Stream Flow Monitoring

By editor

Fort Benton, Chouteau County, Montana

On a bright June day in 1805, Meriwether Lewis, one half of that famous exploring duo, found himself squinting at the Missouri River where it met the Marias. Standing on the muddy bank near what would one day be called Fort Benton, Lewis measured the rivers with the kind of precision that only a man with a surveying chain and a determined spirit could muster. He wrote in his journal: "we took the width of the two rivers, found the S. fork 372 yards and the N. fork 200. The north fork is deeper than the other but it's courant not so swift; it's waters run in the same boiling and roling manner which has uniformly characterized the Missouri throughout it's whole course so far; its waters are of a whitish brown colour very thick and tebid, also characteristic of the Missouri; while the South fork is perfectly transparent runs very rappid but with a smooth unriffled surface its bottom composed of round and flat stones like most rivers issuing from a mountainous country."

Lewis’s measurements, scribbled with a hurried hand, marked the beginning of a long American obsession with knowing what the rivers were doing at any given moment. It’s a habit that the United States Geological Survey (USGS) picked up more than seventy-five years later in 1881, when they installed the first stream flow monitoring station in Montana at this very spot. That station has been keeping an eye on the Missouri River ever since.

The Missouri, by that time, was no longer just a wild waterway winding through an untamed land. By the late 19th century, it had become a crucial artery for commerce and settlement. Fort Benton itself, perched on the river’s edge, had grown into a gateway town for steamboats laden with goods, miners, speculators, and hopeful settlers pushing westward. The steamboat captains needed to know how high the water was, or they'd find themselves stranded on sandbars or dashed against hidden rocks.

The USGS station was part of a nationwide system that today includes roughly 7,000 streamflow stations scattered across the country. It’s a network funded by a patchwork of federal, state, local, and tribal governments, all united by a practical concern: understanding the flow of water because water is the lifeblood of economies and communities. The river’s behavior affects everything from irrigation and flood control to navigation and power generation.

The instruments used in Fort Benton might look like a modest collection of pipes and floats to the untrained eye, but they’re clever contraptions. A float rides on the water’s surface inside a stilling well -- a quiet chamber connected to the river. As the river rises and falls, the float moves up and down, and this motion is converted into an electrical signal recording the water surface elevation, known in the trade as "stage." This stage is measured in feet and logged every hour, then blasted via satellite to a USGS computer database. The data is precise enough to calculate the volume of water flowing past in cubic feet per second, a figure that can swing wildly depending on the season and weather.

Back in Lewis’s day, the only way to measure a river’s flow was to measure its width and depth and guess at the speed of the current -- all while trying not to get swept away. Today’s hydrographers use current velocity meters lowered from a bridge 0.6 miles upstream, depth-sounding weights, and computer models to create “rating curves” -- graphs that relate the water’s stage to the actual flow. They plot these measurements on graph paper and generate tables that tell them exactly how many cubic feet per second are barreling past Fort Benton at any hour.

But the reason behind all this fuss over a river isn’t just scientific curiosity. It’s tied to the economic fortunes of Montana and the greater West. The Missouri River basin covers about 500,000 square miles, eventually feeding into the Mississippi and on to the Gulf of Mexico. The flow of the Missouri affects agriculture, mining, railroads, and towns all along its route.

In the late 19th century, Montana’s economy was riding the boom and bust of mining and railroads. The Northern Pacific Railroad reached the area in the 1880s, turning Fort Benton from a steamboat port into a railhead. The river was no longer the only game in town, but it remained critical for irrigation and as a source of water for growing communities. Accurate knowledge of the river’s flow helped farmers plan irrigation schedules and avoid floods that could wipe out their fields and homes.

A 1907 report by the USGS noted: "The Missouri River’s discharge is subject to great seasonal variation, with spring floods resulting from snowmelt in the Rockies and dry summers causing low flow conditions." Those seasonal swings could mean the difference between a bumper crop and total ruin. Knowing when the river was going to rise or fall was as valuable as gold to anyone whose livelihood depended on it.

The data collected at Fort Benton has also played a role in flood control. The Missouri River was notorious for flooding. In 1912, a flood devastated parts of North Dakota and Montana, reminding everyone that the river was a force to be respected. The USGS stream flow data helped engineers and policymakers design dams and levees decades later -- structures like Fort Peck Dam, completed in 1940, which tamed the river to some extent and produced hydroelectric power for the region.

One of the ironies here is that while Lewis’s party marveled at the river’s "boiling and roling" waters in 1805, the river today is managed and measured with such precision that its natural rhythms have been greatly altered. The river that once surprised travelers with sudden floods and droughts is now a carefully monitored and managed resource, its every rise and fall recorded in digital logs.

The measuring station at Fort Benton connects history to the present day in a way that’s more practical than poetic. It reminds us that the river is a working river, one that has shaped the economy of Montana for over two centuries. As Lewis observed, its waters were thick and tepid, a muddy brown that carried the soil of distant mountains on its back. And as one modern hydrographer put it, "Without accurate stream flow data, we would be navigating the Missouri in the dark."

The Missouri River at Fort Benton is thus not just a river, but a subject of constant scrutiny -- measured, monitored, and understood in numbers and charts, from Lewis’s chain and notebook to satellites orbiting overhead. The tools have changed, but the river remains stubbornly the same, flowing on through history and the lives of those who depend on it.

See also

• Stream Flow Monitoring at Fort Benton, Chouteau County
• Lewis and Clark Trail at Fort Benton, Chouteau County
• Priming the Pump at Fort Benton, Chouteau County