Water-Supply Paper 2499 U.S. Department of the Interior
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| Maximum prior to [month] 1992 |
Maximum during [month] 1992 |
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|---|---|---|---|---|---|---|---|---|---|---|---|
| Site no. (fig. #) |
Station no. | Stream and place of determination |
Drainage area (mi²) |
Period | Year | Stage (ft) |
Dis- charge (ft³/s) |
Day | Stage (ft) |
Discharge (ft³/s) |
Dis- charge recur- rence interval (years) |
| 1 | 05551212 | Hypothetical Creek near Town | 21.0 | 1971-92 | 1987 | 11.1 | -- | 10 | 12.22 | 4,200 | 25 |
| 2 | 05555000 | Hypothetical River at City | 1,212 | 1939, 1955-92 |
1939 | 12.12 | 28,200 | 12 | 21.21 | 82,800 | >100 |
| 3 | 06930030 | Hypothetical River near Metropolis |
3,333 | 1919-92 | 1943 | 33.33 -- |
-- 99,900 |
13 | 25.55 | 33,000 | <2 |
| 4 | -- | Hypothetical Ditch at Village | -- | 1992 | -- | -- | -- | 19 | -- | 3,800 | -- |
Drainage area in the summary table is the total area, as measured on a flat projection map, that would contribute surface runoff to the indicated site. The contributing drainage area may be smaller than the total drainage area if the total area includes areas of extremely rapid infiltration rates that do not produce surface runoff, or closed subbasins that retain all their inflow.
The column headed "Period" shows the calendar years prior to the described flood for which the stage or discharge shown in the seventh and eighth columns are known to be a maximum. For most sites, this period corresponds to the period of systematic collection of streamflow data. For other sites, written or oral history may indicate that a flood stage was the highest since people have observed the stream or was the highest since some known date.
The sixth column shows the calendar year in which the maximum stage and discharge for the indicated period occurred. The seventh and eighth columns show the stage and discharge of that maximum. Separate listings are made when maximum stage and maximum discharge did not occur concurrently. An effort was made to use stages that were measured relative to the datum in use at the time of the flood being described or to indicate by a footnote that a different datum was used.
The last four columns present data for the maximums during the described flood or floods. The data include the date on which the maximum occurred, maximum stage, and maximum discharge and, where available, the recurrence interval of the discharge.
The probability of a given discharge being equaled or exceeded in any given year frequently is used as an indication of a flood's relative magnitude and for comparison with floods at other sites. The relative magnitude also can be expressed in terms of recurrence interval, which is the reciprocal of the flood probability. A third way of expressing the relative flood magnitude is the percent chance of occurrence, which is 100 times the flood probability. A discharge that will be equaled or exceeded on an average (over a long period of time) of once in 10 years has a recurrence interval of 10 years, is termed a "10-year flood," has a probability of 0.10, and has a 10-percent chance of occurring in any given year. A 100-year flood has a recurrence interval of 100 years, a probability of 0.01, and a 1-percent chance of occurring in any given year. Because recurrence interval is used most commonly by Federal agencies (for example, in the context of flood insurance), it is used in this volume even though percent chance avoids the unintended connotations of regularity of occurrence that accompany the term "recurrence interval."
Equivalence of flood probability and percent-chance values to selected recurrence-interval values is as follows:
| Probability | Percent change | Recurrence interval |
|||
|---|---|---|---|---|---|
| 0.50 | 50 | 2 | |||
| .20 | 20 | 5 | |||
| .10 | 10 | 10 | |||
| .04 | 4 | 25 | |||
| .02 | 2 | 50 | |||
| .01 | 1 | 100 | |||
| Percent chance for indicated time period, in years | |||||
|---|---|---|---|---|---|
| Recur- rence interval |
5 | 10 | 50 | 100 | 500 |
| 2 | 97 | 99.9 | * | * | * |
| 10 | 41 | 65 | 99.5 | * | * |
| 50 | 10 | 18 | 64 | 87 | * |
| 100 | 5 | 10 | 39 | 63 | 99.3 |
| Bismarck (119 years) |
Cedar Rapids (112 years) |
Manhattan (104 years) |
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|---|---|---|---|---|---|
| Year | Total | Year | Total | Year | Total |
| 1993 | 22.57 | 1993 | 44.34 | 1951 | 43.05 |
| 1915 | 17.59 | 1969 | 27.88 | 1993 | 42.21 |
| 1914 | 17.57 | 1902 | 27.63 | 1902 | 34.71 |
| 1879 | 15.60 | 1990 | 27.55 | 1908 | 34.69 |
| 1927 | 15.14 | 1924 | 25.27 | 1915 | 31.22 |
The rates of accumulation for 1993 daily precipitation are compared with normal values in figure 9. The normal values are the accumulated daily averages for 1961-90 smoothed to pass through the month-end accumulated totals. The precipitation totals for January through September 1993 at Bismarck, Manhattan, and Cedar Rapids were about 200 percent of normal for January through September 1961-90, but the rates at which the precipitation accumulated were different (fig. 9). Although the rate of accumulation at Bismarck was about normal through June, a dryer-than-normal January through March caused the amount of precipitation received to be slightly less than normal until the end of June. Three large storms, June 29 through July 1, July 15 and 16, and July 21 and 22, combined to produce the large seasonal totals. There was little precipitation after the middle of August. Precipitation at Manhattan followed a pattern similar to that of Bismarck, except that several large storms came earlier in the year. Manhattan's large seasonal total resulted primarily from precipitation during four distinct periods--March 29 through 31, May 7 through 11, July 1 and 2, and July 18 through 22. However, precipitation continued to accumulate at greater-than-normal rates through August and September.
