Frequently Asked Questions

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1. How do I obtain sandbags or other flood-fighting material from the Corps?

ANSWER: The U.S. Army Corps of Engineers is only authorized to provide federal flood-fighting supplies such as sandbags or HESCO barriers, polyethylene and pumps during an officially declared flood emergency.  Obtaining Corps supplies during an emergency event can be done through your local emergency management officials, who also follow the federal guidelines to obtain any supplies.

Over the years, the Rock Island District has provided supplies for flood-fighting efforts and will continue to provide them when warranted.  Here are some totals for sandbags provided for three past major flood events:

          2010: 1,630,700 sandbags (valued at $391,368)

          2008: 13,485,000 sandbags (valued at $2,966,700)

          1993: 13,790,000 sandbags (valued at $3,483,000)

2: I keep hearing that the Corps of Engineers is predicting the river to reach a certain level on a certain date.  Does the Corps provide river level forecasts to the public?

ANSWER: The Corps of Engineers does not issue flood forecasts for any river, anywhere in the United States.

The National Weather Service provides weather, hydrologic, and climate forecasts and warnings for the United States.  The website,, is a Corps-managed site that provides National Weather Service river forecasting information and is linked in the left index under River Levels & Forecasts.

3. Why doesn’t the Corps of Engineers raise the gates on the Mississippi River dams to reduce the pool levels (lower the river) before the snow melt and spring rains to reduce the amount of flooding?

ANSWER:  There are many reasons why this idea is not practical.

     The first, and foremost reason is that Mississippi River navigation dams were constructed to maintain a 9-foot-deep channel for the safe transport of commodities by commercial tows and NOT to prevent or cause flooding. Navigation dams have no flood control capabilities and even if Mississippi River dams were used to try and control flooding, the pool created behind each dam would be so enormous that it would flood many communities.

     The reason navigation dams cannot assist with flood control is because the pools behind them do not contain sufficient storage capacity to accommodate flooding events. Raising the gates out of the water in advance of a predicted flood, would have no beneficial effect with regard to lowering the ultimate crest of a potential flood, but would have negative impacts to navigation and other infrastructure.

     This is supported by the Iowa Institute for Hydraulic Research, University of Iowa, in a study conducted in August 1969 following the 1965 flood when the operation of the locks and dams came into question. The study, Effects of Navigation-Dam Operating Procedures on Mississippi River Flood Levels, was conducted to determine the effects of lowering the Mississippi River pools in advance of a flood. The study results state that "the procedure established by the Corps of Engineers for operating the gates at the navigation dams causes no increase in the crest levels of large floods over that which would occur if the gates were completely opened well in advance of the flood."

     Even if each pool was completely emptied prior to an anticipated heavy runoff period, it would take only a matter of hours to refill them and this would not appreciably lower the peak river stages reached by the flood. This is because the amount of storage that could be made available by pool draw downs is extremely small in comparison with flood volumes.

     It is normal to think of a dam as a huge solid structure used to block the flow of a river and form a lake. However, this is not true of navigation dams, like those on the Upper Mississippi River. These dams are not solid but are a series of concrete piers across the river with movable Tainter or roller gates between the piers.

     The rollers and gates at Locks and Dam 15, as well as all other navigation dams, do not reach from the bed (bottom) of the river to above the water line. They are lowered into a portion of the river to cause the water level upstream of the dam to rise and form a slack-water pool deep enough for navigation. The rollers and gates on the dams restrict water flow near the top of the river; in essence, they put a drag on the flow to keep the pool behind the dam high enough to maintain the 9-foot deep navigation channel. While maintaining the 9-foot navigation channel all river flow passes through the dam under the gates.

     The Corps is required by law to maintain the navigation channel and must maintain it to within +/- one-half foot. There are times, usually in spring, when the natural flow provides a channel deep enough for navigation without the use of the dams, such as we are experiencing in certain Mississippi River pools this spring. When this occurs, the gates between the piers are raised completely out of the water and the river flows as an open river.

     A second reason is that since the Mississippi River maintains it flow through water (rain and snow melt) draining into the river from 41 percent of the continental United States stretching from Montana to New York (31 states and two Canadian provinces), it would be very difficult to lower the river and maintain those levels.

     To attempt to lower or empty a pool would require blocking (damming) the flow of the Mississippi River upstream and the inflows from streams and smaller rivers emptying into the pool. If the river was dammed in this way, the Mississippi River, streams and smaller rivers would back up and cause flooding upstream.

     The amount of water in a navigation pool is insignificant during times of flooding and if it were possible to empty the navigation pools it would not last very long. The 1969 University of Iowa study conclusively showed that flood levels are not affected by the time gates of the dams are opened whether that is days, weeks or months before the arrival of a flood.

