Propagation and composition of the flood wave on the upper Mississippi River, 1993

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U .S . GEOLOGICAL SURVEY CIRCULAR 1120-F U .S . GEOLOGICAL SURVEY GORDON P EATON, Director

For sale by U .S . Geological Survey, Information Services Box 25286, Federal Center, Denver, CO 80225 During spring and summer 1993, record flooding inundated much of the upper Mississippi River Basin . The magnitude of the damages-in terms of property, disrupted business, and personal trauma was unmatched by any other flood disaster in United States history. Property damage alone is expected to exceed $10 billion . Damaged highways and submerged roads disrupted overland transportation throughout the flooded region . The Mississippi and the Missouri Rivers were closed to navigation before, during, and after the flooding . Millions of acres of productive farmland remained under water for weeks during the growing season . Rills and gullies in many tilled fields are the result of the severe erosion that occurred throughout the Midwestern United States farmbelt . The hydrologic effects of extended rainfall throughout the upper Midwestern United States were severe and widespread . The banks and channels of many rivers were severely eroded, and sediment was deposited over large areas of the basin's flood plain . Record flows submerged many areas that had not been affected by previous floods . Indus trial and agricultural areas were inundated, which caused concern about the transport and fate of indus trial chemicals, sewage effluent, and agricultural chemicals in the floodwaters . The extent and duration of the flooding caused numerous levees to fail . One failed levee on the Raccoon River in Des Moines, Iowa, led to flooding of the city's water treatment plant . As a result, the city was without drinking water for 19 days .

As the Nation's principal water-science agency, the U .S . Geological Survey (USGS) is in a unique position to provide an immediate assessment of some of the hydrological effects of the 1993 flood . The USGS maintains a hydrologic data network and conducts extensive water-resources investigations nation wide . Long-term data from this network and information on local and regional hydrology provide the basis for identifying and documenting the effects of the flooding . During the flood, the USGS provided continuous streamflow and related information to the National Weather Service (NWS), the U .S . Army Corps of Engineers, the Federal Emergency Management Agency (FEMA), and many State and local agencies as part of its role to provide basic information on the Nation's surface- and ground-water resources at thousands of locations across the United States . The NWS has used the data in forecasting floods and issuing flood warnings . The data have been used by the Corps of Engineers to operate water diversions, dams, locks, and levees . The FEMA and many State and local emergency management agencies have used USGS hydrologic data and NWS forecasts as part of the basis of their local flood-response activities . In addition, USGS hydrologists are conducting a series of investigations to document the effects of the flooding and to improve understanding of the related processes . The major initial findings from these studies will be reported in this Circular series as results become available .

U .S . Geological Survey Circular 1120, Floods in the Upper Mississippi River Basin, 1993, con sists of individually published chapters that will document the effects of the 1993 flooding . The series includes data and findings on the magnitude and frequency of peak discharges ; precipitation ; water-qual ity characteristics, including nutrients and man-made contaminants ; transport of sediment ; assessment of sediment deposited on flood plains ; effects of inundation on ground-water quality ; flood-discharge vol ume ; effects of reservoir storage on flood peaks ; stream-channel scour at selected bridges ; extent of flood plain inundation; and documentation of geomorphologic changes .

