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Channel surveys provide insight into sediment transport and landscape evolution

By Volcano Hazards Program
Sediment-laden White River 11 km downstream of the OR35 bridge. Hea...
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Sediment-laden White River 11 km downstream of the OR35 bridge. Heavy rains resulted in high-sediment loads within this river, which drains Mount Hood, Oregon.

Volcanically disturbed areas are highly susceptible to erosion and redistribution of material – therefore the landscape changes frequently. This requires intensive, long-term monitoring to document sediment type and size, identify areas where sediment is eroded and deposited, and determine the rate and volume of sediment transport. These sediment data are incorporated into computer models to design projects that reduce the impact of sediment redistribution and estimate timelines for channels to fill to dangerous levels. Monitoring surveys are conducted in different ways:

 

  1. Scientists perform on-the-ground measurements of the same stream channel cross sections repeatedly over many years. This is done with surveying instruments that measure distance and angle to a reflector, or with GPS equipment using satellite positioning. Scientists compare resulting cross-sectional profiles through time to assess channel erosion and deposition.
  2. Digital elevation models (DEMs), which produce highly accurate maps of the ground surface, can be created using digital photographs or lidar technology. Such models created at different times can be compared to accurately define areas of erosion and deposition. Data collected remotely permit detailed analysis of basin-scale changes and deeper understanding of how water and geologic features change.

 

 

Annual surveys of water channels in the crater of Mount St. Helens ...
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Annual surveys of water channels in the crater of Mount St. Helens track rates of erosion, aggradation.
Suspended sediment sample collection from the North Fork Toutle Riv...
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Suspended sediment sample collection from the North Fork Toutle River above the Sediment Retention Structure, downstream from Mount St. Helens.

Tracking the amount of sediment and the flow rate of rivers helps mitigate flooding hazards

Instruments at gaging stations measure the water level, or stage, of rivers every 15 minutes, and automated pumping samplers collect frequent water samples for laboratory analysis. By combining a continuous record of a river's stage with frequent measurements of water discharge and suspended-sediment concentration, scientists calculate the total water and suspended-sediment discharge to produce daily, monthly, and annual totals. Long-term records of water and sediment discharge provide key statistics for use in design of mitigation structures such as dams and flood control levees. Technologies using light transmission (to measure turbidity), sound (hydroacoustics), and imagery (from satellites or digital cameras) promise increased accuracy and efficiency in documenting sediment loads.