Construction of Unit I Missouri River Left Bank, Unit II Missouri River Section I and II Left Bank (Levee Alignment A) began in July of 1946 and work including rehabilitation after the flood of 1952, was completed in 1954.
Construction of Unit II Section III, better known as Indian Creek Tieback began in July 1946 and was completed in 1954. Indian Creek Channel Improvements which included the removal and disposal of silt deposits in the channel and the lowering of the control sill at the mouth approximately 5.7 feet were completed in 1963.
Construction of L-627 Missouri River Left Bank north of the South Omaha Bridge started on 21 May 1949 and was completed on 15 October 1950. Construction of L-627 south of the South Omaha Bridge (Approximate Sta. 207+00 to Sta. B376+00) started on 11 May 1950 and was completed on 28 September 1950.
Construction of L-624 Missouri River Left Bank (Approximate Sta. C180+00 to Sta. C294+00) and Mosquito Creek Tieback started on 11 May 1949 and was completed on 15 October 1950. Underseepage management control feature installation started on 18 May 1950 and was completed on 28 September 1950.
Flood of April 1952
The flood of April 1952 occurred due to rapid melt of abnormally heavy winter snow accumulation and ice jams occurring in upper portions of the Missouri River Basin (i.e. Montana, North Dakota, and South Dakota). The rapid melt and ice jams that followed occurred during a period of very warm weather at the end of March. While areas upstream of the City were greatly affected, this flood is notable because L-627 Missouri River Left Bank and Indian Creek Tieback Right Bank were only partially completed. Peak discharge occurred on 18 April at 396,000 cfs with a peak river stage at 30.2 feet. Prior to the flood event, existing levees and floodwalls were raised by the USACE. Parts of the City located in the floodplain were evacuated and approximately 6,000 army troops assisted in levee work and patrols. The levees were raised 3 to 4 feet by adding earth fill upon which wood flashboards were erected and anchored by sandbag weights. The greatest single threat to the levees occurred on the night of 18 April when the area around Grace Street sewer began to breakthrough. An emergency repair was constructed by dropping steel beams and rock across the riverside of the sewer. It was also reported that seepage boils developed and ring dikes were constructed in several areas along the levee.
Flood of 1967
The flood of 1967 was significant because it was the first major flood that occurred after the initial construction of the currently operational Missouri River main stem reservoir system. Above normal runoff originated from three primary sources: plains snowmelt, mountain snowmelt, and intense summer rainfall. Operation of the main stem reservoir system significantly reduced Missouri River flood damages from Fort Peck Dam in Montana to the mouth of the Platte River in Nebraska. Flooding primarily occurred along the Missouri River from Omaha downstream to Rulo, Nebraska.
Flood of 1984
The flood of 1984 began in late spring when heavy, wet snow and rain fell over a large area of southern South Dakota through Nebraska, Kansas and Missouri. The intense rainfall fell over a wide area already saturated from heavy rainfall in April and May. During early June, the heaviest rainfall occurred in the lower basin over northwest Missouri and southeast Nebraska. The resulting runoff produced record and near-record stages on many southeast South Dakota tributaries and produced the highest Missouri River stages since 1952 from Sioux City, Iowa, to Omaha, Nebraska. According to the 1993 USACE Flood Report, the 1984 flood produced stages to within 10 feet of the Levee System’s embankment top.
Flood of 1993
During the flood of 1993, rain fell in the Missouri River basin every day from 14 March through 29 July. During the period of 1 June to 27 July, rainfall occurred 34 out of 57 days at Omaha, Nebraska. These heavy rains caused the most severe flooding on the Missouri River, from the confluence of the Platte River to the mouth, since 1952. Rainfall amounts up to 25 inches were recorded in southeast Nebraska and southwest Iowa between 21 July and 25 July, with a peak stage of 30.2 feet and a discharge of 115,000 cfs at Omaha, Nebraska, corresponding to the 10 percent annual exceedance probability flood. According to the 1993 USACE Post Flood Report, L-627 Missouri River Left Bank experienced no problems and the 1993 floodwaters were within 12 to 14 feet below the levee top. According to the same report, the flood produced stages within 10 feet of the Mosquito Creek Tieback levee top; however, no high-water marks were set.
Flood of 1997
Runoff in the Missouri River basin upstream from Sioux City during the calendar year of 1997 was the highest annual runoff in 100 years of record; it nearly doubled the average runoff record and was nearly 20 percent higher than the previous record runoff that occurred in 1978. At Sioux City and Omaha, the Missouri River remained well below flood stage. However, low-lying agricultural areas adjacent to the river experienced flooding and drainage problems throughout the spring, summer, and fall.
Flood of 2011
The flood of 2011 began in May and lasted through October. The flooding was caused by record snowpack in the mountains and plains of the Missouri River Basin in combination with above-average precipitation in the upper portions of the Missouri River Basin. These conditions caused the main stem flood control reservoirs on the Missouri River, which are regulated and operated by the USACE, to reach flood stage levels. As a result, record-high outflows from these reservoirs were required to be released in order to maintain reservoir water surface elevations that would not jeopardize the structural integrity of these reservoir’s dams. The USGS measured a peak flow of 210,000 cfs at River Gage 06610000 located south of the I-480/Highway 6 Bridge. This stream flow represents the highest recorded flow at the gage since completion of construction on the last Missouri River main stem reservoir in 1964. Using the period of record from 1964 to 2011, the 2011 peak flow is estimated to have an annual exceedance probability of less than 0.5 percent representing a flood event with a return period greater than 200 years. Water surface levels climbed above flood stage (29.0 feet; 977.24 NGVD 29) on 31 May 2011; reached a peak stage of 36.07 feet on 24 July 2011 and remained above flood stage through 10 September 2011. In addition, the average daily flows for every month in 2011 were higher than average. The Levee System sustained damages throughout the system from the flood and therefore required repair and rehabilitation.