Contents

ByJohn A . Moody

The historic flood of 1993 affected the entire 1,358-kilometer reach of the upper Mississippi River from Minneapolis, Minnesota, to Cairo, Illinois, but had no unusual effect on the lower Mississippi River from Cairo to the Gulf of Mexico. The flood was preceded by heavy (200 percent of normal) spring rains that saturated the ground in much of Minnesota, Iowa, Illinois, and Wisconsin . The flooding began in June along the upper 200 kilometers of the river. Intense rains that continued into summer caused the Minnesota River in Minnesota and the Chippewa and the Black Rivers in Wisconsin to contribute simulta neously near-record discharges to the upper Mississippi River from June 22 to 25 . The maximum discharge in the flood wave, which propagated downstream at about 0.58 meter per second, was five to seven times the daily mean discharge at stream-gaging stations on the upper Mississippi River. As the flood wave propagated southward, the rains shifted southward in concert . The phase or propagation speed of the flood wave was influ enced largely by hydrologic factors, such as tribu tary inflow and flood-plain storage, rather than by hydraulic factors, such as water depth, chan nel width, and channel roughness . The Iowa and the Des Moines Rivers contributed near-record discharges to the Mississippi in early July . A record (greater than 100-year recurrence interval) discharge of 12,300 cubic meters per second occurred on the Mississippi River at Keokuk, Iowa, on July 10 . This part of the flood wave probably passed St . Louis, Missouri, on July 20, but was not the major flood peak there ; by late July, the rains had shifted further southward into Missouri and central Illinois, which caused the Missouri and the Illinois Rivers to contribute about 20,900 and 2,220 cubic meters per second, respectively, to the upper Mississippi River at St . Louis from July 29 to July 31 . Not all of this water passed Thebes, Illinois (about 70 kilome ters upstream from Cairo, where the last down stream-gaging and sampling station on the upper Mississippi River is located), because water left the channel of the upper Mississippi River upstream from Thebes through failed levees and by seepage through and under the levees that did not fail and was stored temporarily on the flood plain . The maximum discharge at St . Louis (29,700 cubic meters per second) occurred on August 1 ; however, the maximum discharge at Thebes (27,700 cubic meters per second) did not occur until August 7, which was about 4 days later than the normal travel time of 2 days .

Flood waves that move down the upper Missis sippi River, which ends at the confluence with the Ohio River at Cairo, Illinois, are confined and altered by human and natural processes . Starting in Minneap olis, Minnesota, a moderate flood cannot propagate as a free wave under normal conditions because the free surface of the river is controlled by a drop of 96 meters (m) through a series of stairstep pools created by backwater from 29 navigation dams that regulate the 1,050-kilometer (km) reach of the river to its con fluence with the Missouri River near St. Louis, Mis souri (fig . 1) . Between Minneapolis and Clinton, Iowa (532 km), the flood plain is narrow and con fined by high bluffs on either side ; the average depth of the river is 2 m. Downstream from Clinton to St .

Louis (518 km), the flood plain widens, and a series of levees of different heights has been built by Fed eral, State, and local agencies . The levees normally confine a moderate flood wave to a narrow channel within the wider flood plain ; the average depth of the river is 3 m . From St. Louis to Cairo (315 km), where the Ohio River joins the upper Mississippi River, the Mississippi is channelized by using lateral dikes and revetments to maintain an average depth of 7 m . Because there are no navigation dams on this reach of the river, flood waves can propagate freely .

The flood of June through August 1993, however, was not a moderate flood . For floods this large, the gates in the navigation dams are raised out of the water, which prevents damage to structures and machinery at the dam, and the river surface more closely resembles a natural, smoothly changing sur face than a series of stairsteps . Flows from tributaries also affect the timing anc: shape of a flood wave, depending upon the propor tion of the tributary's discharge to the discharge of the main river . The mean discharge of the upper Missis sippi River, which receives a large percentage of its water from its many tributaries (fig . 2 ; table 1), (m3 increases from 225 cubic meters per second /s) near Anoka, Minnesota, at an average rate of 4 .2 m3 /s per river kilometer . In contrast, the mean discharge of the lower Mississippi River (downstream from the con fluence with the Ohio River), which has fewer signifi cant tributaries, increased from 13,500 m3 /s near Cairo at an average rate of only 1 .6 m3/s per river kilometer .

This report describes the propagation of the flood wave of 1993 through the engineered channel of the upper Mississippi River from Minneapolis to Thebes, Illinois, and the effects of tributary inflow and flood-plain storage on this flood wave . The composi tion of this flood wave, in terms of contributions from tributaries, is described for five U .S . Geological Sur vey stream-gaging stations between Minneapolis and Thebes-Prescott, Wisconsin, Winona, Minnesota, Clinton and Keokuk, Iowa, and St . Louis .

