It’s not rocket science, it’s fluvial geomorphology
February 1, 2012
By Craig Helker
How many bridges have you driven over, and never noticed the river beneath? Most people give rivers little thought. That is, until the river floods. Then, the angry outcry, “Fix the flooding!” rings through the meeting halls of local governments. Do something. Anything! Fix the river! It’s broken!
In the past, fixing a “broken” river meant dredging, deepening, straightening, or lining it with concrete. Those fixes usually meant biological Armageddon for river life, massive infrastructure costs, and ironically, continued flooding. This, of course, was due to the failure to see the river for what it is.
Marty Melchior sees rivers for what they are. That’s his job. Melchior is a river ecologist with Inter-Fluve, Inc., a Madison firm that has done over a thousand river restoration projects, both nationally and internationally.
“The first thing to do when looking at a river,” said Melchior, “is to recognize that every river is different. Never put blinders on, and avoid assumptions.”
“We in the United States have acquiesced to the destruction and degradation of our rivers, in part because we have insufficient knowledge of the characteristics of rivers and the effects of our actions that alter their form and process.”
—Luna Leopold, fluvial geomorphologist and son of Aldo Leopold, in A View of the River, 1994.
The KK is not the Root; the Milwaukee is not the Menomonee. Avoiding assumptions is best done by collecting data. Lots of data.
In the Field
In 2003, Milwaukee County hired Melchior and Inter-Fluve to conduct fluvial geomorphology (the study of river processes and how rivers interact with the landforms they create) assessments on the rivers of Milwaukee County. In the process, Melchior conducted reconnaissance, stream classification, channel geometry surveys, and infrastructure surveys on 34 streams in the county.
This work even got down to determining the size of sediment in the streams. At certain locations in each surveyed stream, Melchior would randomly pick 100 particles from the stream bed and carefully measure the axis length of each one. This method, called the Wolman Pebble Count, allowed Melchior to determine the percentage and size of material making up the stream bottom. For instance, the bottom of the upper portion of the Root River in West Allis is primarily sand and small gravel, whereas Whitnall Park Creek, a tributary to the Root River, has a bottom ranging from boulders in some areas to sand or sand/gravel in others. This indicates a lot of energy is in Whitnall Park Creek, enough to strip the finer silts away.
Rivers are constantly changing and adjusting, seeking what’s called dynamic equilibrium—a state where erosion and the deposition of eroded material are balanced. Rivers naturally remove material from the outside of bends and deposit it on inside bends. Silts are suspended by flow, while sand and gravels migrate downstream. Even boulders move—when flows are high. The beds of rivers aggrade up, filling with sediment from upstream, or they downcut as their sediment is winnowed away. Aggradation can mean the now shallower stream channel can’t convey as much water, so there is more frequent out-of-bank flooding. Conversely, downcutting means the bed is lower, the banks now higher and less stable, and so are more subject to collapse or catastrophic erosion.
A 2007 study by the Milwaukee Metropolitan Sewerage District (MMSD), the Root River Sediment-transport Planning Study, identified the uppermost portions of the Root River, historically ditched near 124th Street, as having downcut about five feet since the ’60s. Similarly, Melchior said, “The KK in Bay View has downcut 10 to 15 feet in places.” Artificially containing a river in its channel concentrates its flow energy, taking more of the bottom material away.
For an urban river, Melchior said, “Without a lot of experience, it is extremely difficult to correctly identify geomorphic problems.” Ditching and floodplain filling push streams into various states of instability, and the result is either erosion or deposition out of balance with natural processes. The streams will then adjust, sometimes vehemently. Infrastructure that was planned and built with the assumption that the stream would stay where it was put can be undermined or destroyed.
Road crossings and culverts, common in urban areas, are another problem. They act like handcuffs, locking a river in place, keeping it from meandering. Naturally, rivers meander back and forth across their floodplains like squiggling rope. But, gripping a squiggling rope changes the squiggles. The squiggles occur, but somewhere else. It’s the same with a river. The sediment-transport study suggests that the Root River’s meander pattern upstream of Rawson Avenue is due to river bank armoring and dolomite bedrock exposure at Rawson Avenue. Further, look at aerial photos of almost any river, and you’ll see the meander pattern change near narrow bridges, typically on the immediate upstream section. A bend will form, threatening to cut into the foundation of the very road the bridge serves.
The MMSD Root River Sediment-transport Planning Study contains a surficial geologic map, prepared by the U.S. Geologic Survey, that shows the glacial deposits (moraines) of Milwaukee County. This map is an important reminder that the Root River—and all Milwaukee rivers for that matter—are moving over landscapes left behind by the retreating Laurentide glacier.
“The Milwaukee, Menomonee, KK, and Root rivers are young,” said Faith Fitzpatrick, fluvial geomorphologist with the USGS. They’ve only been around for 11,000 years or so, and are busy working down into the glacial leavings.
In the case of the Root River, the MMSD study summarizes that the Root River flows north-south on consolidated, erosion-resistant valley bottom called ground moraine. The tributaries to the Root, however, are generally oriented east-west, flowing down ridges called terminal moraines. This means they flow with more energy, cutting into the unconsolidated ridge material, delivering this coarse material to the Root River.
Melchior’s pebble-counts from the Whitnall Park, Dale, Wildcat, and Hale creek tributaries bear this out. The bed of the Root River where these tributaries enter is actually stabilized by the new larger-grained material, which acts as a sort of armor on the bottom.
Over the years, a number of people have scoured over the rivers of Milwaukee County, taking pebble counts, measuring stream bank heights, and calculating access to floodplain. All this information is contained in the final reports for Milwaukee County and MMSD. These reports go on to give recommendations. For the Root River, the main recommendation is to encourage open lands around the river. That’s it in a nutshell. Encourage the river to act like a river, don’t build right up to it and constrain it. Don’t lock it in place with culverts. Give it the ability to flow out of its banks into floodplain when it needs to. See the river for what it is, not what we want it to be.
Craig Helker is a water resources biologist with the Wisconsin Department of Natural Resources.
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