Iowa River basin map

Go to IOWA RIVER & CORALVILLE DAM INFORMATION: preformatted tables version | enhanced tables version



The purpose of the INFORMATION page and this Guide is to draw together current information on eastern Iowa weather, water flow, and the Coralville reservoir that is of interest to those in the Iowa River basin, and to provide enough explanatory material that a layperson can interpret the information. The hypertext links to current data point to files maintained by the Rock Island District of the Corps of Engineers (COE), the Iowa division of the U.S. Geological Survey (USGS), and to the National Weather Service (NWS). The lake and stream data are generated automatically with data from a set of Data Collection Platform (DCPs) satellite uplinks. The descriptions in this guide of Corps policies for dam management are based on publicly available documents from the COE.

This web site was developed in 1995 by Larry Molnar. As of July, 1998, the maintenance of the site was taken over by Charles Newsom. I welcome any comments or questions regarding the contents of this page.

Iowa reservoirs

The Rock Island District of the Corps of Engineers (COE) manages three flood-control reservoirs in Iowa: Coralville (on the Iowa River in Johnson County), Saylorville (on the Des Moines River just upstream of Des Moines), and Red Rock (on the Des Moines River just downstream of Des Moines).

DESCRIPTION: This links to a file that lists basic statistics of the reservoir: the record lake height along with the date it occurred, location (both latitude and longitude as well as a description) of the dam, drainage area of the river basin.


RAW DATA: Every 60 minutes, two lines are added to the top of the table indicating the date, time (central time), the lake height in feet above mean sea level (following the code "/HP"), the heights of the three outflow gates in feet (following the codes "/Y1", "/Y2", and "/Y3"), the outflow from the dam in cubic feet per second (following the code "/QR" and computed from the lake and gate heights), the water stored in acre-feet (following the code "/LF"), and the estimated inflow into the lake in cubic feet per second (following the code "/QI", computed from the outflow and the rate of change of water storage).

NOTE: The outflow values mentioned above (that follow the QR) have never been calibrated against USGS measurements of streamflow. Because of the systematic offsets in these values, the COE uses outflows based on tailwater gages instead for basin summary reports. However, in a flash flood the tailwater gages will also have a significant positive bias. Therefore, despite the systematic offsets, the outflows listed in the raw data are the only reliable indicator of change in outflow during a flash flood.

GRAPH: A graph of the lake height (in feet above mean sea level, a solid blue line), for the preceding week. Note the ranges are automatically scaled, so that any change, whether large or small, may fill the graph equally well. Read the label to determine the significance of changes. The outflow from the dam, perhaps the most important quantity to residents downstream, used to be plotted as well (in February 1997). It is only available now in a tabular form (see RAW DATA above).

BASIN SUMMARY: A somewhat cryptic summary of conditions up and down a reservoir's basin for the preceding 24 hours. This table is of somewhat less value since March, 1997, when the update frequency was reduced from once every 2 hours to once every 6 hours. Perhaps the most relevant entries are three at the bottom of the page: "Rel", "Stor" and "Inflow".

1) "Rel" is the water flow released through the dam (in cubic feet per second or cfs). This is determined from the height of the tailwater gage, and is therefore unreliable in the event of a flash flood. The only reliable information about changes in outflow during a flash flood is the RAW DATA table, described above.

2) "Stor" is the volume of water currently stored in the lake in units of acre-feet. For the Coralville reservoir, this is based on a pool height to storage conversion determined in 1983. Due to sedimentation, the actual storage is less than this. "Prcnt" gives the same information as a percentage of the total flood pool, units that make it easy to see how much storage remains for the the next storm.

3) "Inflow" is total flow into the lake (in cubic feet per second or cfs), computed from the rate of change in the storage plus the release. (Note that 1 cfs over 1 day equals 1.98 acre-ft of water.) There are a number of circumstances in which this number may be inaccurate. Whenever this number is anomalously low (or negative, which is marked with an "underscore" only), the Coralville reservoir telephone answering machine (354-4466) gives the inflow as "not available" or else estimates the value from the flow at Marengo, the first gage upstream of the lake. As no such correction is made for anomalously high values, the answering machine inflows have an artificially high average.

Variable wind conditions are one cause of inaccurate inflows. Since the gage is near the dam rather than the center of the lake, a wind-induced gradient across the lake is interpreted as a surge or dearth of inflow, with the sign depending on the wind direction. For typical lake elevations, the effect is often several inches of elevation. The 24 hour average (given in the last line) averages out this affect somewhat better than the individual 6 hour averages.

The out-of-date height-storage table systematically affects inflows during periods when the pool is being raised or lowered. For example, as sediment fills in the pool it becomes surprisingly easy to lower the pool because there was less water in it than listed in the table. Hence the computed inflows are low or negative the entire time the pool is being lowered (in December and February). By the same token, the inflows are artificially high when the pool is being raised (in June and September).

WEATHER DATA: A table, with entries every 30 minutes as above, indicating the temperature (in degrees Fahrenheit, following the code "/TA") and the cumulative precipitation (in inches, following the code "/PC"). Due to software limitations, temperatures below zero Fahrenheit are represented by the actual temperature plus 26215.

