XIII. Swimming in the epilimnion

Last winter I got my hands on historic water temperature data (20 years worth) of Lake Siljan from a researcher at the University of Uppsala. Between that information, and a variety of anecdotal remarks from my Swedish friends and their friends (who regularly spend a fair amount of time at and on Lake Siljan), I am expecting the lake temperature at the surface to be somewhere between 53 and 69 degrees F (11.5 and 20.5 degrees C), and most likely, I think, will be a temperature of 63 degrees F (17.5 degrees C) or  warmer.  The historic data and the prevailing opinions of these folks hold that the warmest water at the surface will be between July 24 and August 7. They (the people and  the data) would say that after the first week of August, due to colder nights and diminishing insolation, the lake is experiencing daily net losses in heat, and that summer is slipping away.

The water in a lake is very much stratified based upon the temperature of the water. See this picture (model) of typical summer water temperature dynamics in a lake.  This  picture is  illustrating many things,  but for starters, see that a lake  is divided into three categories along the water column.  The epilimnion  is the warm  layer at the  top of the lake, which grows warmer and thicker as summer carries on.  The  coldest water is at the bottom  of the lake, in the layer called the hypolimnion.  This water tends  towards 39.5 degrees F. (4


degrees C.), which is the temperature at which water is heaviest (Yes, as the temperature of water drops below 39.5 degrees, it gets lighter!)  The middle layer is called the metalimnion, and it provides a buffer between the upper and lower layers, and largely prevents mixing  of these two layers as summer unfolds.

The metalimnion is also  called the  thermocline, and this next image shows the relative temperature dynamics of the three layers. The top layer is relatively shallow and has a
temp changes

fairly uniform  temperature;  the thermocline evinces a significant  dropoff  in temperature, and then the hypolimnion shows a slow and steady dropping to 39.5 degrees F (4 degrees C).

All  this  is really just to  say that I am very grateful for the dynamics that create the epilimnion.  It grows fatter and warmer throughout the summer, providing a comfortable portion of the lake in which I can swim.   An added bonus is that this  layer is  located at the surface of the lake, and so  not only do  I  get to  swim in this (relatively) warm  water, but  I can just  turn may head and breathe the air.   Simple  to do  in the epilimnion,  but don’t try it in the thermocline or the hypolimnion.

cold dropping 2

To  make  a stab at empirically manifesting these facts, I  combined  blue food  coloring and water, and  made  some blue ice cubes.  Then  I  carefully laid one of the ice cubes in a plastic cup  with water in it  that was about 60 degrees F (15.5 degrees C).  As you can  see in the photo to the  left, as the ice melted, the cold blue  water streamed down to the bottom  of the glass.  Then I introduced some hot water (colored yellow), using an  eyedropper, into the glass with the melted blue ice cubes in it.  The yellow water was about 135 degrees F (57 degrees C), and as I  eased it onto the surface of the colder water, most of  it stayed right at the surface.  The  following two  images show this.

introducing warm  water2 layers v3

What I  found was that  when  I first put the warm, yellow water on the surface of the cold, blue water, the warm water bounced around and often dropped into  the blue water,  turning  part of it green.  But  as I  gradually, carefully added the warm water,  it  developed into a top layer  that was cohesive, and  was  distinctly yellow (or maybe sort of green), and  obviously separated from, and above, the colder blue  water.  Once this  top  layer was established, I was able  to add to it  with the eyedropper easily.   As it grew, it developed a structural  integrity, which made it behave like a distinct entity from the rest of the water in the glass.

Below is  the  same glass,  annotated to  show  what  might be the three  distinct layers that represent the water column  in a lake.

2 layers v3 annotated

In  an ideal summer, the top layer would accumulate a lot of warmth,  and  grow to  be a couple of  meters thick.  In a windless world, that layer  would keep fattening  up.  However….a couple of days of substantial wind could very much undo the integrity of that upper layer. Wind across the lake’s surface can start gouging down, and displace the topmost water so that other, colder, deeper water comes up to replace the top water (this is called “upwelling”).  The  first graphic in this post depicts how wind can confound  the idealized layers and cause this upwelling motion, which mixes the metalimnion with the epilimnion.  And  the  graphic  below  shows how  the colder,  deeper water, once set in motion, can infiltrate the  epilimnion  and compromise its  integrity, causing  it to mix with cooler, lower water and lose its warmth.


Once the top layer is mixed with the middle layer, it can take days or weeks before the top layer recovers the  warmth  it loses due to  wind. So, I am hoping for a couple weeks of high pressure and light winds only, in the lead up to the swim date.

In terms of some  water temperatures (from  past marathon swims) as points of reference:

  • Anacapa Crossing in 2010 (a 7 hour swim) was 61 degrees F (16 C)
  • Lake  Tahoe in 2011 (a 14 hour swim) was 66 degrees F (19 C) the entire swim
  • Catalina Island channel  crossing in  2013  (a 12 hour swim) was 67 degrees F (19.5 C) until the last 30 minutes, when it suddenly dropped to  62 degrees F (16.5 C)

I  believe that if I encounter 60 degree F (15.5 C) water (or warmer) I will  be able to complete the swim.
I believe if I  encounter 54.5 degree F (12.5 C)  water (or  colder), my  ability to  complete the swim will  be in  great jeopardy.
I  believe that if I  encounter sustained water temperatures between  54.5 F and 60 F, I will have my  fingers  crossed,  hoping that I can sustain my  core  body  temperature  for the twelve hour long swim.



3 thoughts on “XIII. Swimming in the epilimnion

  1. Dave: Thinking of you and sending WARM WATER wishes for your swim. Will be following your progress on this site, which is a wonderful use of technology. Can’t wait to read your post-swim report!!! Sending Love your way from ELKhorn!!

  2. Wow that was interesting! You could change professions and be a science teacher! Will pray for 61 and no winds all the way. Bad wind hasn’t beaten you before so it may as well lay low! Go Dave!

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