From Big River Magazine
Big River is an independent magazine about the people, places, life and events on the Upper Mississippi River.
Before locks and dams were built on the Mississippi River, water levels took care of themselves. In the spring, and sometimes the fall, water ran high, swallowing islands and flooding into low-lying areas. During hot summers, many channels dried up. But since the river was transformed into a series of pools confined by locks and dams and levees, someone has had to manage the water levels.
Today it takes the coordinated work of experts in three federal agencies using satellite and internet communications, electronic monitoring equipment and powerful computers to forecast, monitor and manage the water levels in the Upper Mississippi. The Army Corps of Engineers, the National Weather Service (NWS) and the U.S. Geological Survey (USGS) collaborate on this project 365 days a year.
Many people depend on the results. Shipping companies need a nine-foot channel to float their towboats and barges. Boaters want to be able to get out of their marinas. Property owners, river towns, railroads, utilities and farms need the river to stay within its defined banks. And a lot more people — county and city engineers, emergency response agencies — need to know if and when it will flood.
Daily Data Grind
In the next few hours, both the Corps and the NWS collect and exchange the most recent predictions about weather and precipitation for the watershed, from NWS; the flow forecast for each dam for the next five days, from the Corps; and the water levels of the tributaries in the 189,000 square miles of the Upper Mississippi watershed, from the USGS. They run this data through computer models that predict how water in each pool of the river is likely to behave in the next 24 hours.
“It’s good to use more than one model,” said Jeff Zogg, hydrologist at the NWS in Davenport, Iowa. “That way we can compare results and make adjustments. Sometimes they pick up something we don’t see, and sometimes we see something coming up that they don’t.”
The two agencies call to compare notes and make adjustments, if needed, until the two forecasts agree.
“We always have to mutually agree on the numbers, so we can release consistent information to the public,” said Zogg. “We started doing this after the big flood of 2001.” During that flood, some confusion resulted when the two agencies released slightly different predictions about crests.
Then the Corps’ St. Paul District calculates appropriate gate settings for each dam and sends them to dam operators. It also lets the Rock Island District know what to expect, so it can do its own forecasts. Rock Island, in turn, passes its forecasts and settings to the St. Louis District.
Daily instructions might read something like, “At 0800 hours, open three feet of tainter and one foot of roller.” (Each dam consists of small tainter gates and large roller gates, except for Lock and Dam 15, which is made up entirely of roller gates.)
By 10 or 11 a.m., this routine work is usually complete for the day.
Crest Coming, Crest Past
During a flood, dam gates are lifted entirely out of the water, allowing the water to run freely, but locks will keep operating for as long as possible.
“If the water gets too high for towboats to see the lock gates, we either close the locks or the Coast Guard orders them closed,” said Ferris Chamberlin, water control chief of the St. Paul District of the Corps. “Forecasting helps the tow industry. They need to know when the crests will be, and when the water is going to fall, so they can start shipping again.”
When the water gets very low, as during late summer, the dam gates may be closed completely, turning the pool into an enclosed lake and making water levels flat. The rest of the time the gates of each dam are adjusted to increase or decrease the flow of water into the next pool.
“On the Mississippi, we try to match, not modify, the natural flow of the river,” said Jim Stiman, water control chief of the Corps’ Rock Island District. “The outflow of water from each pool equals the inflow. Most pools have no capacity to store extra water.”
The situation is very different on rivers like the Missouri River, he said. There, dams hold back vast amounts of water in huge reservoirs, and there’s a lot of discussion about how much water gets held back or released.
To measure water in the river, the Corps uses “pool gages” on the upstream side of each dam, “tailwater gages” on the downstream side and “midpoint gages” in the middle of the pools. The Rock Island District manages pool levels to stay within a six-inch-wide band on the pool gages; St. Paul manages to within a 0.4 foot band on the midpoint gages that are used in every pool, except Pool 7. As long as water levels stay within this band, towboats and barges will have at least nine feet of water in the Main Channel.
One person at each dam, called the pool regulator, is responsible for making sure water levels stay within the band.
“Pool regulators carry cell phones 24 hours a day, seven days a week. If a big storm causes the pool level to exceed its limit, the regulator can get more information and change those gate settings. Then he calls the downstream dams to let them know, so they can adapt to the change,” said Stiman.
To track how much water is on its way from the tributaries, the NWS relies on an extensive network of gages maintained by the USGS. The gages measure water depth and flow, then relay the information via satellite.
John Halquist of the NWS North Central River Forecast Center in Chanhassen, Minn., summed up the NWS’s job this way: “We take the precipitation as it falls through the air and follow it until it exits the system at Cape Girardeau, Missouri.” The North Central River Forecast Center is one of 13 in the United States. It creates the hydrologic models that are used to make forecasts.
Even with sophisticated technology and computers, it’s difficult to predict how much water is about to fall.
“It’s still a significant challenge to have an accurate prediction of how much and where rain will fall in the future. If we think one-half inch will fall, and we use this figure, but it doesn’t rain that much, the prediction will be off,” said Halquist.
Like the weather forecasts that many people check once or twice a day on the internet, river data is updated hourly and posted to NWS and Corps websites. When something goes on upstream or upriver, you can see how it affects the river immediately.
Copyright 2010 Open River Press