ARCHIVAL DATA: A page of daily archival data for selected gauges. Each gauge in the page contains one year's worth of 6 a.m. lake elevations. The year and type of output are selectable in the table.

Iowa River basin gages

GAGE DESCRIPTION: This links to a file that lists basic statistics of the gage: the record river height along with the date it occurred, location (both latitude and longitude as well as a description), drainage area of the river basin, and the gage zero (the number to add to river height in order to find the height of the river surface above mean sea level).

The separate "D" page lists damage as a function of river height based on Corps of Engineers (NCR), National Weather Service (NWS), and Iowa Department of Transportation (IDOT) data. These list critical elevations, such as the stage at which the water treatment plant in Iowa City would be shut down. After some errors in the table were reported to the Corps, it was decided (May 1996) to make these files unavailable to the public to avoid further public comments or questions. The files remain on the system, however, and will presumably be used to make critical decisions in the next flood event despite their errors.


RAW DATA: Every 60 minutes, two lines are added to the top of the table indicating the date, time (central time), the river height in feet (following the code "/HG"), the yearly precipitation total in feet (following the code "/PC"), the temperature in degrees Fahrenheit (following the code "/TA"), and the stream flow in cubic feet per second (following the code "/QR"). Some stations do not measure precipitation. Only the most recent 24 hour period is shown.

GRAPH: A graph of the river height (in feet, a solid blue line) and stream flow (in cubic feet per second, a dashed red line) for the preceding week. (Flood stage is sometimes marked with a solid blue line.) Note the ranges are automatically scaled, so that any change, whether large or small, may fill the graph equally well. Read the label to determine the significance of changes. Only the most recent 7 day period is shown

IOWA CITY FLOW TIME: Typical time for water to flow from Iowa City to the gage (positive values) or from the gage to Iowa City (negative values).

The English River flows into the Iowa River below Iowa City but above Lone Tree. Water at the English River/Kalona gage reaches the Lone Tree gage about 10 hours later, and so can be used to predict crests at Lone Tree (see the decription of downstream thresholds below).

FLOOD STAGE (FT): River height (in feet) defined by the National Weather Service to indicate when a stream begins to leave its bank near a given gage.

OUTFLOW TRIGGER (FT): The regulation plan for the Coralville dam has two types of gage height triggers: flash flood alerts (type "a") and downstream thresholds (type "b").

a: River gages with type "a" outflow triggers are equipped to phone the Rock Island answering service automatically when the river height reaches the trigger level. The triggers are turned off in the winter, when the threat of flash flood is low and the chance of false alarm due to ice blockage is high. These systems were installed in the fall of 1993 to alert the Corps of Engineers of the potential for a flash flood. In response to a flash flood alert call, the answering service is immediately supposed to pass the alert on to a Corps hydrologist and to the Johnson County sheriff. There are currently no written provisions for how the hydrologist should respond (e.g., when and how much to cut back the outflow). The Corps announced that a study that will include the writing of a contingency plan will begin 30 May 1996, and should be completed within one year of that date. No contingency plan has been put into place as of the last update of this file, however.

b: A downstream threshold is the peak height to which the Corps will allow the river to rise when the reservoir is low. When heights are forecast to exceed these triggers, reservoir outflow is reduced to as little as 1000 cfs as needed to limit the crest to the threshold. However, when the reservoir reaches 707 ft (5 feet below the spillway or about 3/4 full), the regulations do not permit any flow reductions to limit crests at downstream threshold gages. For those gages with two thresholds listed, the lower value applies after 1 May each year and the higher one before 1 May.

ARCHIVAL DATA: A directory of daily archival data. Each file in the directory contains one year's worth of 6 a.m. river heights in feet.

Corps of Engineers forecasts

The Corps of Engineers computes a 7 day forecast for the Coralville reservoir (inflow, outflow and lake level) and a 35 day forecast for the Mississippi river.

The lake inflow and the Mississippi level are computed by propagating downstream water that is already in the river system only. Hence these predictions represent firm lower limits. In the event of additional rain during the forecast period, actual levels will be higher.

The lake outflow is determined by applying the reservoir regulations to the predicted situation. This is a particularly valuable curve as it communicates the intended actions of the Corps over the coming week. As noted above, however, these actions may be modified in the event of additional rain.

The predicted changes in the lake level are computed from the accumulated differences between predicted outflow and inflow.

National Weather Service river forecasts

The National Weather service has provided a very nice set of forecasts and data pages for quite a large number of rivers and streams. Included here are those most relevant to the Iowa River flows. You will find that the NWS has NOT made forecasts for every data collection station that the Army Corps of Engineers has installed in our area. Also, the NWS will show the river and stream flows but with no forecasts during periods when the risk of flooding is low.

At the top of each page is a graph of the measured and forecast river levels for roughly a one week period. One can also use the menus to the right to select other sites of interest.

Further down the page is a map showing the river and surrounding area as well as the location of the data collection station. This is where river levels are measured and where the predictions refer to.

Finally, at the bottom of the page is a very interesting list of the impacts associate with the various water levels.

Go to IOWA RIVER & CORALVILLE DAM INFORMATION: preformatted tables version | enhanced tables version

Last updated on July 2, 1998 by CRN