The shape of the upper Mississippi River flood wave of 1993 was not the simple, solitary flood wave commonly depicted in textbooks, but was a composite shape that resulted from the summation of many separate tributary flood waves . The length of the composite flood wave was essentially the length of the upper Mississippi River, or 1,358 km (fig . 3A) . The water surface of the composite flood wave for June 25 dropped sharply (600 km upstream from the Ohio River ; fig . 3A) ; this was caused by the hydroelectric power dam at Keokuk, which always controls the flow of the river (fig . 4) . This composite flood wave comprised three secondary peaks that can be seen when the average river slope of 91 x 10 -6 is removed (fig . 3B) . The daily mean discharge of some tributaries was routed into and down the upper Mississippi River at an assumed average water speed of about 0 .7 meter per second (m/s) (Institute of River Studies, 1983) . The shapes of the tributary discharge flood waves for June 25 had abrupt increases at the mouth of the tributary (fig . 3C) . The summation of the tributary discharges in the upper Mississippi River upstream from Keokuk had a shape similar to the composite flood wave (fig . 3B) . The composite flood wave for August 1 (date of peak discharge at St . Louis at river kilometer 289) distinctly showed the inflow of the floodwaters from the Missouri River (fig . 3B) . On August 1, numerous drops in the elevation of the water surface were evident as the river flowed through navigation dams between 800 and 1,358 km upstream from the Ohio River. By this time, some dams were operational and were again regulating the river .

Many methods for routing a flood wave assume unsteady flow in an open channel and no significant tributary inflow, but these two assumptions are not applicable to the upper Mississippi River. During low water conditions when the river is regulated by naviga tion dams, a flood wave cannot propagate freely through an open channel . However, flood waves can propagate freely in an open channel during high water conditions when the navigation dams no longer control the water level . During spring and late fall 1992, when the water discharge was closer to mean conditions, the periods of high water or open channel typically lasted a few days and occurred only in short reaches of the river between some dams . During summer 1993, the periods of open channel were longer and encompassed the entire reach of river from Minneapolis to Cairo, with the exception of the hydroelectric power dam at Keokuk (fig. 4) . Tribu tary inflow to the upper Mississippi River was significant and had two effects on the flood wave. First, tributary inflow produced several secondary peaks and increased the magnitude of the maximum discharge flood peak as distance increased downstream (fig . 5) . This contrasts with the assumption made by many routing methods that there is a single flood peak and that the magnitude of the peak is constant or decreases in the downstream direction (Gilcrest, 1950 ; Lawler, 1964) . Second, the wave-propa gation speed (phase speed or speed of the stage or dis charge peak) was determined by the tributary inflows rather than the hydraulic characteristics of the channel.

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The flood-wave routing method used in this paper for the upper Mississippi River, therefore, assumed a steady, open-channel flow only from June through August 1993 and only along short reaches of the river between tributaries . The travel times within these reaches were based on field data published by the U .S . Army Corps of Engineers for the upper Mis sissippi River (table 1 ; Institute of River Studies, 1983) and the conservation of mass at upper Missis sippi River stream-gaging stations . The steady open-channel flow was increased abruptly by inflow from tributaries or decreased abruptly by outflow to flood A 1992 =

plain storage . Following these abrupt gains or losses of water, steady open-channel flow was assumed to resume until the next tributary downstream entered the upper Mississippi River .

The discharge information used to prepare this report was provided by many U.S . Geological Survey personnel from the Minnesota, the Wisconsin, the Iowa, the Illinois, and the Missouri Districts . Although Q

O H Q O Q z working extra hours during the flood period, they still took time to satisfy numerous requests for the data required to make the calculations of percent composition of the flood wave .

most of the stage information for at least 50 stations along the upper Mississippi River . Several requests required considerable time to fulfill or required search ing of archived records . Some of these very helpful people were E.G . Eaton and K.W . Willus in the St . Paul District ; W .H . Koelner, J .D . Bledsoe, B .J . Goodrum, G .F . Gitter, and K.W . Finch in the Rock Island District ; D.M . Coleman, S .G. Farkas, and R.J . Kopsky, Jr ., in the St . Louis District ; and S.A . Lehr and H.W. Barton in the Memphis District .