The rate of accumulation of precipitation at Cedar Rapids was different from that at either Bismarck or Manhattan. Although large storms, such as that for July 4 and 5, contributed to the excessive moisture, the rate of accumulation was greater than normal after mid-March. Unlike Bismarck and Manhattan, the rate reflected the accumulation of many small-to-moderate precipitation amounts and shows the effects of widespread storms over the entire area. The above-normal accumulation rates continued into late August, but September precipitation was near normal.
Much of the severe flooding in the upper Mississippi River Basin during 1993 was the culmination of the wet spring and a series of storms during July. Daily rainfall totaled more than 4.00 inches at many locations during July. Thus, flooding was affected not only by wet antecedent conditions and large rainfall totals, but also by the way July daily rainfall was distributed. Maximum 1- and 3-day rainfall totals for July at Bismarck, Manhattan, and Cedar Rapids are similar (table 4). Maximum 5-day rainfall totals are similar at Bismarck and Cedar Rapids, but the maximum 5-day rainfall total at Manhattan was about 1.6 inches greater than at Bismarck and Cedar Rapids (table 4).
[Data from National Weather Service]
| Rainfall totals (inches) | |||
|---|---|---|---|
| Location | 1-day | 3-day | 5-day |
| Bismarck | 4.32 | 5.74 | 6.18 |
| Manhattan | 4.81 | 5.56 | 7.70 |
| Cedar Rapids | 4.18 | 5.55 | 6.01 |
The floods of 1993 were of historic magnitude as water in the Mississippi and Missouri Rivers reached levels that exceeded many of the previous observed maximums. Although large parts of the flood plains of both rivers upstream from St. Louis, Missouri, were inundated, water levels would have been even higher had it not been for the large volume of runoff retained in flood-control reservoirs. Most of the total flood-control storage available upstream from St. Louis is located along the main stem and tributaries of the Missouri River; the largest concentration of reservoirs is located within the Kansas River Basin (fig. 10). The Kansas River Basin accounts for about 10 percent (60,000 square miles) of the drainage area of the Missouri River Basin, and reservoirs control streamflow from 85 percent (50,840 square miles) of the drainage area of the Kansas River Basin. Analyses of flood discharges in the Kansas River indicate that reservoirs reduced flooding along the Kansas and the lower Missouri Rivers.
Flood discharges from the Mississippi and the Missouri Rivers combined for a historic peak of 1,080,000 cubic feet per second on the Mississippi River at St. Louis, Missouri, on August 1, 1993. Historic streamflow records show that this discharge was the largest since 1861 and has been exceeded only by an estimated discharge of 1,300,000 cubic feet per second for the flood of 1844. Discharge for the flood of 1903, which had been estimated to be 1,019,000 cubic feet per second was slightly less than that of 1993. However, changes in the upper Mississippi River Basin that have been made in the last 50 years, such as the construction of many flood-control reservoirs, reduced the magnitude of the maximum discharge of the 1993 flood at St. Louis.
