Rock-Bottom Value

There was a time when lots of ponds were set up without gravel, notes Ed Beaulieu. But as he discusses here, there are so many advantages to lining their interiors with rocks of various sizes that it’s time to push that old practice out of the pond-making picture, once and for all.

Through the years, ponds have been made using a variety of lining materials. Some feature concrete, while others come as pre-formed plastic tubs and still others use liners. In many cases, these materials are left to go bare, exposed to the sun and the elements. But much more often these days, pond interiors are covered by rocks and gravel – and that, I think, is a very good thing.

For a long time, gravel was suspect because it made cleaning a pond more of a chore. This might have been a persuasive myth among do-it-yourself pond installers and maintainers of past generations who, on the one hand, didn’t much like the thought of lugging sacks of gravel to the pond’s edge and then putting it artfully in place and, on the other, figured the gravel would get slimy and require regular water-jet sluicing to keep the muck at bay.

But the anti-gravel position is a myth, and we know now that these small bits of stone play an all-important role by providing a natural habitat for beneficial microorganisms – and a whole lot more.

COUNTING THE WAYS

The rocky bottom of a gravel-lined pond is indeed alive and brimming with activity courtesy of the algae, microscopic invertebrates and bacteria that start taking up residence in its nooks and crannies almost as soon as a pond is filled.

Given a thriving population of fish and plants, this layer of a pond’s cross-section is basically a compost pile: When organic debris falls to the pond’s bottom, it is broken down by the water’s benthic-zone (that is, bottom-level) inhabitants – active recyclers who live off uneaten fish food, decaying plant matter and nitrogenous fish wastes.

If this complex layer weren’t present, the pond would quickly die – suffocated by toxic fish waste and organic build-up. But in a gravel-lined pond, that key layer is present, and there are organisms within this matrix that have evolved to consume practically every available bit of food. In turn, the pond’s fish and other swimming or grazing inhabitants will feed on the minute organisms that live within the pond’s rocky floor.

No matter if a pond sits under a tree loaded with falling leaves or how heavy its fish population may be (within limits, of course), rocks and gravel keep the ecosystem healthier: Just remember that this is how Mother Nature cleans up after herself!

For all that, there’s no need to dwell entirely on science here, because there are a number of practical and aesthetic reasons why gravel should be part of every pond environment:

[ ] It protects the lining from the sun. To one extent or another, all plastic and rubber materials are subject to degradation when exposed to the intense ultraviolet radiation that bathes them, courtesy of the sun. In protecting the lining material from its worst enemy by covering it with gravel, the lining’s longevity can be dramatically increased.

[ ] It holds a liner in place. Just as you might use a rock to keep a napkin from blowing away at a family picnic, installing rock and gravel over a pond liner helps keep it in place. If you don’t use rock and gravel to weigh things down, air bubbles can form underneath – and have been known to bubble up sufficiently to create what’s known as the Loch Ness Monster look, with a blob of liner breaking the surface in the middle of an unhappy pond.

[ ] It offers firmer footing. This is good news for anyone who has ever stepped into a pond without gravel on the bottom: A bare liner or gravel-free interior of any type – rubber, plastic or concrete – is very slippery: If there’s ever a need to wade into the water to trim a plant or get up close and personal with a fish, stepping on small gravel is a safer way to go.

[ ] It lends structural stability to a pond. When gravel is placed behind and between larger rocks, it effectively locks them in place by eliminating open spaces that might allow the bigger components to shift. In effect, it’s a free-floating mortar – a fact known to any pond remodeler who has had to use a pry bar to dismantle the rockwork of an old pond or watergarden.

[ ] It enhances the ecosystem. It’s worth repeating: Gravel provides a habitat for beneficial microorganisms that help break down decaying plant matter and fish waste, turning it into usable plant food while the bacteria themselves become part of the diet for fish and other aquatic creatures. It’s the aquatic circle of life and should be encouraged!

[ ] It “naturalizes” a pond. Rocks and gravel are used to make ponds seem more natural, and that’s not only true on the bottom: Trailing flows of gravel help make waterfalls look better, and they host increased biological activity both within and around the water. This naturalizing role for gravel makes it a versatile and valuable design tool.

So let’s consider the myth shattered: Just about every pond deserves a gravel layer – and all it takes to get it right is using a bit of common sense in reaching for a natural look.

DEEPER BEAUTY

To get started in the right aesthetic direction, it’s advisable to obtain gravel of various sizes – such as a blend of grades ranging from three-eighths of an inch up to three inches. The smaller pieces bring a huge amount of surface area with them to the bottom and sides of the pond, and that’s great for bacteria colonies and other beneficial creatures that will take up residence in the gravel bed. The larger pieces bring mass and texture to the bed and help ease the visual transition from the smaller gravel to the pond’s largest boulders.

Next – and this is important on many levels – make certain the pond is equipped with an appropriate mechanical prefilter/skimmer. This serves to remove the gross solids from the water column, allowing the microorganisms the time they need to complete their tasks without being overwhelmed. And this is particularly true if the pond contains a substantial fish population or is positioned under messy trees.

It’s also important to consider the way water moves through the system: Good circulation throughout the full extent of the pond’s volume ensures the availability of dissolved oxygen all the way down to the benthic microorganisms while displacing carbon dioxide and other waste gasses generated during microbial processes.

A separate consideration is the pond’s basic turnover rate, which requires selection of a properly sized pump. A small waterfeature will require a higher turnover rate than a large one, because the volume of water represents a smaller percentage relative to the number of organisms within the system. Thus, for an aquatic ecosystem of 1,000 gallons or less, the water should turn over as often as three times per hour, where a half-acre pond holding 700,000 gallons will do well if the water turns over once each day.

Getting back to aesthetics one last time, gravel also makes a great planting medium for marginal aquatic plants, which can be removed from their plastic containers and placed directly into the gravel substrate. This allows them to draw nutrients from the biological activity found within the gravel bed, while the elimination of their pots allows the plants to work along with the gravel to naturalize the pond.

Bottom line: Working with gravel is the key to making a pond look more natural and behave more naturally as an ecosystem. Isn’t that what most pond people are after these days?

Ed Beaulieu is director of contractor development and field research for Aquascape, Inc., St. Charles, Ill. For more information, visit www.aquascapeinc.com.