How To: Understand the Features and Hazards of the Different Types of Open Water Swimming Locations – III – Estuaries and Man-Made Locations Including Quarries, Reservoirs and Canals

This third and final part follows:

Part One on Oceans.

Part Two on Lakes and Rivers.

I was at the top of the 1000 foot high Cliffs of Moher with the wind was blowing Force 11. I noticed that unsurprisingly most people were unable to move easily. It had never previously occurred to me that somehow over years I’d learned something subconscious about wind and how to move in it, something I didn’t know I knew until that moment.

A similar situation arises with open water locations. Not all locations are equal. Each have different features and therefore hazards which result from those features. When I started to think and write about some of these, I realised that like that Force 11 gale, there was more to impart than was immediately obvious and that comfort or experience in one area does not imply the same for all locations.

I’ve always believed strongly in sharing useful information about swimming rather than trying to be a guru. Out there, indeed some of you reading, are canal swimming experts, reservoir mavens and river gods, even (I shudder at the phrase) “wild swimmers“, all deeply experienced experts in your chosen swimming arena. So this series is merely an attempt at a concise guide.

The sum total should enable you to make your own decisions about swimming. has never been about telling you where and when you shouldn’t swim. I certainly make all my own decisions. I only desire that people make their own decisions about safety based on an accurate understanding of the location and situation.


Estuaries are where rivers join the sea. As such they can share the features and hazards of both but also carry additional significant hazards. Estuaries tend to be centres of population even in low population density hinterlands. One can treat Bays as simply open ocean but estuaries deserve their own consideration and are swimming areas that in general should be treated with the utmost caution.

Fjords, lagoons and river deltas are all specific types of estuaries.


  • Changing water flow direction. Up, down, in, out.
  • Partially enclosed by land. Estuaries can be many miles long. The lack of visible connection to the sea does not negate the potential effects of the sea and tides.
  • Tidal. The highest tidal range areas in the world are estuaries.
  • Waves. Depending on the topography and bathymmetry of the estuary.
  • River erosion and deposition. Rivers erode while also eroding the land. Few natural geographic features change as rapidly as estuaries.
  • Brackish water. The salinity of an estuary is dependent on a huge number of variables and can range to saline to fresh, but generally if inaccurately estuaries are describes as brackish, which is a mixture of both fresh and salt water.
  • Mudflats. Deposition of silt and sand from rivers as their flow speed reduces in widening estuaries leads to large flat areas, and therefore concentrating marine traffic into narrow lanes. Mudflats exposure will often vary with tides, and are regularly dangerous areas where drowning occurs due to people being caught by unexpected tides, such as the UK’s 2004 Morecombe Bay tragedy.
  • Wildlife. The change from saline to fresh water means that estuaries are an ecological challenge for some creatures. However they can also be areas of high food supply. The range of wildlife can therefore be unpredictable and broader than either local sea or river habitats.