The function of flood-control reservoirs is to temporarily store a part of the flood discharge for later release so that the flood peak downstream will be reduced. In an uncontrolled stream, the flood discharges of the tributary streams are added to the discharge in the main stem. As a result, the total flood volume increases in the downstream direction, as does the maximum discharge (fig. 11A). In the case of the controlled stream, all or part of the flood discharge is stored in a reservoir for later release at a reduced flow rate (fig. 11B). Downstream from the reservoir, additional flood discharges in the tributaries enter the main stem, which add uncontrolled flood discharges to the controlled discharge. In the actual operation of a flood-control reservoir, the uncontrolled flood discharges from the drainage area downstream from a reservoir need to be considered before reservoir releases are made. If uncontrolled flood discharge from areas downstream from the reservoir produces a flood on the main stem, then reservoir releases can be reduced to near zero to minimize additional flooding downstream, provided storage capacity is available in the reservoir.
Most flood-control reservoirs in the upper Mississippi River Basin are of the multipurpose type, which are used to store water for irrigation, power generation, navigation, public-water supply, and recreation. The flood-control, or flood-storage capacity, pool of a reservoir always is above the multipurpose pool level (fig. 12). All reservoirs with provision for flood control are operated so that a minimum amount of water in the flood-control pool is maintained prior to flooding to maximize flood protection. The flood-reduction potential of a reservoir is compromised if additional floodwater must be stored before the previously stored water can be released.
Flood-control reservoirs are constructed with an emergency spillway to protect the dam from being overtopped, which can cause severe damage to or failure of the dam. Flow through the spillway can be uncontrolled or can be controlled by gates that regulate the releases up to a certain elevation in the reservoir. Once the water level in the reservoir rises to the top of the closed spillway gates or the sill of an uncontrolled spillway, water stored above this elevation in the reservoir is in the surcharge pool. Outflow of surcharge in the reservoir is determined by the depth of water and the geometry of the spillway or the spillway gate opening.
There are 34 major flood-control reservoirs within the Missouri River Basin that drain areas greater than 100 square miles (table 5). Table 5 also includes 11 reservoirs in the Mississippi River Basin upstream from its confluence with the Missouri River. Of the reservoirs in the Missouri River Basin, water levels in 13 reached historic elevations, 3 came within 1 foot of their records, and 6 exceeded their spillway elevations. Water levels in reservoirs on tributaries of the Mississippi River upstream from its confluence with the Missouri River, including Saylorville Lake, Coralville Reservoir, and Lake Red Rock, all in Iowa, also reached record elevations. Several reservoirs in the Arkansas River Basin, just south of the Kansas River Basin, had record and near-record water-level elevations. The number of reservoirs with record water-level elevations is an indication of the magnitude and wide extent of the floods of 1993.
The six-reservoir system on the main stem of the Missouri River from Montana through North Dakota and South Dakota is used for power generation, storage for navigation and public-water supply, and flood control. When the 1993 water year began (October 1, 1992), the total storage content in the reservoir system was about 43,900,000 acre-feet. By April 1, 1993, the total system content had increased to 45,468,000 acre- feet (U.S. Army Corps of Engineers, written commun., 1993). The additional 1,568,000 acre-feet resulted from runoff produced by melting snowpack in the mountains during winter and early spring. The total increase in storage contents of the six-reservoir system from April 1 to midnight August 1, 1993, was 10,293,000 acre-feet and the result of excessive snowmelt and rainfall during this period. During July 1993 alone, reservoirs on the main stem Missouri River stored nearly 5,369,000 acre-feet of floodwater. If this water had been released at a constant rate, the daily average discharge of the Missouri River downstream during July would have been about 87,000 cubic feet per second larger than the observed average discharge of 291,000 cubic feet per second on the Missouri River at Kansas City, Missouri (map reference P, fig. 10).
The Kansas River Basin is about 60,000 square miles in area, of which streamflow from 85 percent, or 50,840 square miles, of the basin is controlled by reservoirs. Except for the main-stem Missouri River reservoir system, the Kansas River Basin is the largest basin under flood control in the Mississippi River Basin. Eighteen reservoirs, which have a total flood-control capacity of 7,390,000 acre-feet, provide flood protection within the basin and along the Missouri River downstream. From April 1 to August 1, 1993, the reservoir system in the Kansas River Basin stored 4,500,000 acre-feet of water. Of this amount, 4,027,000 acre-feet were stored during July alone. If this water had been released at a constant rate, the average discharge of the Kansas River downstream during July would have been about 65,500 cubic feet per second larger than the observed average discharge of 76,800 cubic feet per second on the Kansas River at DeSoto, Kansas (map reference N, fig. 10). About one-half of the 4,027,000 acre-feet were stored in Milford and Tuttle Creek Lakes (reservoir reference numbers 15 and 22 in figure 10). Both lakes filled their flood-control pools and were required to store floodwater in their surcharge pools. Tuttle Creek Lake stored 97,000 acre-feet in its surcharge pool, and Milford Lake stored 207,000 acre-feet in its surcharge pool.
