How To: Understand the Features and Hazards of the Different Types of Open Water Swimming Locations – II – Lakes and Rivers

This is the second of a three-part series covering the features and hazards of open water swimming locations, the first of which covered the ocean.The Ocean is sufficiently complex and changeable to require its own article. I had intended in this second part to cover the remaining locations but the article was simply too long. Part Three will cover Estuaries, Quarries, Reservoirs and other man-made locations.

While the largest lakes can display most of the features of the ocean, none can truly display the same complete range. However these locations do have other challenges to the ocean.

That perfect moment: Unbroken water, and a swimmer in flight

That perfect moment: Unbroken water, and a swimmer in flight. Three hundred metre high cliffs and hidden rocks!


The terminology of lakes is confusing. In Newfoundland, for example, almost every lake is called a pond, whereas in Wisconsin, almost every pond is called a lake. In Scotland lakes are called lochs. In Ireland lakes are called loughs. Both are pronounced lock. To add to the confusion in Ireland and Scotland, we also have sea loughs/lochs, which are “arms” or inlets of the sea, and which may otherwise be called lagoons. Loughs range from the almost completely enclosed Lough Hyne to the much larger natural inlet  of Belfast Lough or the fjords of Killary or Carlingford.

There is no accepted definition of a lake and no single agreed feature to which you can’t readily find exceptions.


  • Colloquially, people generally mean lakes are bounded by land and not part of the ocean, (recalling all the above exceptions for Ireland and Scotland).
  • No tides. Most inland lakes have no discernable tides. Even the largest inland lakes are still too small to have noticeable tides. Tides are present but small and wind waves are larger.
  • The majority of lakes are fresh water (again with the exceptions of Sea Loughs and Salt Lakes like the Dead Sea or Great Salt Lake).
  • Different wildlife. Few marine creatures live in either fresh or salt water habitats. More birds share the habitats. Mammals and reptiles vary by region and latitude, as with the ocean.
  • Fresh water lakes have more predictable currents than the sea. The largest lakes are subject to local current variation due to wind, temperature variation, topology, bathymetry, depth and structures.
  • Quicker and greater temperature variation. Since lakes are smaller than the sea, their heat changes more rapidly, depending on the weather, in the way a small bowl of water heats and cools more quickly than a large bowl of water. Freshwater lakes are usually colder than the ocean in winter and usually warmer than the ocean in summer
  • Smaller waves than the sea. Large lakes can have large wind waves but even the largest local wind waves cannot match the size of ocean waves.
  • Reeds and plants. The largely still nature of lakes means that the littoral area can have significant plant growth.
  • Mud.  Unlike the sea, the floor of lakes can be very soft and standing in it can make it difficult to wade or move.
  • Buildings.

Lough Dan start Donal starting IMG_1340_01.resized

Lough Dan, in the Wicklow Mountains where Fergal Somerville holds his International Ice Mile swim, due to lower temperatures than the nearby sea.


  • Visibility. Lakes can range in visibility to crystal clear to impenetrable black. Mountain stream fed  lakes or those surrounded by peat can be terrifyingly unchangingly opaque, leading to Ireland at least, the numerous legends of bottomless lakes.
  • Clarity of water should never be taken as a sole indicator of water cleanliness. A perfectly clear lake can be so because it’s also completely lifeless, though these cases are rare. Similarly, water that is black from peat runoff can still be extremely clean.
  • Lack of buoyancy. Lesser buoyancy means most people will swim more slower in fresh water. Over long distances this will also lead to increased tiredness, especially in the shoulders and back.
  • Pollution. Due to size, lakes can become more easily polluted than the sea. Agricultural runoff can be a separate issue to industrial or unban pollution.
  • Refuse. From plastic wrappers through shopping trolleys to cars.
  • Duck Itch, aka Swimmer’s Itch and Lake Itch. Dermatitis reaction to parasitic microscopic larvae primarily from water fowl, also from sea lice.
  • Marine craft. Pleasure and fishing craft are regular features of lakes.
  • Fishing. Closely associated with the small craft and also shore angling.
  • Plants and mod. While getting trapped in plants is usually an over-rated hazard, it is the case that bullrushes can get very large and some reeds can interfere with a swimmer’s movement. The soft floor that an  occurs
  • Wind waves. Some  sites and sources say that lake waves are different to sea waves. This is incorrect. Lake waves are merely a subset of the range of waves encountered in the sea. Since lakes are smaller most waves present will be short period waves and chop which can hit the swimmers every two or three seconds. (These type of waves also occur in the sea).which are the same as the sea,
  • Currents. Apart from the aforementioned Tidal Currents, there are other types of hazardous currents including:
    • Rip Currents. These localized currents are escape channels for breaking waves, and can be extremely strong, running directly or obliquely away from the shore.
    • Longshore Currents. These run parallel to the shore and can be longer than rip currents. They can also combine with rip currents.
    • Channel currents.  The general term for water flow compressed  into a stronger flow in a narrower space.
    • River Inflows. Rivers entering the lake can be quite strong depending on the river volume, width and fall height. In lakes the entry of  river is generally called an Inflow. In the sea it’s called an estuary or outflow.
    • River Outflows. Rivers flowing out of a lake can potentially cause sudden local increase in flow. See previous note on terminology.
Under the first bridge in Clonmel, one of five.