  • Tidal. Tidal or Estuarine Forcing is where a significant tide pushed into a long narrowing estuary can result in a higher than the local ocean tide as the area is reduced, forcing water upward. Depending on shape and width and local tides, tides may vary greatly and the local tidal cycle may be more complex than a basic high/low tide.
  • Concentrated Tidal Currents. The shape of an estuary can greatly concentrate tidal current far beyond that possible on exposed sea coasts. A tide that flows into a wider area through a narrow gap (a lagoon) may have an extremely powerful tidal current. Just in my local area in South East Ireland this is apparent in Dungarvan Harbour, Youghal Bay and Tramore’s Saleens (all estuaries, all with lagoons) where the current of the flowing tide is stronger than any swimmer and similar to a strongly flowing river.
  • Flow direction. The mouth of an estuary will share a tidal pattern similar to the open sea. The width and length of the estuary, the width and or volume of the river flow, will all affect the flow. Further inland of estuaries flow patterns may deviate from the tidal pattern. For example down river flow may be longer than upstream tidal flow, high tide slack may be shorter than low tide slack water. Swimming should only be done with a good understanding of the time, strength and duration of local flow conditions.
  • Tidal bores. Bore are somewhat rare occurrences in areas with a high local tidal range where on certain spring tides a surging front of water is pushed upstream against the down-flowing river, causing a significant and turbulent wave front. For example the Amazon, the Severn and Morecombe Bay in the United Kingdom, the Shannon in Ireland (and I believe I’ve seen a small bore on the Suir), Savannah in the US, plenty in Canada, Daly and the wonderfully named Styx rivers in Australia, and more. Due to the intense turbulence and debris in the flow bores are extremely hazardous for swimming.
  • Pollution. Similarly to rivers, but due to higher population density combined with the flow of rivers, estuaries can suffer introduced pollution such as sewage, and toxicity from industrial pollution.
  • Population. The warnings of dangers of drowning in urban open water areas in the second part on rivers also applies here.
  • Marine Craft. Estuaries are harbours as often as not. This lead to concentrated marine traffic through navigation channels, and more berthed marine craft.
  • Visibility. Ocean sand and river borne material, along with material stirred up by boats, means estuaries often have poor water visibility.
  • Unpredictability. The combination of all the range of factors mean estuaries can share the unpredictability of the open ocean and with the added hazards of urban or river environs.
  • Turbulent water. The movement of conflicting bodies or flows of water can lead to turbulent and locally unpredictable water movement. One of the challenges of the Manhattan Island Marathon Swim is that unpredictability in the area of Hell Gate at the confluence of the Harlem River, Long Island Sound and New York Harbour via the East River.
  • Rapidly varying or unpredictable depth. Tramore’s Back Lagoon is a large area of water at high tide, and almost empty at low spring tide. The volume as noted above empties rapidly through a narrow gap. The prevalence of mudflats and silted up areas can make depth highly variable.
  • Flotsam and jetsam. Waste, fallen trees, dead animals are all commonly washed downriver into estuaries.
  • Wind Against Sea. While I didn’t mention this weather and water condition specifically as a hazard of sea swimming where it also occurs, in estuaries onshore breezes coupled with out-flowing tides makes Wind Against Sea a regular difficult occurrence.

QUARRIES and Tailings Ponds (and other man-made water-filled abandoned locations)

Far more dangerous than often appreciated and regular scenes of fatalities. I am more wary of quarries than any other open water swimming location and I do not swim in them.

Tailings ponds are the toxic result of mining operations and contain soluble dangerous mining by-products, and therefore likely to be lethal or at least very harmful.

While I’m not aware of a significant number of people swimming in water-filled abandoned factories or similar, the cautions and understanding are still worth taking note, because a few is not none.


  • Still water. Abandoned quarries fill from rainfall and run-off. Rivers don’t as a rule flow into them. Level variation is through evaporation and seepage.
  • Deep water. Quarries can be very deep in comparison to rivers or lakes.
  • Dark or Coloured water. Since quarries are the de facto result of mining, on occasion the water can be a colour that is unrelated to the state or cleanliness of the water.
  • Steep sides. The sides of many quarries are steeper than coastal cliffs.
  • Man-made. Quarries and tailing ponds are not made to be decorative or to be used for recreational pursuits. They are by-products of other processes.
  • Still Water.
  • Contaminated or toxic water. Toxicity is a feature as well as a hazard of tailings ponds.


  • Deceptive water colour. I put this as the number one hazard as it can either mask other problems or be a problem itself. Because quarries are by definition a product of mining, metal leechates in the water, particularly copper, can make the water look the same colour as what we associate with clean healthy water. (Some older swimming pools can add copper to water to make it look blue). Turquoise water that looks inviting can actually be very unhealthy or even dangerous. Unlike the sea where turquoise water is clear, in quarries  it can be completely opaque. This problem is so defined that there are environmentally safe dyes available for local councils to add to water to discourage swimming by turning the water a forbidding inky black.
    Golden evening light makes a dangerous water filled slate quarry seem magical

    Beautifully Dangerous. – A local slate quarry, where at least two people have died