The Chariton River is a tributary of the Missouri River and flows from Iowa through northern Missouri. Lake Rathbun in Iowa and Lake Longbranch in Missouri (reservoir reference numbers 28 and 30 in figure 10) are flood-control reservoirs in the Chariton River Basin and stored 269,000 and 9,000 acre-feet, respectively, during July 1993. The water level in Lake Rathbun reached a record elevation of 927.20 feet above sea level on July 28, thus requiring the storage of 27,000 acre-feet of water in its surcharge pool.
Streamflow from the nearly 15,000-square-mile Osage River Basin is almost completely controlled by Melvern, Pomona, and Hillsdale Lakes in Kansas (reservoir reference numbers 25, 26, and 27 figure 10). and Stockton, Pomme de Terre, and Harry S Truman Lakes and Lake of the Ozarks in Missouri (reservoir reference numbers 31, 32, 33, and 34 in figure 10). The reservoir system in this basin stored 3,547,000 acre- feet of water from April 1 to August 1, 1993; of this total, 3,289,000 acre-feet were stored during July. The effect of Harry S Truman Lake on discharge in the Osage River was significant because the lake stored more than 3,000,000 acre-feet of water during July. The storage in Harry S Truman Lake and that of the other reservoirs in the Osage River Basin system reduced the average discharge of the Osage River at its confluence with the Missouri River for July by 53,500 cubic feet per second.
As severe as the flooding was during 1993, stream and river levels could have been even higher had a system of flood-control reservoirs not been in place throughout the Missouri River Basin. About 10,300,000 acre-feet of potential floodwater were stored in the upper Missouri River main-stem reservoirs in Montana, North Dakota, and South Dakota from April 1 to August 1, 1993. In the downstream sections of the Missouri River Basin, the quantity of water stored from April 1 to August 1 in reservoirs on the Kansas River was 4,500,000 acre-feet, while reservoirs in the Platte, the Chariton, and the Osage River Basins stored 3,900,000 acre-feet. If the total 18,700,000 acre-feet stored in the system had been allowed to flow to St. Louis, the average discharge of the Missouri River would have been 77,300 cubic feet per second greater for this 4-month period. During July alone, the combined storage of about 13,000,000 acre- feet in the Missouri River Basin--5,400,000 acre-feet in the Missouri River main-stem reservoirs, about 4,000,000 acre-feet in the Kansas River Basin reservoirs, and about 3,600,000 acre-feet in the reservoirs of the Platte, the Chariton, and the Osage River Basins--reduced the average discharge of the Missouri River at Hermann, Missouri (map reference Q, fig. 10), from about 587,000 to 376,000 cubic feet per second, which is a difference of 211,000 cubic feet per second. An analysis of the storage of flood volumes in the Missouri River Basin from April 1 to September 1, and specifically during July, enables a comparison of discharges at various points along the river and tributaries with and without the protection of the reservoirs.
The discharges of streams and the changes of storage in reservoirs in the Kansas River Basin during July 1993 were analyzed to estimate the discharges that would have occurred in the absence of the reservoir system. Floodwater that was stored in a particular reservoir was routed down the river valley under high-discharge conditions and added to the observed discharge downstream. This simulation process was iterative because several streams had more than one reservoir. Routing times were determined from observed high discharges before reservoir construction. The Muskingum routing method (Viessman and others, 1972) was used to allow for flood-discharge storage along the river valley as the flood discharges moved downstream. Using this method, daily mean discharges were estimated for selected gaging stations in the Kansas River Basin by using daily reservoir storage and daily observed stream discharges. The computer program BENEFITS (U.S. Army Corps of Engineers, written commun., 1993) was used to estimate the uncontrolled instantaneous maximum discharge at selected gaging stations on the Kansas and the Missouri Rivers. The uncontrolled instantaneous maximum discharges are compared with the observed instantaneous maximum discharges and the simulated uncontrolled maximum daily mean discharges (table 6).
The total effect of the Kansas River Basin reservoirs can be seen in the analysis of the flood discharges on the Kansas River at DeSoto, Kansas (fig. 13). The simulation of uncontrolled discharges resulted in the highest daily mean discharge of 252,000 cubic feet per second on July 10. A secondary simulated uncontrolled discharge of 233,000 cubic feet per second would have occurred on July 26. An observed instantaneous maximum discharge of 172,000 cubic feet per second occurred on July 27, while the instantaneous uncontrolled discharge was 266,000 cubic feet per second on July 27. Many other cities and hundreds of thousands of acres of farmland along the tributaries and main stem of the Kansas River benefited from the flood-control reservoirs as flood discharges were reduced by 30 to 70 percent.