Approaching a bridge. Suddenly shallow on the far side.


River are generally natural freshwater streams flowing in a natural channel which may be permanent or impermanent. But because there really isn’t enough confusion with lakes above, the Kenmare River in Ireland isn’t a freshwater river but rather a large sea bay. We use a lot of different words in English for rivers depending on their size and location.


  • Moving fresh water in a channel.
  • Wildlife. Like ocean and lake wildlife, this subject is too broad to discuss. Lakes and rivers in the same region will share the same animal and plant populations.
  • Changing scenery.
  • Marine craft.
  • Quickly changing depth. Even rivers that are placid on the surface can suddenly increase or decrease depth.
  • Winding course. Few rivers are straight for substantial distances.


  • Flow speed. The centre of a river is always faster moving than the sides. Rain can dramatically and suddenly affect a river’s flow speed and strength through flash floods. Rivers will also vary in speed due to overall and local fall height or local narrowing or widening due to geology or man-made intrusions.
  • Insufficient depth. Its possible while swimming in a river to suddenly have insufficient depth to swim.
  • Rocks. Rivers cut through the landscape and rocks  can be exposed underwater in an area that outside the river seems rock free.
  • Weirs. Weirs are primarily designed for fish. We are not fish. Weirs can generate eddys and undercurrents and limit passage and cause impacts and lacerations.
  • Flotsam and Jetsam.The constant flow means that even the cleanest and best kept rivers will move obstacles along them like branches and even whole trees.
  • Hidden obstacles.  Thier passage through urban areas brings an increasing possibility of dumped and hidden obstacles, especially when combined with reduced visibility.  I narrowly missed being disembowelled while swimming from Clonmel to Carrick-on-Suir by a single hidden metal spike erupting from the river bottom. It was narrow enough to cause no visible surface disruption or to be visible in advance.
  • Storm and flash floods can change a river in minutes and sweep dangerous items along. and change a placid river to a raging torrent.
  • Pollution. River are rarely without urban settlements. With this settlement comes pollution. Sometimes there may be old forgotten sewage outlets or industrial outlets. Pollution can occur upstream in apparently clean rivers.
  • Reeds and plants. Similar to lakes.
  • Bridges. Bridges can affect rivers by compressing the flow or decreasing depth. Items dropped from above can cause hazards. Eddy currents can occur in the lee of stanchions and pylons.
  • Other swimmers. While drowning is of course a risk in any body or type of swimming location, urban rivers are the most dangerous location and site of most drownings. This is partly because of increased use of alcohol, and partly due to people with less experience around the water.

 Related lake and river swimming articles on


Never Swim A Metre More Than Necessary – Lough Allua Swim.

Lough Dan Ice Mile Swim Attempt.

Summer in Ireland – Mountain Lake Swimming 1.

Summer in Ireland – Mountain Lake Swimming 2.

Swimming the River Suir.

Swimming Glendalough Upper Lake.

MIMS 2012 – Part 4 – The East and Harlem Rivers.

MIMS 2012 – Part 5 – Hudson River and Finish.

11 thoughts on “How To: Understand the Features and Hazards of the Different Types of Open Water Swimming Locations – II – Lakes and Rivers

  1. Pingback: 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 | LoneSwimmer

  2. Thanks for this and all of your postings.

    With regard to rivers I would add what we used to call “snags”. Fallen trees and other plant life at the rivers edge can be life threatening hazards. If you get caught up in the snags the rivers current will pull you under the snag.

    River kayakers know about this risk. Even wearing a PFD the current will take pull you under the water. If you’re lucky you’ll get flushed out the bottom. If you’re unlucky the current will just leave you pinned against the snag.


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