  • Submerged hazards. Quarries are often treated as local illegal waste disposal locations. The range is a wide as a criminal polluter’s imagination.
  • Toxicity. Also another variation of the previous points. You may also not know what raw material for which the quarry was originally mined, nor the control of the company mining it nor what has been dumped in it not what the quarry was mined for nor what may have been a by-product of the mining or the abandonment. Tailing ponds will be toxic more often than not. Heavy metal contamination such as mercury or lead, or toxic substances like arsenic are common in tailing ponds.
  • Pollution. Similar to the previous point, but with the addition of deliberately dumped hazardous waste, or the remaining by-products of the mining operation itself.
  • Sharp rocks. Submerged or even exposed rock faces of quarries have gotten that way through cutting, and haven’t had sufficient exposure to weathering to blunt sharp edges. Imagine walking across a submerged bed of scalpels.
  • Depth. Due to the man-made nature of quarries, depth will vary dramatically without any intermediate gradients.
  • Cold. Ponds heat up quickly because they are shallow, AND because they expose a greater surface area to the Sun. Quarries can have deep water that will be slower to warm, and the ability of the Sun to shine of the surface can be restricted by deep sides. While considering cold water is part of the nature of this site, I know of one local instance of a heart attack in a quarry caused by unexpectedly cold water at the height of what passes for summer here.
  • Unstable sides. Quarries, especially old disused quarries, are not designed for structural stability. The sheer sides that often attract cliff jumpers can also be completely unstable.
  • Stagnant water. Malaria and dengue fever are two problems associated with stagnant water in warmer climates than northern Europe. But parasitic and bacterial infection are highly correlated to stagnant water everywhere. Not least is faecal contamination. Without flowing water over gravel or reed beds, stagnant water will act as a bacterial breeding ground. Blue-green algae is also toxic and common to stagnant water.


Canals are entirely artificially constructed waterways designed specifically to enable marine traffic to pass and travel between areas of different height. Depending on the country, swimming may be entirely forbidden (as it is in Ireland) or heavily restricted with some local exceptions, (such as the UK). Water doesn’t flow continuously through a canal as it does in a river.


  • Man-made.
  • Shallow. Barge canals will be extremely shallow, often only waist deep. Shipping canals will be deeper but still relatively shallow. Locks will be deep.
  • Narrow and straight.
  • Marine traffic. Canal traffic is mostly dictated by its intent. Older barge canals will generally only have narrowboat barges, shipping canals will have ships.
  • Divided into sections by locks and gates.
  • No continuous current. When locks are in operation water flow is lethallly overwhelming.


  • Pollution. Due to the lack of continuously flowing water, and also the fact that most canals pass through urban and/or industrial areas.
  • Marine craft.
  • Flooding and evacuation of locks makes them utterly unsuitable and  dangerous locations for swimming
  • Steep sides. Due to the artificial nature, canals will always have vertical or very steep sides.
  • Toxicity. The stationary water means that along with pollution, toxic substances or even diseases such as Weil’s Disease could be more prevalent.


Inishcarra reservoir

Inishcarra reservoir

Reservoirs are used for different principle reasons: hydroelectric power generation, water supply, to control a river’s flow or for irrigation control, usually providing more than one function. Reservoirs are usually either completely or partially restricted.

Inishcarragh Reservoir in Cork is a good example of a popular safe reservoir location that looks like a natural lake, and is widely used for permitted swimming, sailing  and other water activities. The restricted section is the area below and above the dam and prior to the introduction of a new exclusion zone, local marathon swimmers (including myself) would swim right up the dam wall.


  • Often restricted or forbidden for swimming.
  • Dams.
  • Possibly steep sides and difficult or impossible exits.
  • Turbine intakes on upstream side of dams.
  • Clean water. This is not an invitation.
  • Deep.
  • Weirs and turbulence prior to and downstream of dams.


