Portrait of a Watershed
From: The Otter - Winter 2020
A river begins with snowfall that melted, or with raindrops that drizzled or pelted the earth on some faraway hillside or plain. Almost all headwaters are situated in sparsely populated places. It’s only after a river gathers volume and force that human settlements dramatically transform its channel, shorelines, bluffs and floodplains.
In the case of the Big Sioux River, that volume and force is delivered via numerous tributaries that drain all corners of a basin –or watershed- measuring about 8,500 square miles and touching three states. The Big Sioux begins in the high hills near the town of Summit, South Dakota. You can walk the windswept terrain of the Big Sioux’s birthplace, and observe that early on the young river is merely a crease in prairie sod carrying flows from gently sloped headwater wetlands.
As the crow flies, the Big Sioux travels southward for a distance of about 200 miles, though its loopy, ever-growing channel meanders approximately 420 miles before merging with the Missouri River near Sioux City, Iowa.
Understanding the characteristics of a watershed helps you better understand the river that drains that watershed.
Much of the Big Sioux’s watershed corresponds to the geological formation known as the Coteau des Prairies, a flatiron-shaped rolling plateau that rises from the flatlands of the James River valley on the west and the Minnesota River lowlands to the east. This plateau contains numerous lakes, ponds and potholes. It measures roughly 200 miles from north to south, and reaches up to 100 miles wide.
A river’s length, scale, and slope are the product of its geology and weather, and the Big Sioux’s watershed includes varying landscapes, topographies and ecosystems. Study a watershed map, and you’ll notice a lattice-like pattern of waterways feeding the Big Sioux’s main channel. According to the U.S. Geological Survey (USGS), there are approximately 12,000 miles of streams, creeks and slow-flowing sloughs in the Big Sioux system, and like a family tree these tributaries are all related to the river.
How land is used within a watershed also influences a river. In recent decades, particularly in the upper region of the Big Sioux’s watershed, many thousands of acres of perennial grasslands have been destroyed and converted to row-crop agriculture. Perennial prairie requires no fertilizers or pesticides, and prairie sod better holds rainfall and snowmelt than land hosting annual crops like corn or soybeans. This land use transformation –and agriculture’s overall impacts including increasing use of tile drains, more industrialized farming, and the impact of confined animal-raising operations- affects the river’s chemistry and hydrology. Climate change is another factor, adding significant storm events, including weather that now varies more radically within the seasons.
If you’re studying a topographic map you’ll notice the change in height of the land from where the river begins to where it ends. Watersheds tilt to the sea, toward sea level. The source of the Big Sioux is situated at an elevation of about 2015 feet above sea level. By the time the river enters the Missouri, its elevation is about 1060 feet. That’s a drop of approximately 955 feet through a 420 mile-long channel. The swift and steady current you witness in the river is a product of that drop. All rivers flow downhill, even if the slope isn’t easily perceived.
Rivers grow as they approach their mouth because tributaries contribute flows. Stray Horse Creek adds its modest volume to the upper Big Sioux near Castlewood, South Dakota. Hidewood Creek arrives with runoff from its namesake hills. Skunk Creek, a surprisingly large, fast-flowing stream, merges with the Big Sioux in central Sioux Falls. These and innumerable other streams throughout the watershed add water to the Big Sioux, connecting the Big Sioux to all of its watershed.
To illustrate the growth of the river, consider water flow/volume data at two locations; near Dell Rapids, South Dakota, and at the mouth of the river, near Sioux City, about 175 river miles downstream from Dell Rapids. USGS instruments show that during 2016 river flow levels near Dell Rapids averaged 487.7 cubic feet per second (CFS). In 2017 the same measuring station showed flows averaging 686.9 CFS, and in 2018 that number jumped to 1,312 CFS. CFS is a unit used to measure moving volumes of water. Think of a box containing water that is one-foot square. Many “boxes” pass by a certain spot on the river during any given second. One CFS equals 448.83 gallons of water per minute.
Below the Dell Rapids location, and before the Big Sioux empties into the Missouri River, a number of tributaries enter the Big Sioux, including its two largest tributaries, Skunk Creek and the Rock River. This translates into a larger, more powerful Big Sioux River, with daily flows at the mouth during 2016 averaging 2,880 CFS, 2,797 CFS in 2017, and 5,703 CFS in 2018. Each of these numbers is more than four times the flow level recorded at Dell Rapids.
To add appreciation about Big Sioux River flow volumes, understand that the average discharge of the Big Sioux into the Missouri River during 2018 was approximately 2,559,677 gallons per minute. Friends of the Big Sioux River can work to purify those flows, or we can inactively watch as those flows are continually degraded, and their massive detrimental impact moves downstream on the Missouri River.
It would be difficult to identify a single citizen-activist specializing in environmental protections who doesn’t advocate for expanding and strengthening the federal Clean Water Act (CWA). Much of the impetus for this perspective hinges on a recognition that rivers are intimately linked to ALL of their tributaries and the entire watershed. We can’t protect and heal a river without protecting and healing its tributaries.
But the CWA does not protect many small tributaries, especially headwater streams, nor does it protect some important wetlands. This exposes thousands of miles of streams and other surface waters within the Big Sioux watershed to pollution.
Another shortcoming is the CWA’s failure to regulate non-point pollution, the pollution entering rivers and streams through the landscape rather than a specific discharge outlet or pipe. Non-point pollution is the source of much agricultural pollution in our rivers. CWA weaknesses have been exacerbated by new provisions promoted by the Trump administration. How can we resurrect the Big Sioux River if we’re slipping backward, not moving forward, regarding regulations protecting water resources?
Gaze at the Big Sioux River, and realize its flows smoothly sweeping by having come through time from some distant place. They passed by meadows and seeped from springs. In summer, on a gravel beach far upstream, a child waded in the shallows. Or in winter, on some snowy stretch with a deep, stable pool of icy water, an otter family played. This river comes from many settings and locales. It shapes lives with its water and with its unpredictable might. It surges and it sleeps. It speaks a language unique to other natural features on this earth. We are listening, and we are watching. Rivers and their tributaries connect the here and now to the there and then.