All simulated uncontrolled discharges on the Kansas River would have been contained by the Federal levee system, except in Kansas City where backwater from the flooding Missouri River on July 27 might have caused the river stage to overtop the levee system there. However, without the control of reservoirs on the main-stem Missouri River, the combined uncontrolled discharges of the Kansas and the Missouri Rivers would have overtopped the Kansas City levees (Flood Insurance Administration, 1981).
To maintain storage capacity in flood-control reservoirs, stored floodwater is released as soon as the river downstream can accept it without additional flooding, as indicated by figure 14, which shows water-level fluctuations during the 1993 water year at selected reservoirs in the Kansas River Basin. Water levels in many of the reservoirs in the Kansas River Basin at the beginning of the 1993 water year were above multipurpose-pool elevation, but all were lowered during the 1992-93 winter. However, the snowmelt and precipitation of February through May 1993 resulted in fluctuations and steadily increasing discharge in streams in the Kansas River Basin as summer approached. An example is Tuttle Creek Lake, where, beginning in February, monthly increases in storage were followed by controlled releases to lower the lake level back to multipurpose-pool elevation. At the same time, other reservoirs in the Kansas River Basin were releasing stored water, and many uncontrolled streams were flooding. This combination resulted in many streams being at bankfull capacities for extended periods of time.
This cycle of precipitation, flooding, and resulting releases of water from reservoirs was interrupted during July when intense rains fell somewhere in the basin nearly every day of the month. With most uncontrolled streams at or above flood stage and the lower Missouri and Mississippi Rivers flooding, the flood-storage capacity of the Kansas River Basin reservoir system was nearly completely filled. Some floodwater was released as water levels in Tuttle Creek and Milford Lakes reached surcharge storage elevations, but the reservoir system performed effectively to reduce the flooding.
From spring through summer of 1993, severe flooding in the upper Mississippi River Basin resulted from intense, persistent, widespread rainfall from January through September. The flooding was unusual because it came so late in the spring-summer runoff season and because of the large number of streamflow-gaging stations that had record or near-record maximum discharges. Record maximum discharges were recorded from mid-June through early August at many U.S. Geological Survey (USGS) streamflow-gaging stations in the Minnesota River Basin in Minnesota; in the Skunk, the Des Moines, the Little Sioux, and the Nishnabotna River Basins in Iowa; on the Mississippi River at Keokuk, Iowa; in the James River Basin in North and South Dakota; in the Platte River Basin in Nebraska; in the Kansas River Basin in Kansas; in the Grand River Basin in Missouri; and along the Missouri River from St. Joseph to Booneville, Missouri. Unusually high flood discharges were recorded at other locations throughout the area of flooding. The flooding also was unusual for its long duration and widespread and severe damage. At St. Louis, Missouri, the Mississippi River reached flood stage on June 26 and remained above flood stage until late August. Millions of acres of agricultural and urban lands in the upper Mississippi Basin were inundated for weeks, and unofficial damage estimates exceeded $10 billion (Parrett and others, 1993).
For comparative purposes, flood-maximum discharges are referenced to a specific recurrence interval or probability of occurrence. The recurrence interval is the average number of years between occurrences of annual maximum discharges that equal or exceed a specified discharge. For example, a discharge that has a 100-year recurrence interval is so large that an equal or greater annual maximum discharge is expected, on average, only once in any 100-year period. Because of the random nature of flood events, the times between annual maximum discharges of a certain magnitude are far from uniform; a large flood in 1 year does not preclude the occurrence of an even larger flood the next year. In any given year, the annual maximum discharge has 1 chance in 100 of equaling or exceeding the 100-year flood (U.S. Interagency Advisory Committee on Water Data, 1982).
Recurrence intervals for the 1993 flood peaks presented in this report are generally determined by using the most current published USGS flood-frequency reports for States in the area of flooding. Recurrence intervals for the 1993 maximum discharges on the Kansas River, the Missouri River, and the Mississippi River are based on unpublished flood-frequency analyses completed by the U.S. Army Corps of Engineers (Gary Dyhouse, St. Louis District, U.S. Army Corps of Engineers, written commun., 1993; Jerry Buehre, Kansas City District, U.S. Army Corps of Engineers, written commun., 1993).
