The Missouri River once carried impressive amounts of suspended sediment. That sediment found its way into the stream from the prairie-pothole region of North Dakota, the badlands of South Dakota, the short-grass plains of northeastern Nebraska, and the tall grass prairies of Iowa. It came in many forms, including fine granules of sand, tiny particles of clay, bits of pulverized coal, and even pea-sized gravel. It floated, rolled, and spun its way downstream. A portion of it spilled onto the valley lowlands during the Missouri’s annual floods. Some of it became sandbars or islands. A percentage of it flowed all the way to the Mississippi and beyond. The Missouri acted as a conveyor belt, moving soils from the Rocky Mountains and northern plains to and thru the agricultural Midwest. According to the U.S. Army Corps of Engineers, the annual suspended sediment load past Omaha equaled 100,375,000 tons. That was enough goop to fill 5,500 railroad cars (with fifty ton capacities) every single day.
After the Army erected the large earthen dams in the Dakotas, the Missouri experienced a dramatic decrease in its suspended sediment load. The reason was simple. Upstream reservoirs acted as silt traps, capturing the Missouri’s sediment load in their calm, deep waters. After 1955, and the closure of Gavin’s Point Dam’s embankment, the Army noted that the river’s silt content along the reach in eastern Nebraska and western Iowa diminished from over 100 million tons per annum to a mere 4 million tons. The river had long been known as the Big Muddy. But in the post-dam era, it became something altogether different. Gerald Jauron of the Iowa Conservation Commission (the predecessor organization to today’s Iowa Department of Natural Resources) observed firsthand the change in the river. He remarked, “They closed the dam [at Gavin’s Point) last summer. For a couple of days the Mo practically stopped running, and you could almost spit across the river when they shut off the flowage. Then it began to flow over the spillway and the river began to rise again. When it did, I’d never seen the Missouri cleaner.”
The water emanating from the dams was clear and cold, unlike the murky and warm water of old. Army engineers referred to it as hungry water. Hungry water seeks sedimentary material to slow its momentum. In other words, hungry water is highly erosive. Once Gavin’s Point Dam blocked the Missouri, the hungry water exiting its powerhouse tunnels began to tear away the Missouri’s bed – picking up sand and gravel and carrying the detritus off to points further south. Degradation occurred from Gavin’s Point Dam to at least the Kansas City area.
By 1980, the Missouri had degraded its bed 8.5 feet at Sioux City. Although degradation slowed in subsequent years as the Missouri increasingly flowed over coarser bed materials, such as large, erosion-resistant boulders, degradation continued into the 1990s and first decade of the twenty-first century. By the beginning of 2011, the dammed and channelized Missouri had eaten away an additional three feet of its bed at Sioux City. Then came the Great Flood.
Because the 2011 flood originated in Montana and the Dakotas, the deluge first passed through six reservoirs before it reached the lower valley. In the process, the reservoirs filtered out the bulk of the river’s silt load. Even though the floodwaters looked muddy to observers, hydrologists with the U.S. Geological Survey noted the paltry amount of silt in the water.
Because the huge volume of floodwater originated in the reservoirs, it behaved like hungry water on steroids. It had a rapacious appetite for bed material. The high flows of 2011 blew out still more of the Missouri’s bed. At the end of the deluge, hydrologists discovered that the river had degraded its channel another three feet at Sioux City. The Missouri here now flows 14.5 feet below its pre-dam elevation.
To some, the additional degradation might be considered a good thing because it increases the river’s carrying capacity. When the next flood comes barreling down the Missouri (and there will be another flood), it’s going to take more water for it to reach flood stage. But before anyone celebrates this news, he/she should consider the negative repercussions of the recent streambed degradation.
An additional three-foot drop in the Missouri’s elevation equals a three-foot drop in the valley’s water table. A lower water table means municipalities and farm operators will be forced to dig deeper water wells. The foundations of buildings located in the valley will be more likely to shift or subside with the drop in the water table. Power plant operators will have to extend intake pipes to the lowered river. City and county governments will have to construct longer riverside boat ramps.
All of the Missouri’s tributaries south of Yankton will adjust their own elevations to meet the new, lower main stem. Tributaries such as the Big Sioux, Little Sioux, and Soldier rivers in western Iowa will down cut to join the bigger river. This will increase their slopes, speed up their current velocities, and increase erosion along their banks. Agricultural lands across the valley and especially within the Loess Hills will experience greater soil erosion as the lowered streams in the region push snowmelt and rainwater off of farm fields at a faster rate.
But the costs of degradation do not end there. Thousands of rural bridges in western Iowa and eastern Nebraska will face the risk of structural damage as down-cutting streams collapse their banks and blow out their beds. The last remaining valley wetlands will face further water losses as they drain into the degraded Missouri. Wetland losses will adversely effect duck and geese populations, which will mean diminished hunting opportunities.
Degradation affects the aesthetics of the Missouri River. At Sioux City, the Missouri flows between high, steep riprapped banks. There is little to no life along the stream within the metroplex. As degradation has progressed unabated, the Missouri is taking on more and more the appearance of a denuded, rock-lined urban drainage ditch.
But we can halt degradation. Years ago, the National Research Council noted degradation could be stopped through the repositioning or dismantlement of pile dikes and revetments. If the Army widened the navigation channel through this area a mere 400 feet, degradation would end. But would valley residents, and particularly the members of the politically-powerful Farm Bureau and Corn Growers Association, be willing to give the Missouri a little bit more wiggle room to solve the problem? Probably not.