     Reason number three is that there are many other economic and environmental factors as to why we wouldn't lower the navigation pools. Those factors include loss of water intakes for municipal water supplies, manufacturing industries and power generating plants; loss of commercial navigation channel; impacts to docks, commercial riverboats, and recreational craft; bank sloughing and impacts to levees and shoreline; negative impacts to fish and wildlife habitat; loss of recreational opportunities (fishing, boating, etc.); to name a few.

     To attempt this would require coordination with the Fish and Wildlife Service, the U.S. Coast Guard, the Departments of Natural Resources, numerous other federal, state, local and non-government agencies, and the public; and would require compliance with a variety of federal and state regulations to include the National Environmental Policy Act, the Endangered Species Act, the Clean Water Act and the National Historic Preservation Act.

     A fourth reason why the Corps doesn't lower Mississippi River pools is that it is illegal to do so above Lock and Dam 15 in Rock Island in accordance with the Anti-Drawdown Law. This act of Congress, dated March 10, 1934, is entitled, "An act to promote the conservation of wildlife, fish, and game, and for other purposes," as amended by Public Law 732 on August 14, 1946, and again by Public Law 697 on June 19, 1948.

     The "Anti-drawdown Law" directs that in the management of facilities (including locks, dams and pools) on the Mississippi River between Rock Island, Illinois, and Minneapolis, Minnesota, administered by the U.S. Army Corps of Engineers, full consideration and recognition is to be given to the needs of fish and other wildlife resources, including habitat. To the maximum extent possible, the law directs that the Corps regulation of the navigation pools take the needs of these natural resources into account, while maintaining navigation, without causing damage to property, and without creating additional liability to the government. The law also directs that the Corps shall generally operate and maintain pool levels as though navigation were carried on throughout the year.

4.  Why doesn’t the Corps of Engineers lower the levels at its three reservoirs (Coralville Lake, Saylorville Lake and Lake Red Rock) in anticipation of spring flooding?

ANSWER: Reservoir pool levels are maintained for authorized project purposes which include flood control, water supply (Saylorville Lake only), low flow augmentation, fish and wildlife management and recreation.  Lowering pool levels an additional amount in the spring could result in bank sloughing, increases the risk for fish kills and significantly increases the potential for ice jams at the controlling works which could impact reservoir releases and cause pool levels to rise more rapidly.  Additionally, lowering reservoirs beyond their authorized level does not afford significant flood storage capacity as empty reservoirs would fill to flood storage capacity within 18 to 33 hours during flood events similar to 2008.

     Saylorville Lake in Johnston, Iowa, is maintained at the normal pool elevation of 836 feet in anticipation of spring rain and snowmelt runoff.  The normal conservation pool represents 11.5 percent (23.9 billion gallons) of Saylorville Lake’s total storage capacity of 184.7 billion gallons.  Fluctuation of the pool level results from snowmelt and rainfall entering the reservoir.

     The Saylorville Lake project encompasses 25,515 acres of land and water and provides 13 recreation sites.  An estimated 1,300,000 visits occurred in fiscal year 2010 with an economic impact of approximately $27,000,000.  Since its completion in 1977, the reservoir has prevented more than $183,500,000* in flood-related damages.

     Coralville Lake in Iowa City, Iowa, is maintained at its spring conservation pool elevation of 679 feet in anticipation of spring rain and snowmelt runoff.  The spring pool is four feet below Coralville Lake’s normal (summer) pool elevation of 683 feet and provides approximately 4 billion gallons of additional flood storage.  The spring pool represents 3.7 percent (5.11 billion gallons) of Coralville Lake’s total storage capacity of 128 billion gallons.  Fluctuation of the pool level results from snowmelt and rainfall entering the reservoir.

     The Coralville Lake project encompasses 24,591 acres of land and water and provides 11 recreation sites.  An estimated 1,138,090 visits occurred in fiscal year 2010 with an economic impact of approximately $22,400,000.  Since its completion in 1958, the reservoir has prevented more than $184,000,000* in flood-related damages.

     Lake Red Rock in Knoxville, Iowa, is maintained at its normal pool elevation of 742 feet in anticipation of spring rain and snowmelt runoff.  The normal conservation pool represents approximately 11.6 percent (61.59 billion gallons) of Lake Red Rock’s total storage capacity of 467.92 billion gallons.

     The Lake Red Rock project encompasses 50,300 acres of land and water and provides 11 recreation sites.  An estimated 741,250 visits occurred in fiscal year 2010 with an economic impact of approximately $14,300,000.  Since its completion in 1969, the reservoir has prevented more than $559,000,000* in flood-related damages.