Route 3 in Alexander County, Illinois, behind the East Cape Girardeau levee, was provided by K .L . Bartelsm eyer and B .G . Stout, Jr., of the Illinois Department of Transportation, Division of Highways .

Department of Natural Resources were instrumental in the conception of this chapter .

FLOOD-WAVE PROPAGATION

flood-plain storage, were more important than hydraum

The U .S . Army Corps of Engineers provided

The water-level information along Illinois

Discussions with J .F . Sullivan of the Wisconsin

Hydrologic factors, such as tributary inflow or lic factors, such as water depth, channel width, and channel roughness, in determining the flood-wave propagation speed . Water speed is not the same as wave-propagation speed . The wave-propagation speed was determined from the times and locations of maximum stages or discharges in the regulated reach of the upper Mississippi River. The general observed wave-propagation speed was 0 .58 m/s for open-channel conditions during the 1993 flood (fig . 6) . This compares closely with the wave-propagation speed of 0 .52 m/s calculated from measurements given by Seddon (1900) for the upper Mississippi River flood of April 1881 when no navigation dams were present .

Tributary Inflow

The peak inflow from the tributaries of the upper Mississippi River upstream from the Illinois and the Missouri Rivers was nearly synchronous with the arrival of the flood wave from upstream because the areas of intense rainfall moved southward as the summer progressed (Wahl and others, 1993) . This pattern of inflow contrasts with the inflow from the Illinois and the Missouri Rivers, which lagged behind the arrival of the upper Mississippi River flood wave by about 2 weeks and caused the wave-propagation speed to decrease from 0 .58 to about 0 .08 m/s (fig . 6) . The average wave-propagation speed in the open-channel reach from St . Louis to Thebes increased to about 0 .42 m/s (fig . 6) ; this still was much, less than the average of 1 .6 m/s for previous high-water events in 1993 (table 2) . Along some reaches, however (for example, 300-400,700-800,900-1,100, and 1,100- 1,200 km upstream from the Ohio River), the wave propagation speed was much higher than 0 .58 m/s . This higher speed usually was caused by the arrival of the tributary inflow at the downstream end of a reach of the upper Mississippi River at the same time that the flood wave was arriving at the upstream end of the same reach . Consequently, the time of maximum stage or discharge coincided nearly simultaneously along the entire reach of the river and resulted in high wave-propagation speeds . Figure 6 . Times and locations of maximum stages and discharges during the 1993 flood on the upper Missis sippi River. Maximum stages are shown by open circles, and maximum discharges, by solid circles . The times and locations of maximum discharges for the tributary are labeled . The three segments of the dashed line were fit visually to show the general trend . The slope of the three line segments is an estimate of the flood-wave propagation speeds and are equal to 0 .58, 0 .08, and 0.42 m/s . An estimate of the wave-propagation speed for a simple kinematic wave in an open channel is the slope of the regression line for a plot of discharge as a function of cross-sectional area (Gilcrest, 1950) . These estimates of the predicted kinematic wave-propagation speed for stations on the upper Missis sippi River (table 2) generally differ from the speeds observed during the flood of 1993 and the speeds cal culated from measurements during previous high-water events . This is because the significant tributary inflow, not the hydraulic factors, determined the prop agation speed of the flood wave, and numerous levee breaks and seepage under and through the levees delayed the arrival of the flood wave downstream by removing water from the river and storing it on the flood plain . During the flood of 1993, water was stored on flood plains because of two mechanisms-levee breaks and levee seepage . No levee breaks occurred upstream from Clinton, a few occurred between Clinton and St . Louis, but the largest levee breaks occurred downstream from St. Louis and caused a delay in the arrival of the discharge flood peak at Thebes .

