Transport and Channel Bed Changes in a Proglacial Mountain Stream

Bed load transport and channel change were measured during May to July 1987 in the proglacial stream of Bas Glacier d'Arolla, Switzerland. Over sampling periods of several hours, variations in bed load transport show little apparent relationship with discharge; however, over the 67-d study period, cross section and long profile surveys suggest that steady-state transport is maintained, except during floods. Bed load transport occurs over approximately two-thirds of the channel width, but the majority of the load is transported in bands of narrow width corresponding loosely to the maximum thread of flow. Bed load movement in narrow bands causes transport rates to vary by up to an order of magnitude within the space of 1 m (in a cross channel direction). Large errors associated with bed load sampling make it very difficult to estimate the true variability in the sediment transport process. Short-term daily changes in channel sediment storage are minor during periods of low to moderate discharge, but during floods the channel shifts dramatically from a slightly aggrading state to a state of considerable aggradation or degradation. On the basis of field observations and sediment transport measurements, a three-phase model of bed load transport is proposed for steep coarse-bed streams where bed material is highly heterogeneous. Phase one generally involves the flushing out of fines deposited in the channel during low flows; phase two transport is associated with the break-up of the gravel-bed surface layer; and phase three transport results in the destruction of the step/pool topography during flood.

Channel evolution and hydrologic variations in the Colorado River basin: Factors influencing sediment and salt loads

Suspended-sediment and dissolved-solid (salt) loads decreased after the early 1940s in the Colorado Plateau portion of the Colorado River basin, although discharge of major rivers — the Colorado, Green and San Juan — did not change significantly. This decline followed a period of high sediment yield caused by arroyo cutting. Reduced sediment loads have previously been explained by a change in sediment sampling procedures or changes in climate, land-use and conservation practices. More recent work has revealed that both decreased sediment production and sediment storage in channels of tributary basins produced the decline of sediment and salt loads. Sediment production and sediment storage are important components of incised-channel evolution, which involves sequential channel deepening, widening and finally floodplain formation. Accordingly, the widespread arroyo incision of the late nineteenth century resulted initially in high sediment loads. Since then, loads have decreased as incised channels (arroyos) have stabilized and begun to aggrade. However, during the 1940s, a period of low peak discharges permitted vegetational colonization of the valley floors, which further reduced sediment loads and promoted channel stabilization. This explanation is supported by experimental studies and field observations. Both geomorphic and hydrologic factors contributed to sediment storage and decreased sediment and salt loads in the upper Colorado River basin.

Impact of riverine wetlands construction and operation on stream channel stability: Conceptual framework for geomorphic assessment

Wetland conservation is a critical environmental management issue. An emerging approach to this issue involves the construction of wetland environments. Because our understanding of wetlands function is incomplete and such projects must be monitored closely because they may have unanticipated impacts on ecological, hydrological, and geomorphological systems. Assessment of project-related impacts on stream channel stability is an important component of riverine wetlands construction and operation because enhanced erosion or deposition associated with unstable rivers can lead to loss of property, reductions in channel capacity, and degradation of water quality, aquatic habitat, and riparian aesthetics. The water/sediment budget concept provides a scientific framework for evaluating the impact of riverine wetlands construction and operation on stream channel stability. This concept is based on the principle of conservation of mass, i.e., the total amount of water and sediment moving through a specific reach of river must be conserved. Long-term measurements of channel sediment storage and other water/sediment budget components provide the basis for distinguishing between project-related impacts and those resulting from other causes. Changes in channel sediment storage that occur as a result of changes in internal inputs of water or sediment signal a project-related impact, whereas those associated with changes in upstream or tributary inputs denote a change in environmental conditions elsewhere in the watershed. A geomorphic assessment program based on the water/sediment budget concept has been implemented at the site of the Des Plaines River Wetlands Demonstration Projection near Chicago, Illinois, USA. Channel sediment storage changed little during the initial construction phase, suggesting that thus far the project has not affected stream channel stability.