5. What is the average speed of the Mississippi River near the Quad Cities?

ANSWER:  Mississippi River velocities vary widely depending on flow (rain and snow melt) draining into the river from 41 percent of the continental United States stretching from Montana to New York (31 states and two Canadian provinces).  Since flows vary greatly, the Rock Island District Hydrology & Hydraulics Branch provided the following estimates using the one-dimensional unsteady state flow (UNET) model for high, low, and average flows near Mississippi River Mile 487 (at Bettendorf, Iowa, and Moline, Illinois).

  • Low Velocity:  0.6 feet per second, based on Oct. 2000 low flow.
  • Average Velocity:  1.6 feet per second, based on average annual flow.
  • High Velocity:  4.9 feet per second, based on peak 1993 flood flow.

     In-depth calculations.   To calculate miles per hour, the velocities can be converted to statute miles; the type of miles per hour you would drive in your car.

  • Low Velocity:  0.6 feet per second, based on Oct. 2000 low flow = 2,160' per hour / 5,280' (statute mile) = 0.409 statute miles per hour
  • Average Velocity:  1.6 feet per second, based on average annual flow = 5,760' per hour / 5,280' (statute mile) = 1.09 statute miles per hour
  • High Velocity:  4.9 feet per second, based on peak 1993 flood flow = 17,640' per hour / 5,280' (statute mile) = 3.34 statute miles per hour
6.  What does a “100-year flood” mean?

     ANSWER: The term “100-year flood” is often used as an attempt to simplify the definition of a flood that statistically has a 1-percent chance of occurring in any given year.  Likewise, the term “100-year storm” is used to define a rainfall event that statistically has the same 1-percent chance of occurring.  Just because it rained 10 inches in one day last year doesn’t mean it can’t rain 10 inches in one day again this year.

     It is not a safety standard, and it has been set as the level that flood insurance is not required if the 1% annual chance flood can be excluded from the floodplain.  Although a 1% annual chance flood sounds remote, keep in mind that over the life of an average 30-year mortgage, a home located within the 1% flood zone has a 26% chance of being inundated by this size flood.  This same home has less than a 1% chance of fire damage during the same period.  What is more significant is the house in a 10-year flood area is almost certain to see a 10-year flood (96% chance) in the same 30-year mortgage cycle.  In many areas the difference in flood heights between a 10% and a 1% event is less than one foot.


7. What is meant by the term “flood stage”?

ANSWER: The National Weather Service, based on the desires of the local community, establishes the “flood stage” gauge height for any given community. The flood stage gauge height is often the stage where damages begin to occur.

     Many communities desire to use the flood stage gauge height as an early warning alert, prior to the onset of significant damages. Significant damages may not occur until river levels are several feet above flood stage.

     Additionally, conditions along some rivers may have changed since the gauge and flood stages were established and reaching the flood stage may or may not result in actual damages. Again, stages are site specific, so feet above flood stage at one location can’t be compared to another.

8. What is meant by the term “river stage”?

     ANSWER: A site-specific measurement of river-level referenced as the height in feet above a designated zero reference point, called the gauge zero, at the site. The zero reference point is sometimes, but not always, chosen as the elevation of the river bottom. Normally, stage values are always positive.

     Drought conditions could cause the river level to fall below gauge zero, and the stage reading at that time would be negative. Since each gauge was established independently at each location, the stage reading is good for that location only and cannot be compared to other locations.

The only way direct comparisons between two gauges can be made is by converting river stage to elevation by adding the stage to the gauge zero elevation.




9.  What does the Corps of Engineers do to assist before and during a flood?

ANSWER:  The Rock Island District provides flood-fighting assistance and supplies to communities within its 78,000 square-mile area covering the eastern two-thirds of Iowa, the northern half of Illinois and corners of Minnesota, Missouri and Wisconsin.  This area includes 314 miles of the Mississippi River and 268 miles of the Illinois Waterway and their tributaries.  The District is the national supplier of Innovative Flood Fight Products for the U.S. Army Corps of Engineers and home to the National Flood Fight Materiel Center

     Prior to a flood event, the Corps can provide contingency planning support and technical assistance to levee districts, communities and local, state and federal agencies.  The Corps can also provide advanced flood-fighting measures such as temporary levees if the situation warrants such federal actions.

     During floods, when all available local and state resources have been exhausted, the Corps can also provide flood fighting supplies, such as sandbags, polyethylene and pumps, as well as contingency planning support, technical assistance and emergency, temporary levee construction.

     In preparation for spring flooding, the District has reviewed and updated its flood response plan and is assisting local and county agencies with updating their flood plans.  In March 2011, approximately 100 specially trained Corps' flood area engineers received refresher training and flood-assistance teams have been re-structured to provide flood-fighting assistance to communities within the District's 78,000 square-mile area of operations.

