Proceedings of the U.S. Geological Survey (USGS) Sediment Workshop, February 4-7, 1997


G.F. Koltun, M.N. Landers, K.M. Nolan, and R.S. Parker

Information needs for sediment data are changing rapidly. Historically, the Water Resources Division provided data and analyses regarding sediment transport and geomorphology issues that dealt primarily with engineering structures in the river environment. Indeed, these early sediment studies contributed substantially to the reputation of the United States Geological Survey (USGS) as a leading agency for assessing the Nation's water resources. Today, the most pressing sediment-related problems are associated with environmental questions such as the transport and fate of attached pollutants, effects of sediment on aquatic biota and their habitat, and changes in sediment transport related to land use change. In short, current sediment issues require that sediment studies now address multiple objectives in water resources management. This change places new demands on sediment transport and geomorphology studies in the USGS and on traditional methods which served the organization so well in the past.

Present and future work in sediment transport and geomorphology in the Water Resources Division is based on the following tenets:

  1. Sediment is the largest contaminant of surface water by weight and volume and has been identified by the United States Environmental Protection Agency as the number one problem threatening America's waterways.

  2. Understanding the fate of sediment-bound pollutants requires an understanding of the transport and deposition of sediment.

  3. The physical system of a river provides the framework for understanding the chemical and biological systems. Increasingly sophisticated studies of the chemistry and ecology of river systems place additional demands on our understanding of the physical system.

  4. Hazards such as debris flows, landslides, and channel changes during floods continue to be a nationwide problem.

Listed below are some current issues confronting the USGS and some examples (listed with bullets) of on-going or recently completed projects that address those issues:

Predicting response to channel or watershed modification -- Channel and (or) watershed modifications can be identified as the primary cause of most anthropogenic sediment issues in the United States. Specific problems associated with this issue include prediction of the impacts of urbanization, agricultural practices, and stream channelization or restoration.

Instream mining -- Current issues concerning instream sand and gravel mining are the effects of extraction method, and the amount and timing of extraction on channel response.

Forest fires and floods -- Forest fires are known to play a role in altering the hydrologic regime of an area. Increased flooding and sediment transport is one common consequence. Although research funds are sparse, interest remains in the issue of off-site impacts of forest fires.

Flood plains -- Flood plains are of interest as a conveyor of water and sediment and as a repository of sediment and associated contaminants.

Regulated rivers -- Sedimentation problems on regulated rivers are considerably different from natural rivers. Problems include: sedimentation in reservoirs, channel effects downstream of dams, dam removal, and in-stream flow requirements for maintenance of channels and fisheries. Reservoirs can profoundly affect the geomorphology of streams that have a large natural sediment load, as the reservoir traps sediment and releases clear water. The resulting downstream geomorphic effects of clear water releases from dams typically includes channel instability (as the channel and banks are eroded to satisfy the sediment carrying capacity of the waters), channel bed armoring, and alteration of habitat (Collier and others, 1996).

Channel stability / biological impacts -- Channel stability has long been a impact of channel stability on structures, such as bridges, located in or near the channel. That focus has shifted somewhat in recent years to include concern for biological communities living in or near the channel. Examples of problems with biological implications are: predicting the size of flushing flows for spawning sites and maintenance of backwater areas, identifying the temporal changes in bedload transport for substrate mobility, and analyzing bank stability for riparian environments.

Cost efficient methods -- The WRD collects basic sediment data that support interpretive studies conducted by WRD and other agencies. In 1996, sediment data were collected approximately daily at 153 stations across the U.S. That number represents a decrease of about 30 percent in the number of daily sediment stations that were operated a decade ago. The reduction in basic data collection has occurred despite the fact that many difficult questions regarding sediment transport and deposition continue to be asked. One reason often given for the decline in basic data collection is the associated cost.

Sediment as a conveyor of contaminants -- Evaluation of water-quality conditions and mass transfer of chemical constituents in the Nation's rivers are important parts of the USGS programs. Fluvial sediment is possibly the continentŐs most important contaminant and the medium by which large amounts of toxic substances are transported. The association between the transport of trace metals and sediment is so strong that the seasonal and annual transport of most trace metals can be estimated from sediment flux.

Sediment-associated contaminant transport is of concern within all four WRD regions. Nutrients, metals, pesticides, organics, and (or) radionuclides bound to sediments pose ecological and human health risks in watersheds throughout the Country. Several active interpretive studies in the U.S. have the determination of sediment and associated contaminant loads as a major element. The objectives of these studies are varied and include such goals as to (1) characterize the hydrogeology and water chemistry of watersheds, (2) assess the trophic state of lakes, and (3) provide information on which to base stream restoration strategies.

Natural hazards -- Life-threatening hazards that result from the occurrence of rare events remain an important focus of the USGS. The public relies on the USGS to assess hazards before, during, and after they occur. Such hazards include debris flows triggered by heavy rainfall or following volcanic eruptions, landslides on slopes destabilized by road or building construction, dam-break floods, surging glaciers, and earthquakes.

Coastal processes -- Erosion of coastal areas and loss of adjacent wetlands are important management issues. Accelerated coastal erosion results from both natural and man-induced factors including: saltwater intrusion, subsidence, sea level rise, wind-blown surges, upstream channel modification, mineral extraction activities, navigation and drainage projects, and agricultural practices. Information integral to the understanding of this erosion problem is the movement and storage of suspended sediment in the coastal estuaries and slow-moving rivers that border the coast.

Wetlands interactions -- The maintenance of wetlands and their role in trapping and removing sediment and other contaminants are important water resource issues. Determination of wetland sediment budgets is paramount to our ability to understand and maintain wetland functions.

Current sediment issues, such as those discussed above, suggest that much remains to be learned within the topic areas of sediment and geomorphology. Consequently, there is continued need for high-quality sediment data of both a long-term and short-term nature. In order to better address current (and future) sediment and geomorphology issues, we must seek out improved methods of data collection and analysis to meet the broader sampling and data requirements imposed by their multidisciplinary nature. At the same time, we must pursue every efficiency in order to gather necessary data and operate within the present budgetary climate.


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