dams and the missouri river -- 9/29/21
Today's selection -- from Holding Back the River by Tyler J. Kelley. Thousands of dams were built in the U.S. in the early 20th century. None were built with an awareness of the need to manage sediment. As a result, the functions of many are being compromised:
"Inquiring after an unfamiliar tributary of the Mississippi, seventeenth century French explorers recorded an Algonquian word that meant 'muddy water,' a fact William Clark knew from unpalatable experience. In his journal of June 21, 1804, Clark wrote, 'The water we Drink, or the Common water of the missourie at this time, contains half a Como Wine Glass of ooze or mud to every pint.' When Clark recorded those words, his expedition was approaching the site of modern-day Kansas City, and the ooze they didn't swallow was flowing down the Missouri, into the Mississippi, and on toward the Gulf of Mexico. Half the Missouri's watershed is semi-arid, and although its basin stretches from the Rockies to St. Louis, and from Saskatchewan to Kansas, the river carries comparatively little water. What it does carry is sediment. At least it used to. In pre-dam times, the Missouri supplied more than half the Mississippi's sand, silt, and clay. Now, thanks to dams on the Missouri and Arkansas Rivers, the Mississippi's sediment load has declined by almost two-thirds.
"Gavins Point Dam is the smallest and southernmost of the six dams that slow down and back up an eight-hundred-mile stretch of the Missouri River. Spanning the river near Yankton, South Dakota, it consists of a mile-and-a-half-long grass-covered slope, a powerhouse with three turbines, and a concrete spillway topped with fourteen Tainter gates. Gavins turned a shallow, fast-moving river into Lewis and Clark Lake, a twenty-five-mile-long reservoir that flooded the valley from bluff to bluff. As soon as Gavins began holding back water, in 1955, the newly formed lake began filling with sediment.
"Another river also disgorged into this lake -- the Niobrara, which drains some of the driest land on the Great Plains, including the Nebraska Sand Hills. It, too, carries a lot of sediment. Before the dam was built, a small delta of pure sand would form where the Niobrara flowed into the Missouri. When the Missouri flooded, the delta was washed away. But sand is heavy. To carry it, water must move swiftly. When Gavins Point was completed, the Niobrara was suddenly dumping sand into a still lake instead of a fast river. Because other dams were operating upstream, the Missouri lacked the force to scour the sand away. It piled up in the riverbed and stayed. Deltas form where rivers hit still bodies of water -- usually oceans, but in this case, a man-made lake.
|Gavins Point Dam on the Missouri River, impounding Lewis and Clark Lake.|
"The Missouri dropped its own heavy sediments at the same spot, and, as this mass of material accumulated, it advanced into the reservoir. It broke the surface. Islands formed. Trees and grass took root. The delta expanded and the lake contracted. By 2018, only fifteen miles of the original twenty-five-mile reservoir could be called a true, open-water lake. The rest was a textbook delta, a maze of tentacular channels and hummocky shoals. Except for the bluffs, it could pass for the mouth of the Mississippi, the Danube, the Ganges, or the Mekong.
"As the Lewis and Clark delta grew larger, a useful reservoir grew proportionately useless. For lack of a better word, specialists in the still-new field of sediment management declared Lewis and Clark Lake 30 percent 'full' in 2018. Full of sediment, not water. Any dam that isn't moveable like Olmsted must gradually surrender its capacity for storing water to the relentless assault of sediment. The reservoirs behind all seventy-nine thousand dams in the United States are filling at different rates. Reservoirs in the arid American West, where man-made systems are often the only source of water, are filling the fastest. Thirsty for the power and wealth to be gained from controlling these rivers, American engineers and politicians built thousands of dams in the first half of the twentieth century. None were built with a way to manage sediment."