How the Lower Missouri Became Flood-Prone

Ten days after Lewis and Clark started up the Missouri on their epic voyage westward, their expedition nearly came to an inauspicious end.  On May 24, 1804, while steering the fifty-five-foot long keelboat across mid-channel, the full weight and force of the Missouri struck the keelboat broadside, almost tipping the ship over.  Had the keelboat gone down in the Missouri that day, the explorers would have had to turn back, since the vessel held the supplies necessary for the long journey ahead.  This incident revealed to the captains, and the crewmen manning the keelboat and two pirogues, that the Missouri’s currents were not only powerful, but shifting and difficult to decipher.  The explorers recognized that the Missouri was an unpredictable, potentially dangerous stream that could sink a keelboat in an instant and take a man’s life just as quickly.

Following the near loss of the keelboat, Lewis and Clark navigated the Missouri more cautiously.  Their healthy fear of the river’s power helped them reach the Missouri’s headwaters without losing a single man to the Missouri’s cold, silty waters.

When the expedition returned down the Missouri in the summer of 1806, the explorers encountered a different river than the one they had experienced two years earlier.  Some river reaches had changed so much the explorers had trouble recognizing any former features.  Mud flats lay where timber tracts once stood, the main channel, or thalweg, flowed through previously dry land, and sand dunes reclined where willows had once been firmly anchored to the bank.

Lewis and Clark’s Missouri River was what hydrologists refer to as “meandering,” meaning it was characterized by long, graceful bends and a relatively deep, narrow, main channel.  But by the mid-nineteenth century, that meandering river began to disappear, replaced by an altogether different kind of river.

In the 1840s and 1850s, a wet precipitation cycle began across the Missouri drainage basin.  Heavy winter snows, and torrential spring and summer rains, caused the Missouri to flood more frequency and at greater heights.  Agricultural settlers played a part in the flooding too.

By the early 1850s, European-American settlers occupied the prairies of western Missouri and western Iowa.  These settlers had a vociferous appetite for wood.  Consequently, they felled the extensive forests growing along the Missouri and its tributaries.  They also plowed under the prairie.  Those two actions allowed still more runoff to enter the Missouri.

During the era of agricultural settlement, major floods struck the Missouri Valley in 1844, 1857, 1858, 1867, 1872, 1874, 1875, 1878, and 1881.  The 1881 deluge represented the greatest Missouri River flood of the nineteenth century; it devastated communities from Montana Territory through the state of Missouri.

In addition to inflicting severe damage on the farm economy, the floods dramatically altered the Missouri.  In order to accommodate the larger volume of water descending its channel, the river became straighter and wider, with multiple channels – or what is known in hydrological terms as “semi-braided.”  By the end of the nineteenth century, the semi-braided Missouri looked and behaved little like the meandering river of Lewis and Clark.

In the twentieth century, the Missouri underwent yet another major change – this one brought on by human action.  Employing a system of pile dikes, willow mattresses, and stone revetments, the Army Corps of Engineers constructed a nine-foot-deep navigation channel in the Missouri from the river’s mouth to Ponca, Nebraska – a distance of 753 river miles.  The navigation channel narrowed the average width of the Lower Missouri from 2,363 feet to 739 feet; it also eliminated the river’s islands, sandbars, and side channels.  For all intents and purposes, the Army turned back the clock along the Missouri, forcing the semi-braided river to once again flow as a meandering stream.

Defying nature carries risks.  Specifically, the Army’s new, narrow, channelized river lacked the conveyance capacity (the ability to move water downstream) of the former semi-braided river.   But in the Dirty Thirties, when the Army extended the navigation channel to Sioux City, this wasn’t a problem.  In that dry decade, the Missouri was starved for water.  However, in the 1940s, the rains returned to the Missouri Basin.  As a result, the Army’s engineered, meandering Missouri could not safely haul away the large volume of water entering it.  In consequence, the Lower Missouri jumped out of its navigation channel, and attempted to reestablish its former semi-braided channel area, every year between 1941 and 1952.

In 1943, Colonel (later General) Lewis A. Pick, who oversaw the management of the Missouri from his office in Omaha, believed the solution to the navigation channel’s reduced conveyance capacity was not to reestablish the Missouri’s semi-braided character, but to build dams along the Missouri main-stem in the Dakotas.  Those dams, Pick argued, would keep high flows out of the flood-prone navigation channel.

Pick got his dams.  Between 1946 and 1966, the Army built five massive dams in North and South Dakota.  Those dams did reduce the amount of spring and summer runoff entering the river south of Ponca, but they did not completely stop the flooding.  The channelized Lower Missouri rose above its rock-lined banks in 1971, 1973, and 1984.  Then, in 1993, a colossal flood ripped through the lower valley.  That flood should have led to a serious re-examination of the navigation channel, but it did not.  The farm lobby, as well as river navigation interests, killed any attempt to address the navigation channel’s vulnerability to high flows.

Since 1993, climate change has increasingly influenced the Missouri’s hydraulic regime. Four of the highest runoff years on record occurred in 1997, 2011, 2018, and 2019.  As the earth warms, the Missouri is becoming more voluminous, more flood-prone, and more dangerous.  Yet, the Army, at the behest of powerful interest groups, has not effectively responded to the changing climate or the higher volumes of runoff entering the Mighty Missouri.  Instead, after every major flood, the Army has rebuilt the navigation channel, which only primes the river for another flood.

For almost 130 years, the Army has been engaged in a war against the Missouri.  Army engineers have dammed the river, redirected it, deepened it, dumped millions of tons of rock into it, and even dynamited it – all in an attempt to confine the Lower Missouri to a meandering channel.  Ultimately, all of those efforts have failed.  And although the Army will never admit it, it has lost its long war with the Missouri.  The spectacular floods of 2011 and 2019 are irrefutable proof of that fact.

In most wars, a defeated army either retreats or surrenders.  But the Army Corps of Engineers can do neither along the Missouri.  Rather than roll up their battle flags and humbly march back to their comfortable, air-conditioned offices in Kansas City and Omaha, the Army engineers need to stay along the Lower Missouri and re-make it yet again – into what it should have been since the late nineteenth century – a semi-braided river capable of safely carrying a greater volume of water.

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