The difference between a leaking dam and a seeping dam

The short answer: a leaking dam is losing water through a structural failure — a crack, a hole, or a failed outlet that needs urgent repair. A seeping dam is losing water slowly through the embankment as water moves through the soil itself, and while some seepage is normal, unchecked seepage can destroy an embankment over time. Knowing which one you have determines exactly what you do next.

Why the distinction matters

I get calls from landowners all the time who say “my dam is leaking.” When I get out there, about half the time what they actually have is seepage. The two problems look similar on the surface — the water level drops and you see wet ground somewhere you don’t want it — but the cause, the urgency, and the fix are completely different. Treating seepage like a leak, or a leak like seepage, wastes money and can make things worse.

What a leaking dam looks like

A leak is a discrete pathway where water escapes through a defined gap or failure point. Think of it like a hole in a bucket — water moves through fast, and you can often trace the path.

Common causes of dam leaks

• Cracked or failed pipe outlets and their surrounds
• Animal burrows — wombats, rabbits, and yabbies are the main culprits in southeastern Australia
• Tree roots that have penetrated the embankment
• Settlement cracks in the crest or batters
• Erosion channels cut by overtopping or spillway discharge

How to spot a leak

• A concentrated wet patch or stream of water coming from a specific point on the downstream face or toe
• Turbid (muddy) water exiting downstream — this is a red flag that soil is actively moving through the embankment
• A sudden drop in water level that doesn’t match evaporation or usage
• Visible cracks running across the crest

If you see muddy water coming through the embankment, treat it as an emergency. That’s called piping — the embankment is eating itself from the inside, and it can lead to failure within hours. Call someone immediately.

What a seeping dam looks like

Seepage is water moving slowly through the pore spaces of the embankment or foundation material. Unlike a leak, there’s no single defined pathway — water percolates through the mass of soil itself. Some seepage is normal, especially in new earthen dams that haven’t fully consolidated.

When seepage becomes a problem

A slightly damp toe on a full dam isn’t unusual. Seepage becomes a problem when it appears high on the downstream face, the wet area is growing, the soil is becoming saturated and unstable, or you see cloudiness in any water exiting the embankment — that last one means soil particles are being carried out, which is serious.

In most cases I see, excessive seepage comes down to one of three things: the dam was built with the wrong material (too sandy or gravelly), compaction wasn’t done properly, or the original design had no core or drainage system. Older dams built without engineering oversight are the most common offenders.

Seeping dams can often be fixed without major earthworks. I’ve repaired many seeping dams using bentonite clay sealing — a technique where powdered or granular bentonite is applied to the upstream face or introduced into the water. The bentonite swells on contact with water and fills the pore spaces in the embankment. It’s one of the most cost-effective fixes available for a seeping earthen dam.

How I diagnose the problem on site

When I inspect a dam, I start by checking the water level against any historical records — is the drop consistent with evaporation rates and usage, or is it faster than that? Then I walk the full embankment, looking at every metre of the downstream face and toe. I check the condition of all outlet works — pipes, headwalls, and valve surrounds — because these are common failure points that are easy to miss.

If I can’t make a clear call from surface inspection, I sometimes use dye testing — introducing a non-toxic dye near a suspected entry point and watching for it to appear downstream. It confirms a discrete leak path where surface observation alone isn’t conclusive.

For a thorough assessment before you spend money on any repair, a proper dam consultation is worth doing. Getting the diagnosis right saves significant cost down the track.

The repair approaches are different

Fixing a leaking dam

Leaks need targeted repair of the failure point — replacing a failed pipe outlet, filling animal burrows with compacted clay, cutting out and repacking a settlement crack, or removing and replacing damaged sections of embankment. Any earthwork repair needs properly compacted clay. Loose, uncompacted fill will fail again.

Fixing a seeping dam

Seepage repairs focus on reducing permeability or improving drainage. Common approaches include bentonite application to the upstream face, installing a clay blanket, adding a toe drain to safely collect and discharge seepage, or in severe cases, constructing a clay core by reworking the existing embankment. For full details on what’s involved, the dam repair page covers the main methods I use.

Which is more dangerous?

Both can be dangerous if left alone, but an active leak producing muddy water is always the higher urgency situation. Piping can bring down an embankment fast — treat it as an emergency. Seepage is slower but still serious. A heavily seeping dam with a saturated downstream face is at risk of slump failure, which can be just as catastrophic. Don’t ignore seepage because it seems gradual. It can progress quickly once the soil structure starts to break down.

FAQ

Can a dam seep and leak at the same time?

Yes. A failed outlet pipe can create a discrete leak while the surrounding embankment material is also too permeable and seeping. I always check for both during an inspection, because fixing one without addressing the other won’t solve the water loss problem.

How much water loss is normal for an earthen dam?

In hot Australian summers, evaporation alone can account for 10 to 20mm per day on an exposed dam. Some seepage is also normal in new dams. If your water level is dropping faster than evaporation and usage can explain — say, more than 30 to 50mm per day with no clear water draw — that warrants investigation. Measure the drop with a staff gauge over a few days to get a reliable rate before calling.

Will bentonite fix a leaking dam?

Bentonite works best on seeping dams where water is moving through the soil matrix. It won’t reliably seal a discrete structural leak — a cracked pipe, a burrow, or a settlement crack. For those you need targeted earthworks or mechanical repair. Applying bentonite to a leaking dam may reduce seepage around the failure point but won’t address the underlying structural problem.

If you’re not sure what you’re dealing with, give me a call and we can talk through what you’re seeing. Call free on (02) 7229 4866.

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