photo by Garry Brandenburg The Iowa River east of Marshalltown curves and twists in a sinuous pathway. This is typical of any broad floodplain with little topographical relief. Each outside bend of the river is where the fastest current exists, and where natural erosion of soils takes place. All of the sediments taken from outside curves is water sorted and re-deposited downstream to many sites along inside curves. Sand bars are the result. Now, during very low river flow times, those sandbars appear larger and very conspicuous. So sit back as we explore another facet of natural history right in front of our eyes.

DROUGHT-LIKE conditions occur periodically, to be followed years later by excess rain fall events, and all become intertwined with “normal” years when a stream like the Iowa River does not flood. All seems well if the river water levels stays below bank full. It is to be expected that the great weather machine of earth’s atmospheric air masses will sometimes bring us more rain than we want, or little if any rain when we need it most. So us humans have to adapt as best we can to natural variances in year-to-year weather.

When you look at today’s image of the river, you will note that the course it takes is a meandering pathway. It has sinuous back and forth loops, bends and curves, all typical of a stream flowing over very flat floodplain lands. Those bends and curves manage water flow energy by increasing resistance and reducing channel gradient. This natural application of water flowing over the land is how a river maintains its equilibrium.

Rivers are transport machines, moving water from the landscape of its watershed, and taking sediments of all sizes for a ride suspended within the water. All of this is taking place very slowly, as during drought conditions, or very fast during flood events, or at intermediate levels when neither drought or flood conditions are happening. Those meandering loops and curves take away material from the outside bend and redistribute/sort it by size downstream. The smallest particles of silt may stay in suspension all the way to the Gulf of Mexico. Sand sized material, is heavier and cannot be held in suspension during low energy flow rates. So the sand makes new deposits onto sandbars. Small pebbles and rocks will also move if the current is fast enough. But when the current is very slow, particle size movement is also very low.

Meanders of a river can eventually cut close enough to another curve to take a shortcut during the next big flood event. The result is an old channel segment cut off from the new main channel. Those cutoff channels are called oxbows. They get water during high water flow times or flood times, but they are not the main channel anymore. A good way to see old river channels is from aerial photos of the floodplain made in late winter when snow melt is taking place and with no leaves on forest land trees. All kinds of old oxbows and former river channels appear as overlapping curves superimposed on other former channels. The story being told by such images are one of a relentless interplay over centuries of time of the river following and making new pathways as the water finds its way to a lower elevation. A meandering river needs room to roam. And geologic evidence tells us that the entire width of the floodplain is nature’s definition of room to roam.

Room to roam across the floodplain changed often during post glacial episodes. As Wisconsinan ice began its big melt back and retreat 15,000 years ago, huge volumes of water came flowing off each summer from the ice margin. For our locality in what is now Marshall County, the ice margin was along the Story/Marshall County line, and water escaping from the melting glacial ice had to find its way to lower elevations. The broad floodplain lands of Linn Creek, Minerva Creek and the Iowa River were running bluff to bluff full of high energy fast flowing water each summer. That high energy water faltered each late fall and winter as super cold winter seasons greatly reduced glacial melting. Now all the exposed sands and silts, rocks and pebbles, were vulnerable to erosion of different sort….wind.

Returning to the discussion ofdrought conditions, I looked up the data from U.S. Army Corps of Engineers records from 1977 and 1988. Why those years? Because those were true drought years in many places in the Midwest. And our Iowa River reflected very low flow rates all year long. For the month of August, 1977 river water levels were noted at about 861 feet above sea level and cubic feet per second rates of under 100. In 1988, data shows August river levels stayed virtually steady at 860.3 feet above sea level. Now I compared the Iowa River water of August 17th, 2021 and found the elevation at 861.6. Our 2021 drought, from the river’s perspective, is not as low as the years 77 and 88, but we are very close to minimal flow rates.

Which brings up another point, why do our Midwest rivers never go dry each summer? Good question. The answer is our location on the North American continent where long term averages of rains and snow events are never at extreme levels. Some portions of earth’s surface do not fare as well. They may have a long rainy season followed by a long dry season. Or they may have one or the other but not both. Midwesterners do not see those extremes. Our soils tend to work like big sponges to hold water in many subsoil layers where plant roots can access that moisture. And geologists tell us that the water we see in a low flowing river is the result of what they call base flow. Essentially it is the slow and steady leaking of water out of the base of the soil column.

Weather is short term day-to-day happenings. Climate is different as it is a compilation of geological time scale trends. Keep in mind the ultimate driving forces of our climate. Long term evidence written in the strata of rock layers paint a picture of the land we now call Iowa being at the bottom of warm shallow oceans more often than we were dry land above sea level. Plate tectonics move and alter landscapes. Glacial systems numbering at least 33 episodes during the past 2.6 million years have altered landscapes. And our Sun’s energy, though mostly constant, has its maximums and minimums, that even from 93 million miles away have effects upon long term climate cycles. Planet earth has an orbit around the Sun of almost circular to elliptical on a time scale of 100,000 years. Earth’s axial tilt varies every 40,000 years between 21.5 and 24.5 degrees. Now add precession, or wobble of the earth’s axis, on a 19,000 year cycle, and what we get are unstoppable long term geologic time frame climate shifts of huge proportions.

Nature rules. There is nothing we can do but adapt. We will have dry years, drought years, normal years and wetter than normal years. We will see Iowa river low flows and periodic flood events. Mother Nature’s system is an incredibly powerful and dynamic mix of seasonal changes. We can only admire and try to understand the awesomeness of nature’s workings. Until next week, stay cool.

Garry Brandenburg is the retired director of the Marshall County Conservation Board. He is a graduate of Iowa State University with a BS degree in Fish & Wildlife Biology.

Contact him at:

P.O. Box 96

Albion, IA 50005