Among the most dramatic pictures seen on television and in the newspapers during the upper Mississippi River flood of 1993 were those of floodwaters raging through levee breaks and flooding adjacent flood plains . The floodwalls that protect St . Louis and East St . Louis, Illinois, held throughout the flood and confined the river to a narrow channel near the Gateway Arch at St . Louis . However, many levees failed downstream from St. Louis (table 3) and allowed water to flow onto the flood plain, thus decreasing the discharge of the river at Thebes where natural bluffs again confined the floodwaters to a narrow channel . The plot of water discharge at St . Louis (fig . 7) has been lagged 2 days (on the basis of the wave-propagation speed of previous high-water events in 1993) so that the two red shaded areas (in July and August) between the water-discharge graphs for St. Louis and Thebes represent the water volume stored on the flood plain between St. Louis and Thebes . These volumes of water stored on flood plains (fig . 7) between St . Louis and Thebes were estimated to be 1 .36 x 10 9 m3 during July and an additional 1 .54 x 10 9 m3 during August . The volume of water stored on flood plains also can be estimated by using the approximate area protected by the levees that broke and an assumed average depth (table 3) . A generous estimate of the average depth is 6 m, but this is still not enough to account for all the water stored on the flood plain, which is based on the difference in the discharge graphs for St . Louis and Thebes (fig . 7) . The levee breaks, therefore, probably do not account for all the water stored on the flood plain ; additional water was stored by seepage through and under levees that did not fail.

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were dry on August 7 were flooded on September 25 . No measurements of the water level were made after September 25, but the water level probably rose a lit tle higher when a minor flood peak passed Thebes on October 2 . The river level did not drop sufficiently below the water level behind the levees until Octo ber 11, when the gates in the levees were opened, and the water began to drain back into the river (R .R . Colyer, East Cape Girardeau Levee District, oral com mun ., 1993) . If a conservative mean water depth of about I m is assumed, then the volume of water stored on the flood plain in these levee districts must have been about 0 .1 x 109 m3 . If seepage in other levee districts along the 217 km of river between St . Louis and Thebes is allowed for, then an additional 0 .2 to 0 .3 x 10 9 m 3 of water may have been stored on the flood plain . This volume, when added to the water stored from the levee breaks, may account for the difference in discharge between St . Louis and Thebes (fig . 7) . The storage of water on the flood plain explains the observation that the wave-propaga tion speed between St . Louis and Thebes was slower than the predicted speeds or the measured speeds of previous high-water events that had no discharge losses caused by levee breaks or seepage (table 2) .

FLOOD-WAVE COMPOSITION

A water budget was constructed by computing the discharge contribution from upstream tributaries to the flood wave and comparing it to the measured dis charge at five primary stream-gaging stations on the upper Mississippi River-Prescott, Winona, Clinton, Keokuk, and St . Louis . For computations of tributary contributions during open-channel conditions from June through August discharges of tributaries and the dis charges at stream-gaging stations on the upper Missis sippi River upstream from the primary gaging station were lagged relative to the downstream primary gaging station (see travel times in table 1) . The tributary and upstream contributions were computed as percentages of the reported daily mean discharge at the primary sta tion (table 4) and not as percentages of the sum of all tributaries and the upstream lagged discharges . There fore, if some of the water that passed an upstream tribu tary gaging station was later stored on the flood plain and did not reach the downstream primary station, then the sum of the percentages could be greater than 100, and the amount that is greater than 100 percent represents stored water (figs . 9-13) . The error in this percentage computation is a function of the error in estimating the lag time, the error in the rating curve for each stream-gaging station,

Minnesota and the St. Croix Rivers, accounted for 50 to 90 percent of the daily mean discharge at Prescott during June and July ; the maximum daily mean discharge (3,680 m3 /s, or 7 .5 times the mean discharge) at Prescott was measured on June 27 (fig . 9) . The maximum daily mean discharge of the Minnesota River near Jordan, Minnesota, was 24 times the mean discharge, but the maximum daily mean discharge of the St . Croix River at St. Croix Falls, Wisconsin, was only 4 .3 times the mean discharge . About 3 percent of the unaccounted discharge of the Mississippi River at Prescott is from ungaged tributaries (J .H . Hess, U.S . Geological Survey, oral commun ., 1994) .

