Removing Silt from your Pond

By Bob Lusk

After reading that headline, the first question you should ask is, “Why is a fisheries biologist writing about silt?”

The answer, “Because he’s scraped a fair share of that gooey stuff from under his toenails over the years.” Plus, he’s been stuck mid-thigh deep in a quagmire of pond muck. That qualifies, doesn’t it?

All kidding aside, for the moment, and to quote a longtime friend, Forrest Gump, “Silt is as silt does.”

Silt, by definition, is rock, mineral or clay fragments 1/20 millimeter diameter or smaller. These tiny pieces of geologic wonder are easily moved by water flow and quick to settle once the movement stops, depositing itself in waves where the water slows.

When a pond or lake is built, siltation begins with the first rain, especially when that precious wet stuff falls from the sky onto freshly disturbed soils. Next time you drive by a new pond…or a freshly renovated old one, look at the ruts cascading from top to bottom where water sought its path of least resistance. What was in those ruts now sits less than contentedly beneath the surface of that water body or deposited somewhere downstream of its prior resting spot.

Over time, silt deposits can completely change the landscape. If you don’t believe me, ask New Orleans. If it weren’t for the biggest river in North America depositing its silt on the downstream side of Louisiana, New Orleans might have been settled just south of Minneapolis.

When a pond or lake is first built, it has a life span. Take, for example, the thousands of flood prevention sites built by the Federal Government in the countryside around cities in the late 1950’s through the early 1970’s. These dams were purposely overbuilt for each site, tall, straight, with lots of compacted soils, with the sole purpose of relieving pressure on nearby rivers and streams. During that era, the mission was to stop flooding in expanding cities and suburbs. Each dam was designed to capture excess flow from giant rain events and then slowly, precisely, release that roiling water in an orderly fashion. That’s what they do. Each of those reservoirs was built to have about a fifty to sixty year lifespan. Fast forward to now and that’s exactly what’s happened. Their lifespan has ended. Each one has tons and tons of silt. As a professional fisheries biologist and lake management consultant, I have the distinct pleasure to meet with happy new landowners on their recent purchases to discuss what to do with these lakes-on-their-last-legs, so to speak. Typically, they’ll ask, “What do we need to do to make this a good fishing lake?” Even though I don’t show it, my whole body winces inside. Might as well ask me how to make Tiger Woods into a preacher. It could happen, but we’ll need some pretty strong divine intervention.

Most of these bodies have impressive dams…and when we drive atop the dam to see the lake…voila’, there it is, a massive earthen structure which yields to a flat plane of shallow, muddy water, typically with an average depth of two to three feet beneath the surface. Less than impressive…unless you wanted a flood prevention lake.

Even small ponds have this dilemma. Over time, Nature being what it is, ponds and lakes receive a portion of silt. It’s that “lifespan” thing.

There were literally millions of ponds and lakes built with heavy equipment all over the United States during the last fifty to sixty years. Millions of them. Some estimates today claim there to be 4.5 million private ponds and lakes dotting the landscape of our fair country. Other estimates push that number beyond 6 million. Each of those ponds has a lifespan, the length of which primarily determined by how much silt flows in, and how fast, to fill that hole and turn it from a pond to a marsh to dry land again.

Inevitably, the questions are now turning to, “How can we get this silt out of my pond?”

The best first answer is, “Should you?”

Ask that question, first. Then, figure out the best methods to remove silt…if it makes good sense. For example, if you live in an urban area where ponds are the focal point of a neighborhood and a homeowner’s association runs your show, removing the silt could make sense on a variety of levels. Quite a few developments were built around existing ponds and lakes which had already outlived half their lifespan. When the homes were built, if good construction practices weren’t properly exercised, even more siltation occurred, reducing the pond’s lifespan even quicker. Odds are property prices are heavily influenced by that shining water body behind your house. Look at removing silt to keep property values up. No one wants to live beside a mosquito infested, heavily vegetated, mud hole.

At the other end of that spectrum sits a small rural farm pond that was built in the 1960’s for livestock water. So many times, I’ll hear from the new landowner, “I grew up near here and fished that pond when I was a kid. It was a great place to fish and swim back then.” Today, that formerly pristine-looking memory-filled pond is nothing more than a silted in, cattail covered marsh, with a pot of water in the middle. The dam is covered with scrub trees and the pond is well past its prime, on its way to pasture. That landowner wants to explore reviving this pond to its former luster…and have it teeming with giant fish. My first advice? See if we can build another pond somewhere nearby, preferably downstream.

Why advise that?