     Technical expertise is only a part of the District's public assistance.  Flood-fighting supplies are also being procured for distribution to local organizations, cities, counties and states.  As of April 18, the District has a stock of approximately 5 million sandbags, miles polyethylene sheeting, and 125 pumps ranging in diameter from 4 to 16 inches with the capability of pumping up to 18,000 gallons of water per minute.

10. What flood protection projects have the Corps of Engineers built in the Quad Cities?

ANSWER: In the Quad Cities area, the U.S. Army Corps of Engineers has constructed multiple flood risk management projects and completed repairs to several levee systems after they were damaged by flooding.

     The Bettendorf, Iowa, Flood Protection Project, consists of approximately three miles of levee, 2,200 feet of concrete floodwall, two pump stations, two new railroad bridges, eight gatewells and two ponding areas, together with an early warning system for the entire project. This project includes a unique folding floodwall which USACE won an award for when it was designed. The floodwall can be lowered during non-flood times for the public to view the river, and raised during the threat of rising waters. The damage prevented since the completion of this project is more than $55,000,000.

     Rock Island, Illinois, also has a flood protection project built by USACE and it protects about 650 acres of the city's extensively developed industrial, commercial and residential land was subject to Mississippi River flooding. Constructed as part of the project were levees and floodwalls, including closure structures along the left bank of Sylvan Slough and along the Mississippi from the Chicago, Rock Island and Pacific Railroad embankment downstream to 18th Avenue. This project was completed in 1974 and has prevented more than $60,000,000 in flood damage.

     Another project is the East Moline, Illinois, Flood Protection Project. The project focuses on protecting a 1,300-acre industrial area which also includes commercial sites and more than 1,000 residences, schools and churches. About 2.4 miles of levee, railroad raises, street raises, a closure structure, gravity drainage outlets, open ditches, ponding area and pumping plants were constructed.  This project was completed in 1984 and has prevented more than $4,500,000 in flood damage.

     The Milan Local Flood Protection Project was originally authorized by the Flood Control Act of 1968. Construction of the Milan‐Big Island LFPP was completed under five construction contracts from June 1981 to July 1988 with an original estimated cost of $5.65 million. After completion of construction, the project was turned over to the Village of Milan, which has subsequently been responsible for operations and maintenance of the project. The portion of the levee system known as the Big Island Local Flood Protection Project is jointly maintained by the Big Island Conservancy District and the Village of Milan. The project consists of 10.6 miles of levees, 1120 feet of floodwalls, numerous closures, ramps, and interior drainage facilities.

     The  Davenport, Iowa, Local Flood Risk Management Project, Reach 1. is a portion of the original Davenport flood protection project that was authorized for construction on Dec. 31, 1970. For a variety of reasons, to include the cost of the project, poor economic conditions at the time, and maintaining connectivity with the Mississippi River, the city declined to participate in its construction. Record flood levels along the Mississippi River at Davenport, Iowa, in 1993 were nearly matched in 2001, causing extensive flood damages and attracting national attention. The water treatment plant was threatened by flooding, but remained in service during the floods due to emergency flood-fighting actions. A Limited Reevaluation Report was completed in April 2002, to update the 1982 project costs and economic analysis. The report determined that a federal project to protect Reach 1, including the water treatment plant, was justified. All property within the area protected by the project is owned by Iowa-American Water Company, except for an I & M Rail Link railroad line that transects the site and some city of Davenport property parcels adjacent to the river. The Iowa-American Water Treatment Plant at Davenport, Iowa, provides the only source of water for 131,000 people in Davenport, Bettendorf, and portions of Scott County in Iowa. Several hundred businesses and governmental organizations also rely on this water for their day-to-day activities and the water treatment plant is the sole source of water to the area population for the foreseeable planning horizon. Design of the project was completed March 2010, a construction contract was awarded September 2011, and construction was completed October 2013. 


11. What does NGVD29 mean?

ANSWER: The Sea Level Datum of 1929 was the vertical control datum established for vertical control surveying in the United States of America by the General Adjustment of 1929. The datum was used to measure elevation (altitude) above, and depression (depth) below, mean sea level (MSL).

Mean sea level was measured at 26 tide gauges: 21 in the United States and 5 in Canada. The datum was defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. The adjustment required a total of 66,315 miles (106,724 km) of leveling with 246 closed circuits and 25 circuits at sea level.

Since the Sea Level Datum of 1929 was a hybrid model, it was not a pure model of mean sea level, the geoid, or any other equipotential surface. Therefore, it was renamed the National Geodetic Vertical Datum of 1929 (NGVD29) in 1973. NGVD29 was superseded by the North American Vertical Datum of 1988 (NAVD 88), based upon an equipotential definition and a readjustment, although many cities and Corps of Engineer projects with established data continued to use the National Geodetic Vertical Datum of 1929.