The magnitude and timing of several rainstorms during late June and July, combined with wet antecedent climatic conditions, were the principal causes of the severe flooding in the upper Mississippi River Basin. To illustrate the effect of the timing of runoff from these storms on the maximum discharge in the Mississippi River, the maximum discharges and their dates of occurrence for selected streamflow-gaging stations in the general area of flooding are shown in figure 15.
During June 17-18, 2 to 7 inches of rain fell throughout southern Minnesota, northern Iowa, and southwestern Wisconsin. Runoff from this storm caused flooding on the Minnesota and the Mississippi Rivers in Minnesota and the Chippewa and the Black Rivers in Wisconsin. As a result of these floodwaters, the discharge of the Mississippi River at Clinton, Iowa, peaked on July 8, 1993.
Two separate storms during early July caused large-scale flooding in Iowa. During the first storm on July 5, 2 to 5 inches of rain fell in central Iowa and caused lowland flooding on the Iowa, the Skunk, and the Des Moines Rivers. During the second storm on July 8-9, 2 to 8 inches of rain fell in central Iowa. Rivers throughout central Iowa had not receded from the July 5 storm, and the three major reservoirs in this part of the State were at capacity. The runoff from this storm, combined with the runoff from the July 5 storm, caused record or near-record maximum discharges at streamflow-gaging stations throughout the Iowa, the Skunk, the Raccoon, and the Des Moines River Basins. The floodwaters from these rivers entered the Mississippi River at about the same time as the flood peak from the late June storm in northern basins reached Keokuk, Iowa. The coincident timing of the flood peaks from these tributary rivers increased the maximum discharge on the Mississippi River and aggravated flooding on the Mississippi River from Davenport, Iowa, to St. Louis, Missouri. The discharge on the Mississippi River at St. Louis that resulted from these combined floodwaters peaked on July 20.
On July 15-16, 2 to 7 inches of rain fell in eastern North Dakota and western Minnesota and caused flooding in the upstream reaches of the Minnesota River Basin in Minnesota and the James River Basin in North Dakota. Although maximum discharges from this storm were not as large in the downstream reaches of these basins as the maximum discharges of late June, the floodwaters from the James River added to the flooding of late July on the Missouri River.
From July 22 to 24, 2 to 13 inches of rain fell in parts of Nebraska, Kansas, Missouri, Iowa, and Illinois. The runoff from this storm caused record maximum discharges on the Platte River in Nebraska and contributed large flows to previously filled reservoirs in the Kansas River Basin in Kansas. Maximum discharges on the Kansas River were the largest since 1951, which is before significant river regulation began. Discharges also were near-record on the Nishnabotna River in Iowa and the Illinois River in Illinois.
Before the July 22 to 24 storm, the Missouri River was at or near flood stage as a result of large tributary inflows earlier in the month from the James River in North and South Dakota, the Big Sioux River in South Dakota, and the Little Sioux River in Iowa. As a result, floodwaters from the Platte and the Kansas Rivers caused record or near-record maximum discharges on the Missouri River at streamflow-gaging stations downstream from the confluence of the Platte River. The flood peak on the Missouri River reached Hermann, Missouri, on July 31. The maximum discharge from the Missouri River caused a second and greater maximum discharge at the streamflow-gaging station on the Mississippi River at St. Louis on August 1.
Flood conditions on the Mississippi River differed upstream and downstream from the confluence of the Ohio River. At Thebes, Illinois, just upstream from the confluence, severe flooding on the Mississippi River peaked on August 7. Downstream from the confluence, flooding on the Mississippi River was not severe because of less-than-average discharge contributed by the Ohio River and a substantially larger channel capacity in this reach of the Mississippi River. The discharge of the Ohio River was less than average during July and August as a result of generally dry conditions and low reservoir outflows throughout the Ohio River Basin.
Flooding during the spring and summer of 1993 in the upper Mississippi and Missouri River Basins was widespread, encompassing nine States. Many streams in this nine-State area had historic floods, while some streams had only moderate flooding. Tables 7-15 include a compilation of flood information for selected streams within each of the states of Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, North Dakota, South Dakota, and Wisconsin (U.S. Geological Survey, 1994). Figures 16-24 provide the location of the streamflow-gaging stations within each State. Only streams within the upper Mississippi and Missouri Basins are listed. Flooding outside of these basins or other than the spring and summer of 1993 are listed in the sections "Summary of Floods of 1992" and "Summary of Floods of 1993."