  • Difficult or Impossible exits.  Reservoirs can often have steep concrete sides. Swim too far and you may be unable to exit, and invisible to people on land. Ardnacrusha Reservoir in Limerick is a good example.
  • Limited or restricted access. Possibly associated with civil fines or penalties.
  • Terrifying sucking inlet pipes of certain death. Aka Stupidity Accelerator.
  • Vortices and submerged weirs on downstream side of dams. Vortex is a such a lovely word.
  • Cold water. Again, since reservoirs are usually dammed rivers, they can be quite deep. In temperature they are not outside the range discussed on this site, but the problem here is unexpected cold in warm weather  by inexperienced bathers.

Storm Drains, Fish Ponds, Fountains etc.

If there is water, and you are an open water swimmer, you will think of swimming in it.

Features & Hazards

  • Not designed or built for swimming.
  • Shallow.
  • Water jets or fountains.
  • Ornamental fish.
  • Concrete and stone.
  • Syringes and broken glass.
  • People throwing coins.
  • Fast food packaging.
  • Pollution.
  • Remote controlled model boats.
  • Stagnation.
  • Swans and ducks. Children and dogs.
  • Drunken students.
  • Sober students.
  • Security guards.

6 thoughts on “How To: Understand the Features and Hazards of the Different Types of Open Water Swimming Locations – III – Estuaries and Man-Made Locations Including Quarries, Reservoirs and Canals

  1. Interesting about tailing ponds. I have seen them looking tropical blue and enticing on a summers day..doubt general public are aware of the fatal dangers within. Often as they are abondended warning notices will be faded or vanished.


    • This has been on my mind for while. I think as long as the freak wave message continues, then people will think, no matter how many times they hear it. ” it’s a freak waves, so there’s no chance of it happening to me”. Th situation is even worse with urban swimming locations and some of the other locations.


      • Donal I hear you on the freak wave delusion it was the first thing that came to mind on hearing the news. We learnt to observe the water for 10 – 15 minutes through surfing. The bigger the surf the longer we’d be watching the water to see how it was behaving and where and when those bigger waves were coming. Call it what you like but this is a fundamental survival technique. I’ve not much knowledge on the urban environment and will make a point of finding out more. What did we learn at school about crossing the road? Seems pretty simple right? STOP – LOOK – LISTEN. Well I am going to say that these same principles need to be applied to water safety as they can be applied both in silence and sound. It doesn’t take a genius to be creative and think up a hundred or so scenarios where these principles could be applied so that people can avoid danger. If you don’t believe me just ask I’d be happy to share examples. I’d like to also say that even though a lot of great work is done with water safety and saving lives I strongly believe that a lot of whats out there is simply not adequate. I have seen water safety brochures with cartoons that do not instil or convey the seriousness of the situations being depicted. They seem lighthearted about it and it is not a lighthearted subject. So again it is my belief that what is currently in place is simply not adequate. I wish there was a way to change it and to make a difference.


  2. I’d be interested to see some detailed stats on our canals here, in terms of water quality. A cursory glance at some EPA reports would suggest a rehash of one of those god-awful political phrases; ‘a lot done, more to do’. As you know, the summit levels of the Grand and Royal Canals are in the midlands and if you take the Grand, for example, it’s fed by one of the cleanest sources of water anywhere: Pollardstown Fen and the Milltown Feeder (one of the coldest inland swims you’ll ever have). No doubt as the levels drop towards either the Shannon or Dublin, canals pick up some bogey material, but on the whole, I would imagine their levels in relation to the surrounding land makes them less of a target for general leaching of agricultural pollution and run-off than your average river. This is all educated guesswork on my part, but I love the canals around Kildare and Meath and have swam in them a fair bit over the years without any ill effects. I appreciate of course that in addition to my admitted layman’s knowledge, the lack of experiencing any problems to date does not prevent any possible future ones from happening, plus I am referring to the two main Irish canals, and have no experience of any further afield. But just thought I’d give them a quick shout out. On the whole, the Royal and Grand appear to be healthy ecosystems, and that starts with water quality. For other factors, such as access and clarity, that can vary of course. Just stay out of the locks. They’re up with your vortices and inlet pipes of death for an off-the-scale stupidity reading.


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