Mississippi River at Winona, Minnesota

sissippi River between Prescott and Winona are ungaged ; the percentage of the discharge contributed by these tributaries is unknown and probably explains why the total percentages at Winona usually are less than 95 percent (fig . 10) . The Vermillion, the Cannon, the Zumbro, and the Whitewater Rivers, which have stream gages on either their main stems or some of their tributaries, accounted for 1 to 7 percent of the water that flowed past Winona. The major tributary that entered the upper Mississippi River between Prescott and Winona is the Chippewa River, which reached a maximum daily mean discharge (2,400 m3/s, or 11 times the mean discharge) on June 23 . This discharge flood peak arrived at Winona on June 24 and accounted for 56 percent of the water that passed Winona on that day . Discharge of the Mississippi River at Prescott accounted for 43 to 79 percent of the water that passed Winona in June and for 71 to 83 percent in July .

Mississippi River at Clinton, Iowa

be considered in an accounting of all the water that flowed past Clinton . The Black and the Wisconsin Riv ers are the main tributaries between Winona and Clinton, but the combined discharge from the Trempealeau, the Root, the upper Iowa, the Turkey, the Grant, the Platte, and the Maquoketa Rivers accounted for 6 to 20 percent of the water that flowed past Clinton . Total discharge contributions were greater than 100 percent of The daily mean discharge of two tributaries, the

Many of the small tributaries of the upper Mis-

The discharge from many small tributaries must s

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the actual discharge at Clinton between June 26 and July 2 (fig . 11) . The sharp daily mean discharge peak of the Black River for June 21 (1,540 m3/s, or 31 times the mean discharge) was expected to reach Clinton about June 27 ; however, much of the peak discharge may have been stored on the flood-plain delta near the mouth of the Black River (J .F . Sullivan, Wisconsin Department of Natural Resources, oral commun ., 1993) and did not reach Clinton until after July 2 (fig . 11) . The Wisconsin River had a broad daily mean discharge peak (1,670 m 3/s, or 6 .8 times the mean discharge), which accounted for 16 to 28 percent of the daily mean discharge at Clinton between June 16 and July 2 . The Mississippi River at Winona contributed from 49 to 80 percent of the daily mean discharge at Clinton during June and July .

Mississippi River at Keokuk, Iowa

The Rock River reached a maximum earlier (June 11) than did most tributaries of the upper Missis sippi River. The maximum daily mean discharge (986 3 /s,. or 5 .7 times the mean discharge) of the Rock m River represented 17 percent of the daily mean dis charge at Keokuk (fig . 12) . The combined discharge of the Iowa, the Skunk, and the Wapsipinicon Rivers in Iowa accounted for 17 to 48 percent of the daily mean discharge at Keokuk from June through August. The individual maximum daily mean discharges of these three tributaries arrived at Keokuk on or about July 10, July 11, and July 19 . The discharge of the Mississippi River at Clinton accounted for less than 50 percent of the discharge at Keokuk after July 19, and the com bined discharges of the Iowa, the Skunk, and the Wap sipinicon Rivers accounted for slowly increasing percentages of the discharge at Keokuk through August .

Mississippi River at St . Louis, Missouri

The discharges of the Des Moines and the Illi nois Rivers each accounted for 5 to 12 percent of the daily mean discharge at St . Louis from June through August . The Mississippi River at Keokuk accounted for about 50 percent of the discharge at St . Louis with a maximum of 54 percent on July 13 and 14 . In early August, the Mississippi River at Keokuk accounted

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for only about 21 percent of the discharge at St . Louis due to large contributions from the Missouri River . However, in late August, this percentage again increased to about 50 percent due to decreasing con tributions (about 30 percent) from the Missouri River. The Missouri River contributed a maximum percentage (70 percent) of the discharge at St . Louis on August 1 and 2, which are the dates the flood peak passed St . Louis . Significant amounts of water (10 20 percent of the discharge measured at St . Louis) did not reach St . Louis as a result of flow through major levee breaks onto the flood plain at upstream sites (table 3) . The Meyer and the Indian Grave South and North levees were breached on July 9 and July 13, respectively, and flow through these breaks proba bly accounted for some of the water that did not reach St . Louis between July 10 and July 17 (fig . 13) . The Sny levee was breached on July 25 after a long battle to save it (Stewart, 1993) . Flow through this breach probably accounted for the additional water that was stored and did not reach St . Louis between July 28 and August 4 . Some of the water stored on the flood plain drained back to the river (fig . 13) and represented the "missing" water that could not be accounted for by the Illinois, the Des Moines, and the Missouri Rivers or by the Mississippi River at Keokuk.

