The Interaction of Floods and Droughts in the Missouri River Basin

Following the 2011 flood, the Corps conducted two post-flood analyses. The first analysis (USACE 2012a) focused on a review by an independent panel of experts that examined the causes and response to the flooding and provided recommendations to mitigate or prevent impacts from similar or larger floods in the future. The second report (USACE 2012b) examined whether keeping more flood storage in the Missouri River mainstem reservoir system would reduce flood risk for subsequent floods. Flood control in the Missouri River Basin is based on runoff entering the reservoirs during the spring/summer when runoff is at its peak. Under ideal conditions, the mainstem reservoir system begins the runoff season with its conservation storage full, which is

78 percent of the total storage, and the flood control storage empty, which is the remaining 22 percent of total storage. The large volume of conservation storage was provided in the system's design so that the reservoir system could serve authorized purposes, such as navigation and water supply, through long extended droughts, such as the 1930-1941 drought. The water stored in the flood control zone in the spring/summer is then released at slower rates over the remainder of the year to meet authorized purposes. The water in the reservoir system's flood control zone is not saved from one year to the next, and efforts are made to evacuate all stored floodwaters prior to the next runoff season. Evacuating stored floodwaters during the fall and winter, however, is complicated by the fact that during the winter months, large releases are not possible because of ice, which reduces the capacity of the river channel. This limits the number of days the Corps has to release large amounts of stored flood waters prior to the runoff season and results in decisions about how much water to evacuate being made in the fall, before ice begins to form, and prior to knowing the amount of precipitation in the upcoming water year.

This presents a problem for both floods and droughts in that the Corps must balance releases from the mainstem reservoir system to ensure enough flood control storage space is available to capture runoff during flood events but not so much that it cannot meet the other authorized purposes of the reservoirs: navigation, hydropower, water quality control, water supply, irrigation, fish and wildlife, and recreation. The significance of the Corp's post-flood analysis (USACE 2012b) on creating more flood control storage space was that in addition to determining whether additional storage would reduce flood risk, the analysis also assessed the potential economic impacts of increasing flood storage space at the expense of some of the other authorized purposes. The analysis concluded that the economic impacts to the other authorized purposes would be significant if more flood control storage space was made, but when single large flood events were considered, like 2011, the benefits of more flood control storage space would be an important consideration. The ability of the Corps to reflect on singular large runoff events to make this trade-off between flood control storage space and meeting the requirements of the other authorized purposes, though, is predicated on the ability to forecast their occurrence at lead times of 6 months or more. To understand the feasibility of using forecasts at such long lead times, the Corps commissioned the NOAA Earth System Research Laboratory and its Physical Sciences Division (Hoerling et al. 2013) to assess the meteorological conditions in which the flood occurred and whether it could have been predicted. The report concluded that the record spring rainfall was the most critical variable in the series of events that led to the flooding. The report also noted that the below-normal temperatures and above-normal precipitation in the winter of 2010 were consistent with what would be expected in a La Nina year, but it could not explain the wet spring in the upper part of the basin. The report also concluded that there was no way to predict, in the fall of 2010, the record spring rainfall, which would have been necessary for the Corps to have had enough time to evacuate additional water prior to the runoff in 2011. The lack of a skillful seasonal forecast was confirmed in a subsequent study by Pegion and Webb (2014) that focused specifically on the seasonal forecast skill for the 2011 flood as well as the predictability of the 2012 drought. The authors concluded forecast skill was only good at short lead timescales for the lower Missouri Basin and only during El Nino events, and that neither the 2011 flood nor the 2012 drought could have been predicted at seasonal lead times using existing forecast methodologies and models. Based on this study and the Hoerling et al. (2013 and 2014) assessments, forecasts alone would not be sufficient to inform the Corps' water management decisions at the necessary timescale and improve their ability to anticipate these large annual extremes in runoff.

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