Groundwater and Flood Monitoring
It is the mission of the U.S. Geological Survey to monitor water resources across the country. We talk with Chris Wilkowske, a hydrologist with the USGS at the Utah Water Science Center, about the science and methodology of measuring stream flow. We talk about how stream gauges operate and the process of validating and adjusting data, particularly after events like floods. Local examples from the Moab area and the Colorado River highlight the dual purposes of stream gauges for flood monitoring and groundwater assessments.
Meet the Scientist: Chris Wilkowske
Hydrologic Technicians are the backbone of the USGS mission of collecting reliable water data. Chris is a senior technician providing guidance to the Moab Field Office. He developed the center’s quality assurance plan and continues to be pro-active with continued development and oversight.
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Interview Excerpt - Monitoring Stream Flow
Today we’re talking about measuring stream flow with Chris Wilkowske, a hydrologist with the US Geological Survey. Chris is part of the Utah Water Science Center based here in Moab.
Science Moab: How do you measure the stream flow?
Wilkowske: There’s two really two things that go into stream flow measurements. First thing that a stream gauge does is measure the height of the water, the stage of the river or the creek. It does this by measuring the pressure of the water and then determining the height of the water over that instrument.
For the second measurement, we use an acoustic Doppler current profiler. It uses the Doppler effect just like radar, but it uses sonar instead of radar to penetrate the water column. We drag it back and forth across the river and it gives us a whole profile of what the velocity is in the river. So when we’re measuring stream flow, we’re measuring the width, the depth, and then the velocity of the river.
The stream gauge is designed to be automatic. It’s recording all the time and it transmits that data via satellite. All that is doing is telling us how high the river is. We use that data to tell us what the flow is. We make a relationship table that says if the stream is 10 feet high, that equals a hundred CFS. If the stream is 12 feet high, that equals 200 CFS like that. In order to make that table, we’re going out at one point in time with our instrument measuring the actual flow.
For instance, with the Colorado River, we’d start in the early spring when the water’s low, and then try to measure right at the peak. We put all those measurements in the table.
Then, as the stream gauge works, it uses the table to calculate CFS from the height of the water. When a stream gauge is in operation, we’ll continue to make flow measurements at least every six weeks through the life of the stream gauge. So some of our long-term stream gauges have thousands of measurements.
Science Moab: So your concept of the width and depth of the channel are consistent unless there’s some kind of flood event or a reshaping of the channel where you might have to go out and measure that again.
Wilkowske: Exactly. That’s why we measure it on a set schedule, because river channels are dynamic. We want to be reporting the flow as accurately as we can, so we go out there on a routine basis, remeasure it, adjust that table if need be. On our webpages, we have a statement that says provisional data, subject to change because it is subject to change.
Science Moab: how do you decide where, in a stream or any waterway where to put a gauge? What factors do you consider?
Wilkowske: The main thing that we look for is something that we call the control.
you can think of a control, like you think of a series of rapids. You might have a pool and drop on the rapids, we would put the gauge in the pool. And then the actual rapid or the drop is that would be the control. So the rapid or a riffle or some kind of cobble bar or something like that.
It’s a feature in the river that’s backing up water behind it. Hopefully it’s something that’s fairly stable over time. So that’s what we look for.
Science Moab: if we look locally here in the Moab area, we have both Mill and Pack Creeks that have had pretty substantial floods in the last few years. What have you had to go out and do for those two creeks specifically in terms of gauges?
Wilkowske: We made a lot of corrections, especially on Pack Creek, but Mill Creek as well. After the first floods in 2022, we surveyed high watermarks and cross sections of the creek. We put that data into a simple flow model which computes a theoretical value of discharge…because we weren’t able to get out there during the flood event and measure it with our equipment.
So we were able to get numbers on those peaks back in 2022, and we used that to build our stage discharge rating table. They’re called indirect or slope area measurements because they take into effect the slope of the stream and then the cross-sectional area. But we’ve done several of those measurements to try to quantify the peaks on PackCreek.
So then when the flows came in 2024, we had done that work in the past, so we were ready. The flows in 2024 were of similar magnitude, so then we could use that relationship.
Science Moab: Like you said, these are pretty simple instruments. Have they changed much over time?
Wilkowske: The basic fundamentals of stream gauging have really remained consistent since they were first developed during the John Wesley Powell era really is when the stream gauging program with the USGS started. It was Powell’s idea to quantify the water resources of the West, and he thought we needed a unified, consistent way of doing this.
So they came up with these methods. They’ve evolved a little bit over time as technology has changed, but the fundamental practice is more or less the same. We measure how high the river is and we build a table and we measure the velocity in the stream channel. We measure the width and depth and compute flow.
That really hasn’t changed.