Simply because moving silt is tedious and expensive. We can overcome that tedious part, but the landowner must come to grips with the expensive side. Simply speaking, removing silt from a pond entails moving the dirt at least twice, often three times. And, all you get is a cubic yard of water for every cubic yard of silt removed. That isn’t really efficient. Building a new dam, you at least get to typically raise the water level and get lots more water for the cost of construction. But, that’s another story.

So, Job One is deciding whether or not to remove silt. If you do your homework and the decision is “Feasibility, yes” then you need to quantify how much silt to remove. That sounds like an impossible task, but there are companies out there with the capability to calculate with some degree of accuracy just how much silt lies at the bottom of your pond.

Here’s a story to help understand the importance of this concept. I’ll never, ever forget a fishing club where I consulted way back years ago, in the late 1980’s. They were in north Texas, had a nice 30-plus acre lake and six smaller ponds scattered over their 350 acre property. One pond was heavily silted, so the board of directors decided to remove the silt and bring new life to that two acre honey hole. They sought bids from local earthmovers. One guy bid $12,000, another bid $20,000, another said he wanted to cut the dam, take a look and then give a bid. A fourth guy said, “I ain’t touching this job because we don’t know how much silt there is.” The low bidder was chosen, simply because the club knew they could go downstream and build a new pond for $20,000, so the $12,000 bid seemed like a fair bargain to revive the older pond. The crusty old dirt man showed up with a forty year old dump truck, a dragline that seemed older than he was and a bulldozer dripping hydraulic fluid. He cut the dam with the dragline and the pond drained. Best we could, we gathered the fish and moved them to another pond. After the pond sat for several summer weeks, the dirt guy and his assistant went to work on the dirt, when they weren’t working on their equipment. About four weeks into it and with half the pond bottom looking good as new and the other half still up to six feet deep in cracked silt topping with black pudding-like muck beneath that surface crust, the earthmover called the board president for a quick meeting. “Mr. President, we have a problem.” The president, with one eyebrow cocked upward a little bit said, “Oh?” The earthmover spit tobacco juice, pawed at the ground with his right foot, ducked his head a little bit and surmised, “There’s way more silt in here that I thought. You boys should’ve told me that before the bid.” The president wasn’t too eager to hear this story, for good reason. The earthmover went on to explain that he’d gone deeper into the soils than he might have needed, but he was getting to the end of the money and wasn’t going to be able to finish the project for the bid he’d let. He gave the club a couple of options. Either they pay him by the hour until he finished, or he’d have to quit and leave the pond as is. The board conferred, and for some strange reason, paid more money. But, they made him agree to some other dirt work before he left, gratis. At the end, the project was over time, over budget and was such an irritant to the members that the president lost his job. And…there was a big pile of silt left about 200 yards from the pond…not by choice destined to become the new shooting range.

Today, property owners or managers can hire a company for a reasonable fee to come in with sophisticated sonar equipment and map the pond bottom and get a good estimate of how much silt there is. With that map, a manager can project the cubic yardage of silt to be removed, how deep it is, where it sits on the pond bottom and then receive educated, accurate bids to remove the gooey stuff. Then, a more intelligent decision can be made.

How’s the best way to remove silt? Talk to some earthmovers and they’ll tell you to cut the dam, drain the pond and let it sit for months to dry up as much as possible. Those are the guys with bulldozers who have to push dirt to make a living. Other earthmovers suggest removing the water, and then get in there with a trackhoe or a bucket loader and start at the edges and dig it up, load into trucks and haul offsite. Still others with a different train of thought suggest leaving the water in, bring in a dredge on a barge, till that stuff up and pump it to its final resting place or a big bin to be hauled away. Another idea is to simply raise the old dam to increase pond depth and not worry about the silt. I’ve seen several projects where the landowner decided to remove only the silt around the periphery of a pond and leave the deepest muck where it sits. There are a number of ways to skin that silt-laden pond. You, being the pond owner, must decide what the best way is for you, based on your mission…and your pocketbook. Oh, and be sure to check and see if you need any permits.

With a rural pond, it might make the most sense to de-water, mix the silty soils with the dry soils beneath, roll it, pulverize best you can and push it over the existing dam and spread it out as the new back-side slope. It might make the most sense to dig with an excavator, mix with dry soils, and load into a dump truck and haul it off site. And, it could make sense for that excavator to dig, spin and make piles on the shoreline to spread and distribute later, when those pudding-like soils dry up enough to move.

Here’s the bottom line. Silty ponds are common today. Removing that silt may make sense, it may not. If it does make sense, do your due diligence to decide the best methods to move and dispose of the material. After all, you don’t want to get half-way into the project and decide it made more sense to leave nature to do what nature does with that silted-in pond.