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Monitoring Sediment Erosion, Transport, and Deposition
Water and Sediment Discharge Records
The USGS, in cooperation with the U.S. Army Corps of Engineers (USACE), operates four gaging stations to measure surface water flow and amount of suspended sediment in the rivers draining Mount St. Helens. The stations measure river water surface elevation (or stage) every 15 minutes, which can be calculated to determine the volume of water transported over time. Suspended sediment samples are regularly collected and analyzed. Daily values are published in annual water data reports. Provisional data for these stations are available through the Mount St. Helens monitoring map.
Turbidity as a Surrogate for Suspended Sediment
Turbidity probes determine the murkiness of water by comparing the amount of light transmitted to the amount of light received. The measured values can be related to how much sediment is in the water. At Mount St. Helens, measurements are recorded every 15 minutes to determine sediment concentration; an increase in sediment load could signal a lahar or debris flow is occurring. Daily values are published in annual water data reports. Provisional data for these stations are available through the Mount St. Helens monitoring map.
It is important for geologists to understand how rivers and streams change over long periods of time in order identify areas of erosion and deposition, changes to channel location and geometry, and to compute the amount of sediment that is transported through drainage basins.
Channel Cross Section Network
Since 1980, more than 300 fixed locations to measure cross sections of stream channel have been established along drainages around Mount St. Helens. Several of these are measured regularly, which provide very precise documentation of changes to bank and channel elevation and data about changes that happen over time. Whenever feasible, profiles along the stream channel, rather than across, are also surveyed to evaluate changes to the channel-bed slope or migration of material downstream.
Remotely-sensed data are collected using measuring devices that are not in direct contact with the objects being measured (e.g. lasers, light). Data collected remotely are becoming increasingly valuable to monitor hydrologic and topographic change at Mount St. Helens. Digital elevation models (DEMs) are derived from light detection and ranging (LiDAR) data, photogrammetric data, and satellite imagery. These data have provided new understanding of how water and geologic features change.
Gaging stations on Spirit and Castle Lakes are operated by the USGS–CVO in cooperation with the U.S. Army Corps of Engineers (USACE). These gages determine lake elevation, which is monitored by the National Weather Service as part of a warning system that will signal a debris dam failure. Data are recorded every 15 minutes and can be noisy due to wind-generated waves and lake seiche. Daily values are published in annual water data reports.
Recent surveys of the below-water-level topography at Spirit and Castle Lakes allow scientists, land managers, and flood forecasters to relate measured lake elevation or surface area to the volume of water in the lake.
Provisional data for these stations are available through the Mount St. Helens monitoring map.
Lahars and debris flows are monitored along the North and South Fork Toutle River valleys. Acoustic flow monitoring (AFM) stations measure ground vibration caused by passing lahars or debris flows. Data are recorded every second and continuously sent to CVO to provide real-time monitoring of lahar activity. The stations are also part of a lake breakout warning system in the event of debris dam failure.