Why do dams leak? I get asked this every week. I’m Angus from Big Ditch. I’ve built over 341 dams and I’ve repaired plenty that other people built. Most leaks come back to the same handful of problems. If you know what to look for, you can usually work out where the water is escaping and why it started.
In this post I’ll run through the 6 most common causes of leaking farm dams I see in Australia, what each one looks like on the ground, and what I do about it.
Quick note before you start digging
A dam can lose water to evaporation and normal seepage through the ground. A leak is different. A leak is a concentrated path that keeps flowing even when the dam level drops. If you’ve got wet spots that stay wet, muddy runs, or a constant trickle below the wall, treat it as a leak until you prove otherwise.
1) Poor compaction in the core or key
This is the number one reason I see dams leak. It happens when the wall was pushed up too fast, or the builder didn’t lay the fill in thin lifts and compact it properly. You end up with voids, loose seams, or a soft core that water can push through.
What it looks like: The dam might hold water for a while, then start dropping steadily. You can often find a wet patch on the downstream face of the wall, usually mid-slope, not right at the toe. If you dig a small test hole in the wall you might hit damp, crumbly fill instead of tight clay.
What causes it: Too thick a layer, too dry or too wet at the time of compaction, not enough passes with a proper compactor, or building on a key trench that wasn’t cleaned to firm clay.
What I do: I don’t guess. I locate the seep line and then rebuild the affected section with proper clay, moisture control, and compaction. In a lot of cases you can do targeted work without draining the whole dam, but you do need access and the right equipment.
2) The wrong clay, or not enough clay
You can compact all day and still end up with a leaky wall if the material is wrong. Some soils look like clay but don’t seal. They might have too much sand, too much silt, or they might be a reactive clay that cracks when it dries.
What it looks like: Leaks that show up soon after filling are common. You might also see the inside face of the wall sloughing off, or fine sediment clouding the water after rain. If the wall has lots of gravel or visible sand bands, it’s a red flag.
What causes it: Borrow pits taken from the wrong layer, mixing topsoil into the fill, or building with whatever was easiest to push up.
What I do: I test the material and I don’t rely on “it feels sticky”. A simple jar test, ribbon test, and a proper look at the site tells you a lot. If there isn’t enough sealing clay on-site, you import it or you use a liner system. Doing half a fix with the same bad material just buys you another failure.
3) Pipes and penetrations through the wall
Any pipe through a dam wall is a risk. If it wasn’t installed with anti-seep collars, proper bedding, and good compaction around it, water will track along the outside of the pipe. That’s called piping, and it can turn from “small leak” into “wall failure” faster than people think.
What it looks like: A wet patch below the outlet, water bubbling up near the pipe, or fine soil washing out. If the leak water looks dirty, that’s your warning sign that the wall is starting to erode from the inside.
What causes it: Smooth pipes that don’t bond with clay, backfill that wasn’t compacted, or pipes that have moved because the wall settled.
What I do: If I’m building new, I avoid pipes through the wall where I can and use a spillway setup that does the job safely. If there’s already a pipe, I assess it properly. Sometimes you can seal around it. Sometimes you cut it out and rebuild that section. If you ignore piping, you gamble with your whole dam.
4) Trees and old roots in the wall or basin
Trees look harmless until they die or get knocked over. Then the roots rot and leave channels. If the tree is on the wall, it’s even worse. Those root channels can run right through your core.
What it looks like: Leaks that start after a tree dies, after a storm, or after someone clears vegetation. You might see a wet area downslope from the tree, or a sink spot where the root ball used to be.
What causes it: Building over uncleared ground, letting trees establish on the wall, or leaving stumps and roots in the key trench.
What I do: I remove the tree properly and then I excavate and replace the affected clay. Just cutting the tree off at ground level doesn’t solve the problem. The leak is in the root runs, not in the trunk.
5) Burrowing animals: yabbies, rabbits, and the rest
Burrowing is a big one in parts of Australia. Yabbies can honeycomb the floor and batters. Rabbits and other animals can tunnel into the wall. Once there’s a tunnel, water will use it.
What it looks like: Small holes on the inside batter, fresh spoil piles, or a wet patch that appears after you’ve had animals around for a while. You can sometimes see small whirlpools or cloudy plumes in the water near the bank.
What causes it: Poor animal control, a soft wall face, or a dam with shallow water margins that make it easy for animals to work.
What I do: First, you stop the animals or you’re wasting your money. Then you repair the damage with proper clay and compaction. In bad cases, you need to strip back a section of wall and rebuild it. Products that claim to “seal burrows” can help in minor cases, but I don’t bet a dam on a quick pour-in fix.
6) Spillway and overflow problems
Not every leak is a seep through the wall. Sometimes the dam overtops or the spillway erodes and water starts cutting a path around the wall. That erosion can look like “leaking” because you get constant wet ground and rills on the outside.
What it looks like: Scouring near the spillway, erosion channels on the downstream side, or a low point on the wall crest. After big rain, you’ll see fresh wash marks and exposed soil.
What causes it: Spillway too small, wrong location, poor grass cover, or no rock protection where it’s needed.
What I do: I treat spillways as part of the dam, not an afterthought. Fixing it can be as simple as reshaping and armouring, or as involved as moving the spillway and rebuilding a section of wall. The key is to keep uncontrolled water off the wall.
How I diagnose a leaking dam on site
- Track the wet area: I follow wet ground uphill to the highest point I can find. That often puts you close to the leak path.
- Check water level behaviour: If the loss slows or stops at a certain level, the leak is usually at that elevation.
- Look for dirty leak water: If soil is coming out, the dam is eroding internally and you need to act quickly.
- Inspect the wall crest and spillway: Settlement, cracks, and low points tell you where water will try to go.
FAQ
Can a dam seal itself over time?
Sometimes. Fine clay can settle and plug minor seepage, especially in new dams. But a proper leak path through the wall or around a pipe rarely “self heals”. If you see a constant wet patch or a trickle that keeps going, you need a proper diagnosis.
How do I tell evaporation from a leak?
Evaporation drops the level evenly and the ground below the wall stays dry. A leak leaves signs. Look for wet patches, muddy runs, greener grass lines, or water pooling below the wall even when it hasn’t rained.
Is bentonite a good fix for a leaking dam?
Bentonite can work when the leak is through the floor or batters and you can mix it properly into the soil. It does not fix poor compaction, piping along a pipe, or root channels. I treat it as one tool, not a magic answer.
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