SUMMARY

The flood wave that propagated down the upper Mississippi River from June through August 1993 was a composite of individual tributary flood waves . The average wave-propagation speed in the river was about 0 .58 m/s upstream from St . Louis and about 0.42 m/s downstream from St. Louis. The wave-propagation speed was determined primarily by hydrologic factors, such as tributary inflow and flood-plain storage, rather than by hydrau lic factors, such as water depth, channel width, and channel roughness . Flow through levee breaks accounted for most of the flood-plain storage on the upper Mississippi River, but levee seepage also was significant, especially in its impact on human activi-

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ties . Seepage through and under levees continued long after the peak of the composite flood wave in the river had passed, which caused extended periods of inundation . The flood wave peaked twice as it passed St . Louis during July and August . The first discharge peak of 27,600 m3/s on July 19 comprised about equal amounts (40 percent) of upper Mississippi and Missouri River water. A second and larger discharge of 29,700 m3/s peaked on August 1 ; the Missouri River accounted for about 70 percent of the total dis charge, or about twice the discharge contributed by the Mississippi River at Keokuk .

SELECTED REFERENCES

Gilcrest, B .R ., 1950, Flood routing, in Rouse, Hunter, ed ., Engineering hydraulics, chap . 10-Proceedings of the 4th Hydraulics Conference, June 12-15, 1949, Iowa Institute of Hydraulic Research : New York, John Wiley, p . 635-710 .

Institute of River Studies, 1983, Time of travel estimates for selected streams in Missouri, Iowa, and Illinois : U.S . Army Corps of Engineers, St. Louis District, St . Louis District potamology study (S-17), 237 p .

Lawler, E.A ., 1964, Flood routing, pt . 2 of Hydrology of flow control, in Chow, V .T ., ed ., Handbook of applied hydrology : New York, McGraw-Hill, p . 25- 34-25-59 .

Moody, J .A., and Meade, R.H., 1992, Hydrologic and sedi mentologic data collected during three cruises at low water on the Mississippi River and some of its tribu taries, July 1987-June 1988 : U .S . Geological Survey Open-File Report 91-485, 143 p .

Parrett, Charles, Melcher, N.B ., and James, R.W ., Jr., 1993, Flood discharges in the upper Mississippi River Basin, 1993, in Floods in the upper Mississippi River Basin, 1993 : U .S . Geological Survey Circular 1120-A, 14 p.

Seddon, J.A ., 1900, River hydraulics : Transactions of the American Society of Civil Engineers, v . 43, p . 179- 243 .

Stewart, J .B ., 1993, Battle on the Sny : The New Yorker, v . 69, no. 25, August 9, 1993, p . 30-40 .

U .S . Army Corps of Engineers, 1990, The upper Missis sippi River nine-foot channel : Washington, D .C ., U .S . Government Printing Office, 2 p.

U .S. Geological Survey, 1989, Water-resources data for Illinois, water years 1989-1990 : U .S . Geological Sur vey Water-Data Reports IL-89-1 and IL- 90-1, vol umes 1 and 2 (published annually) . 1989, Water-resources data for Iowa, water year 1989 : U .S . Geological Survey Water-Data Report IA-89-1 (published annually) . 1988, Water-resources data for Minnesota, water year 1988 : U .S . Geological Survey Water-Data Reports MN-88-1 (published annually) . .1989, Water-resources data for Missouri, water year 1989 : U.S . Geological Survey Water-Data Report MO-89-1 (published annually) . 1989, Water-resources data for Wisconsin, water year 1989: U .S . Geological Survey Water-Data Report WI-89-1 (published annually) .

Wahl, K.L ., Vining, K .C ., and Wiche, G .J ., 1993, Precipita tion in the upper Mississippi River Basin, January 1 through July 31, 1993, in Floods in the upper Missis sippi River Basin, 1993 : U .S . Geological Survey Circular 1120-B, 10 p .

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