Get the Gravel Out of the River

“If we could just dig the gravel out of the river like we used to, we wouldn’t have these flooding problems.”

It sounds simple and logical. After all, a deeper river will handle more water and move it faster, while harmlessly providing gravel for repairing roads, right?

It turns out that digging gravel out of streams does a lot more harm than good, both immediately and over the long term. In recent decades, we have discovered that rivers build up gravel in their beds as they become stable and attempting to fight that natural process by digging gravel out of streams, “armoring” the banks with stones and straightening the river makes flooding and flood damage more likely, not less.

Fluvial geomorphology, as the study of these river systems is called, relies on physics and can help to predict how streams will respond when gravel is dug from their beds or when banks are lined with rip- rap and berms.

Rivers and Streams Follow a Predictable Pattern when Disturbed

It’s widely believed that taking gravel out of a river allows the water to run deeper – further away from houses and roads – moving it quickly and lessening flooding impacts.

But in fact a river which has been changed by having gravel and stones dug from it goes through a predictable, but sometimes violent, period of infancy and adolescence before eventually returning to a mature and stable adulthood. During that process the stream can quickly and erratically change where it flows.

The Schumm Channel Evolution Model, pictured below, illustrates that various stages most Vermont streams go through.

Let’s look at gravel extraction as an example. Viewed in cross-section, most of the rivers around you are in one of these five stages. Stage I is a stable, mature river. By taking out gravel bars out of such a river, it moves into Stage II – running deep and swift. In this new stage, the river, because it’s running deeper, is deprived of its floodplain. Floodplains act as a pressure relief valves slowing the water down and allowing its energy to be dispersed as it flows over the top of its banks.

Not only does digging out a stream channel or building up the banks create a faster river flow, it also decreases the overall area the river has to accommodate high volumes of water from storms. And, like putting your thumb over the end of a garden hose, doing so increases the power and speed of the water – and the likelihood the streambanks will cave into the river (especially in downstream areas).

Having a deeper, swifter flow without access to the floodplain increases the stream’s power. Streambank failure, road slumping, and loss of vegetation into the streams is inevitable. And once they start to fail, the same areas have to be fixed over and over again. If that’s your property that’s falling into the river that has to be rip-rapped every year, you know how frustrating – and expensive – that can be. And ask your road crew how often they go back to the same places to make repairs, year after year. Interrupting a river’s physical process by repeatedly digging out gravel ensures we never get to a stable system – and money gets poured down the river year after year.

Resisting the Urge

Over the following months or years the river moves into stage III, growing wider, filling back in with gravel, eroding its banks and forming a new floodplain to release some of its energy. This results in an over-wide channel, dumping sediment and gravel as the water begins to slow. The gravel bars in this widened stream concentrate the stream’s flow against the soft, newly exposed banks, further eroding them and anything sitting on them – like your house. The urge to remove these gravel bars returns.

If we resist this urge to remove the gravel, the stream WILL (absent of other impacts such as increased rip-rapping and straightening) move into Stage IV. Gravel builds up in the riverbed and the river regains access to its new floodplain. It now disperses its energy over these gravel bars, dumping the sediment and gravel it is carrying and returning to a relatively stable state as in Stage I unless a new disruption sets it back into one of the other stages.

So, when we dig gravel from a river (or armor its banks) we can knock the river off balance, making it less predictable, not more predictable. By dredging or filling, we are, in one area, unnaturally constraining or otherwise altering the river’s flow. The result can be faster water, major and erratic flow, erosion, and flooding and property damage in other parts of the river over and over in the same locations for many, many years until the stream settles once again back into a stable, natural state.

 

Vermont Rivers Too

Environmental officials measured Vermont rivers from 2004-2010 in order to help to predict the changes to be expected. The data show that after 200 years of landscape changes that include streambed straightening, gravel extraction, restriction of river access to floodplains, and changes in the watershed, 75% of the stream miles measured in Vermont are in an unstable condition, that is, in Stages II, III or IV. It is not surprising we are seeing so many erosional flood impacts.

Stable Rivers Stay Stable

Rivers change based on larger storm events. Stream changes may begin immediately, or stay the same for decades, depending on the sensitivity and geometry of the specific stream, how much gravel has been extracted, and how much rain we get. Jarring a stream from a stable state into the turbulence of stage II, III or IV can be done very quickly. But moving from stage IV back to a stable stage I condition may take one hundred years or more. When we take gravel out of a stream, we set it back to stage II and off we go again.

Experience and science tell us that a stable, balanced river—that is, one that has evolved: it’s just wide enough, deep enough and long enough to move the amount of water and gravel produced in its watershed and it will erode its banks and change course only minimally, even in flood situations.

Allowing gravel to build up in streams may not seem smart, but it lets the river create new, stable channels based on physics that will remain stable for decades and protect us against as much of the flooding impacts as possible.

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This document was created by the Vermont Natural Resources Council and the Conservation Law Foundation based on information from the Agency of Natural Resources River Corridor Management Program. For more information, visit ANR’s website at: vtwaterquality.org/rivers.htm