Puzzled by its bad reputation among some of his clients, Mike Gannon began doing research on gravel. He'd always believed in its value, he says, but found a rich history that now aids in him on occasions when he needs to talk his clients through their moments of doubt.
By Mike Gannon
To this day, and despite the fact it’s basically settled science in the pond community, there are prospects and clients out there who persist in viewing the use of gravel in ponds as a matter of controversy. They’ll tell me out of the blue that they believe gravel makes the water dirtier than they should be; that it harbors components that made fish sick; or that it is an abrasion hazard for those same hapless fish.
I do my best to counter their perspectives by talking up the beautifying effect that gravel can have, particularly within fast-flowing streams but generally in any sort of constructed pond system. Then there’s the fact that gravel can take a good, naturalistic composition and bring it fully to life, making it seem as though it’s been in place since time immemorial.
Finally – and this has always been where I come down with the full weight of my experience and professional credentials behind me – I tell them that gravel plays an invaluable role in water filtration and is the best way I know of to guide water to the sort of gin-like clarity to which I always aspire in designing and building their ponds.
This full-frontal approach generally works, but the debate breaks out so often that I began to wonder why gravel was such an issue – an exploration I began on the practical side.
My own limited research indicates that gravel as a filtering agent has been around for millennia, going back all the way to the earliest known applications in ancient Sumeria.
It makes sense that this sort of technology would emerge 5,000 years ago in the arid Middle East, basically because water was a precious commodity and none of it – brackish, contaminated, stagnant or filthy – could reasonably be allowed to go to waste. I have no clue how they figured out that percolating water through layers of gravel would strip away sediments, break down impurities, remove odors and bring questionable water to the point of usability, but thank goodness for civilization’s sake that they did!
Thank goodness as well that someone additionally determined that, by running water through a bed of crushed charcoal, the same questionable water could be rendered potable with a bit of boiling.
So why, all these centuries later, would anyone question the value of using gravel to filter and scrub the water of modern pond systems? Hasn’t gravel proved itself? The only thing I can think of is that the problem, at least on the practical side, might have something to do with the word “gravel” itself: While smallish crushed stone may be inoffensive on its own, describing it with a word that has so many entangling associations may not be doing the rocky material any favors.
I mean, gravel is what many of us found imbedded in our knees after tripping and falling on childhood playgrounds. Gravelly voices are often discordant and tough to understand, while “getting graveled” is a sure step on the path to an angry outburst. And if the word isn’t negatively freighted, why would prominent Alaska politician Mike Gravel (or his forbears) alter pronunciation of his common-word surname by putting the accent on the second syllable instead of the first?
Or maybe people are thinking about the absorbent stuff they put in their bird cages or a pet cat’s litter box. Others might be conjuring memories of the larger material used to “pave” backcountry roads and how awful these surfaces are to drive across. Personally, the 30 or so different kinds of gravel my stone supplier has to offer is what comes to mind first and presents me with a sometimes uncomfortable need to choose among the three or four types that are actually suitable for pond applications.
My point is, while “gravel” is a perfectly good word, many people tend to free-associate when the word pops up in conversation. Whatever the exact cause, I’ve seen this often enough that I’ve come to call the phenomenon “gravel anxiety.”
CLEARING THE AIR
As mentioned at the outset, this so-called “debate” over gravel, whatever its origins, leads to various challenges I’ve heard clients raise to its use in pond systems.
In entering these discussions, I don’t hide the fact that I’m an unabashed gravel fan no matter whether a pond is huge or tiny, boulder-strewn or unadorned, formal or natural, planted or bare-bones. And no matter whether there’s a waterfall and the surface is active or it’s a single-level pond and the water is calm and mirror-like, I’m a such firm believer that spreading gravel across the interior surface is the key to giving the watershape its basic visual appeal that I can’t let go.
But the debate society won’t let go, either: No matter where or how or when or where they’ve picked it up, this is what I hear:
q “Gravel makes pond water dirty.”
This is a two-headed beast. On the one side, there’s what I call the instant-gratification curse: Yes, when gravel is placed and water is first introduced, no matter how many times the material has been rinsed before addition the water will be cloudy for a while. So every subsequent time the pond gets cloudy, the gravel must be to blame and its reputation takes a hit wherever an Internet comment section is found.
On the other side is a pond-management issue: If a pond’s owner overfeeds the fish, overstocks the pond or mismanages or neglects the vessel in some other way, the water can become cloudy no matter whether it contains gravel or not. It often helps when I explain that the presence of gravel as a filtering bed will help the water clear more rapidly once the real issue behind the clouding is addressed.
q “Gravel will kill the fish.”
