The Missouri is Digging Deep

U.S. Geological Survey hydrologists recently reported that the Missouri’s present flood is carrying far less sediment than previous floods.  The reason is simple; the high flows are coming out of the Dakota reservoirs rather than from the Lower Missouri’s dirtier, uncontrolled tributaries.  The upstream reservoirs are trapping the river’s silt load and discharging clearer water through spillways and outlet tunnels.

The hydrologists also noted that the river is moving at almost 12 miles per hour, which is double its average speed.  The engorged river’s clear water and higher rate of speed is combining to reshape the channel’s morphology.  Since the start of the flood, new chutes, holes, and banklines have been gouged out of the valley’s soft alluvium.  The government hydrologists observed that the river at Nebraska City, Nebraska, normally holds a depth of between 12 to 16 feet.  But with the onset of the flood, the Missouri has dug a hole there that measures 66 feet deep.

Between Gavin’s Point Dam and the mouth of the Platte River, the Missouri River degraded its bed at least fifteen feet since the closure of the Dakota dams in the 1950s and 1960s.  In recent weeks, the Missouri has spread out across its valley.  Near Missouri Valley, Iowa, the river is 11 miles wide.  At Nebraska City, the river is normally 1,100 feet wide, but it now extends 2.6 miles.  The bulk of the floodwater that has overtopped the banks of the river must find its way back to the degraded navigation channel.  The returning floodwaters are digging down to meet the navigation channel’s lowered elevation.  At this very moment, the Missouri is excavating deep furrows through the valley.  The downcutting is washing out tons of prime agricultural land, undermining roadways, destroying the foundations of buildings, and outflanking pile dikes and revetments.  East of Decatur, Nebraska, the Missouri is eroding the east abutment of a bridge that spans the river.

Believe it or not, there is a positive side to the river’s erosive actions.  As the Missouri burrows new channels through its former floodplain, it increases its carrying capacity and lowers its flood crest.  Through erosion, the Missouri is reducing its present flood height and enhancing its ability to carry future floods.

When the floodwaters recede, the Army will almost certainly survey the entire river from the mouth to Ponca, Nebraska, to determine the extent of damage to the navigation channel.  The damage will be extensive.  General John McMahon at the Army Corps of Engineers’ Northwestern Division headquarters, acknowledged that the cost of repairs to infrastructure damaged by the flood will be high.  The military will seek federal funds to reconstruct destroyed piles dikes, revetments, and chevrons.  Congress must not provide funds for those purposes.  The Army’s design channel worsened flooding in the lower valley.  Moreover, its reconstruction would contribute to future flooding.  Finally, in this age of record deficits, the reconstruction of the navigation channel would be an abject waste of taxpayer dollars.  The volume of barge traffic on the lower river does not justify the cost of rebuilding the training structures.  The river is currently dismantling the navigation channel while increasing its carrying capacity.  At the end of the flood, American society would be foolish to again constrain the river behind the Army’s riprap.

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