In 2010, US scientists released dye into Montana’s Missouri River and tracked it across 41.2 miles; one reservoir held the plume for 8.2 hours, helping predict chemical-spill timing
A river can carry a contaminant far beyond the point where it enters the water, but predicting exactly how quickly that material will travel is not always straightforward. In 2010, scientists with the U.S. Geological Survey (USGS), working with the Montana Department of Environmental Quality, conducted a dye-tracing experiment on a 41.2-mile stretch of Montana’s Missouri River to better understand how substances move through the waterway. According to the USGS report, researchers released rhodamine WT dye at two locations and monitored its movement downstream. The experiment revealed significant differences in how quickly the dye travelled through individual sections of the river, including an 8.2-hour residence time in Toston Reservoir.
Tracing the river’s movement
The study focused on the upper Missouri River upstream from Canyon Ferry Lake. The researchers examined a 41.2-mile reach extending from the Missouri River Headwaters near Trident, Montana, downstream to the U.S. Route 12 Bridge near Townsend. Rather than releasing a harmful substance, scientists used rhodamine WT (RWT), a dye commonly employed as a tracer in water studies. The dye was injected at two locations during August and September 2010: Missouri River Headwaters State Park and Broadwater-Missouri Dam, also known as Toston Dam. Researchers then measured dye concentrations at downstream monitoring locations. At the time of the experiments, river flows were relatively steady, ranging from about 3,070 to 3,700 cubic feet per second. This provided researchers with conditions under which they could examine how quickly the dye plume moved and how it spread through the river.
Not all sections moved equally fast
One of the important findings was that the dye did not travel at a single, uniform speed throughout the study area. Researchers calculated the movement of different portions of the dye plume, including its leading edge, peak concentration, center, and trailing edge. The velocity of the plume’s center ranged from 0.80 to 3.02 feet per second across different sections of the river.Outside the reservoir section, the mean velocity for the study reach was calculated at 2.87 feet per second. But the section between the Milwaukee Railroad bridge near Lombard and Broadwater Dam behaved very differently. There, the dye’s centroid moved at just 0.80 feet per second. The slower movement was associated with Toston Reservoir, where the dye remained for approximately 8.2 hours during the experiment. That difference matters because a contaminant entering a river does not necessarily move downstream like a solid object travelling along a straight path. River flow, reservoirs and the physical characteristics of individual reaches can influence how quickly a substance moves and spreads.
Measuring the spread
The USGS study also examined longitudinal dispersion, or the way the dye plume spreads along the direction of the river. Estimated dispersion rates varied substantially across the study area. They ranged from 0.72 feet per second in the section between the Milwaukee Bridge and Broadwater Dam to 2.26 feet per second in the section between the U.S. Route 287 Bypass Bridge north of Toston and Yorks Islands. In practical terms, this means a substance entering the river can become distributed over a longer stretch of water rather than remaining concentrated in one compact location. Understanding that spreading pattern is important when trying to estimate when a contaminant might reach downstream locations and how its concentration could change along the way.
Turning dye data into spill planning
The experiment was ultimately about more than following a coloured plume through the Missouri River. One of its major purposes was to generate information that could help authorities respond to accidental contaminant or chemical spills. The Missouri River is an important water resource in Montana, supplying water for communities and supporting irrigation and recreation. Because transportation and other activities occur around the river, accidental releases of contaminants are a potential concern. The researchers established a relationship between the travel time of the dye’s peak concentration and the amount of time required for the plume to pass a monitoring location. According to the USGS, that relationship can help estimate when the concentration of a potential contaminant would decline to 10% of its peak concentration following an accidental spill.The 2010 experiment therefore provided something difficult to obtain from theory alone: a real-world picture of how a substance moves through a complex river system. By tracking a harmless dye across 41.2 miles and observing how reservoirs and river sections altered its movement, scientists were able to build information that can support water-quality models and emergency-response planning for the upper Missouri River.
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