This one is hard to take because most gravel is pretty passive and wouldn’t hurt a filament on any fin it might encounter. Of course, there’s a possibility that someone might have selected the wrong type of gravel (that is, something from one of the 26 piles I walk right past at the stoneyard to get to the four I use) and that this bad selection might start to disintegrate or react and thereby cause pH fluctuations or clouding. But again, this one has the air of Internet-fueled misinformation.
q “Gravel generates toxic gases that foul the water.”
Again, this is a possibility, but only if the gravel selected is tragically wrong for the task at hand. But in any event, none of the gravels you see in the average stoneyard will be inherently toxic: The real culprit here is actually tragic pond mismanagement.
Most ponds are resilient and can be saved if the problems are caught before the water truly becomes toxic. The rap against gravel here is that it becomes slimy and smelly and is therefore must be outgassing and causing the problem in some way. But actually, what’s happening is that the gravel bed is doing its job and is filtering out the slime and muck, just as it should. The real culprit here is the pondkeeper who has let things slide so far.
q “Gravel cuts the fish and gives them wounds.”
This is another material-selection issue: If the pondowner chooses and the pond installer uses jagged glass pieces as gravel, for example, the fish will certainly be imperiled. Absolutely, gravel can be abrasive even without cutting edges, but I haven’t encountered many fish who voluntarily beach themselves to experience the joy of rolling around on pointy surfaces.
q “Gravel will damage the pond liner.”
So, fine. Yes, this is possible – indeed it’s far more possible than any of the issues mentioned above – but it’s still highly unlikely given the way modern ponds are installed and the quality of the liners we use.
Someone on the crew might start doing the twist in excitement to celebrate the beginning of the filling stage, for example, or a poorly placed boulder might fall from the edge of the pond and down into the water and might have enough heft to weaponize gravel sufficiently to make it penetrate the liner, but that generally doesn’t happen – even in seismically sensitive areas because of care with which rockwork is typically installed in those regions.
All it takes to ease this concern (where it might be valid) is to thicken the gravel layer a bit to distribute the weight of a crew member or a rolling stone. But even more so, today’s liners are tough and are made to take even significant challenges in stride.
THE RIGHT CHOICE
If I do see any of the abovementioned gravel-related problems, they typically arise in ponds installed by homeowners or their do-it-yourself friends. I’ve seen ponds lined with sand, for instance, or decked out in gravel with two- to three-inch grain size – too fine on one hand, too rough on the other. (My personal preference is for grains in the three-quarter-to-one-inch range. I stopped using three-eighths-inch grains after observing how easily fish moved them around.)
I’ve also seen construction gravel used – too craggy and far too homely – and have even come across crushed lava rock, which is tantamount to lining a pond with ground glass. There are so many variations on this “misuse” theme that I could go on for many paragraphs, but suffice it to say that material selection is important and, helpfully, usable options are limited enough in number that it’s hard to go wrong.
So let’s define what an acceptable gravel is for use in filtration. My go-to is typically called “river gravel” with grain sizes between three-quarters of an inch and an inch – and that has been tumbled in processing to remove sharp edges. The only problem with this material is that it’s often pretty dull in appearance, even under water. Nonetheless, if effective filtration is the goal, this material, once rinsed a few times, is a great choice when placed as a two-inch-thick bed.
And at last, we can finally get around to aesthetics, which is often as important a consideration in gravel selection as is its potential as a filter. At a stoneyard, it can be difficult to tell how a material will look in a pond, so I will wet samples and/or place them in a tray or bucket to see how much variation in color and visual texture there might be.
I also work with the stoneyard’s staff, expressing my needs and asking them to help by obtaining gravel with a more colorful wet appearance. But I know this is a hard path: As much gravel as I might buy, the four gravel types I’ll choose from don’t generally sell in the volumes seen with the other 26 types mentioned above. With a large project, I find I have some motivating leverage with my suppliers. For a typical backyard pond? Not so much.
So here’s my thought: All things considered, there are other things I can do to dress up a pond lined with dull-looking gravel by using plants, large river pebbles and rocks to create visual interest and bring a variety of colors to the interior surfaces. As a result, my focus is always on getting a material I know will perform its filtration function as required; if it also looks good, great! It’s a nice bonus.
Bottom line: If the gravel functions as it should by capturing and removing debris, particles and sediments from the water, trapping them between its grains where the contaminants can be further broken down by millions of beneficial bacteria that grow on surfaces in the gravel bed, I’m happy – and my clients will be, too, because gravel filtration works, efficiently and inexpensively.
It was good for the Sumerians, and it’s great for modern ponds as well!