In the News

Curious Iowa: How are river levels measured?

November 27, 2023   The Gazette

View Source

As much of Iowa experiences drought conditions, extra attention has turned to low river levels in the state. According to Iowa DNR’s October water summary update, Iowa has seen improvement in drought conditions but it needs normal to above normal rainfall heading into the winter months.


Curious Iowan Bob Peterson of Cedar Rapids wrote to The Gazette’s Curious Iowa, wondering how river levels are measured. Peterson said at times he can see rocks at the bottom of a river despite the river stage being recorded at 3 feet.


Curious Iowa is a series from The Gazette that seeks to answer Iowans’ questions about the state, its culture and the people who live here. The Gazette spoke with Quad Cities National Weather Service Hydrologist Matt Wilson to answer Peterson’s questions.


What tools are used to measure river stages?

Wilson said that measuring a river stage is like measuring the height. That measurement is used to translate stage into flow. According to the Environmental Protection Agency, flow impacts water quality and which organisms can live in a river. Large, flowing rivers can dilute pollution more effectively than small, stagnant bodies of water.


Wilson said there are a few tools used to measure river stages: a wire weight, a staff gauge, a radar gauge and a pressure transducer.


If you drive along bridges over rivers, you may notice a wire weight box attached to the bridge. Those boxes hold a metal winch with a weight on the bottom of it. Someone gathering the water level will open the box and lower the weight down until it hits the top of the water.


A staff gauge is like a large ruler that measures water surface elevation.


Wire weights and staff gauges are manual measurement tools that are cheap to install and maintain. Because these are manual measurement tools, the readings are subject to human error.


“So we prefer some of our more high tech methods, but we still use those in locations where either flooding is infrequent or funding for an automated gauge just isn’t available.” Wilson said.


A radar gauge is an automated gauge attached to the side of a bridge.


“It shoots a radar beam down to the top of the river, it reflects off the top of the river and back up to the radar sensor,” Wilson said. “And so using the calculations that the algorithm runs behind the scenes in the radar, it tells you how far it is down to the river level and back.”


The third method used to measure water stages is called a pressure transducer. If you’ve seen a little house along a river with an antenna on top, that’s a U.S. Geological Survey house. Pressure transducers run a small tube from the gauge house into the river bed underneath the middle point of the river. Air is pushed through to release a small bubble at the end of the tube. Wilson said the amount of pressure required to release a bubble can be used in a mathematical equation that will show how much pressure is applied to the tube.


“And if we know how much pressure is being applied to that tube, we can tell how much water is on top of that because we know the density of water and then we can tell the thickness of water,” Wilson said. “So basically, instead of measuring from the bridge down to the top of the water, we're measuring from the bottom of the river to the top of the river.”


River gauges can be damaged or vandalized. Although newer gauge houses and bridges are built to withstand current record flooding, old locations may experience submersion. Wilson said erosion commonly damages pressure transducer gauges.


Who measures river stages?

Measuring river stages is a large undertaking requiring many hydrologists and hydrologic technicians to collect data from hundreds of sites around the state. Wilson said the National Weather Service partners with agencies like the U.S. Geological Service (USGS), Army Corps of Engineers and Iowa Flood Center to maintain gauges and share data. Wilson called this system “a self-organized cooperative.”


Wilson estimated that there are almost 500 locations monitored in the state of Iowa between the Army Corps of Engineers, USGS and Iowa Flood Center.


Why can a river stage be recorded as negative?

Wilson explained that river beds change over time and almost all river stages are based on a local datum, or a historical data point. That can be why our visual perception of a river stage may differ from what a reading says, like Peterson seeing rocks in the river bed despite the river stage being recorded at a few feet.


If a river stage is recorded as negative it is because the gauge is reading at or below a historical datum which is the agreed upon zero level. Wilson said data can be 50 to 100 years old.


“Usually it was attempted to be the bottom of the river bed, but over years river beds shift and change, so today they might not be as close as they once were,” Wilson said.


He gave the example of the Mississippi River at St. Louis being recorded as -1.85 feet at the time of The Gazette’s interview with him in October.


“No, the river is not dry there either, in fact there is over 80,000 cubic feet per second of flow at that point,” Wilson said. “It is just that the 0 foot datum at St. Louis is now significantly higher than the stream bed.”


Why do we measure river stages?

Wilson said the National Weather Service’s mission is to protect lives and property and one way to do that is by monitoring water levels.


“We have different metrics and different levels that we look at for certain places that we know that if the river's gonna drop below a certain area that that could indicate that a water intake for a small municipality might be going above water,” Wilson said. “So being able to tell them that the river level is gonna drop below a certain point, can give them a lead time so that they can find some alternate way to get that water into their water intake so that they can continue to offer fresh drinking water to people.”


In early October, Wilson surveyed gauge locations on the Cedar River. Normally, the river is 100 to 300 feet wide in areas.


“Where I was at near Palo, the Cedar River was two little 20-, 25-foot wide sections with a bunch of sandbars in between,” Wilson said. “So you got all the wildlife and everything that would normally have the entire breadth of river to traverse and now you’ve got fish, turtles, amphibians and such kind of pocketed in a couple of little narrow channels.”


He explained the low water levels have led to debris buildup.


“You end up building these little artificial dams here and there, which once you do get a good rain, those can actually back up rivers, especially smaller streams,” Wilson said. “You can get those built up and you can get flooding in places.”


The Gazette previously reported that Iowa’s wetter-than-normal October improved drought conditions. In an October water summary update news release, the Iowa DNR’s Hydrology Resources Coordinator Tim Hall said, “We continue to need normal to above normal rainfall across nearly all of the state to get us into better shape headed into next spring.”


On Nov. 18, the National Weather Service Quad Cities posted to X (formerly Twitter) that the last 30 days of observed rainfall fell below normal for much of Iowa and northern Illinois.


On Nov. 19, the National Weather Service Des Moines posted to X that an 18-day no-precipitation streak had ended. The 18-day streak was the longest stretch without rain since a 20-day stretch from Sept. 30 to Oct. 19, 2015.