A question about thermoclines and fall turnover

[scaleface]

I know what a thermocline is and the fall turnover .

A lake i frequent is 30 foot deep . The thermocline has dropped to that depth . So when a thermocline reaches the deepest part of a body of water, would that be  the same conditions as the fall turnover ?

[A-Jay]

You may actually be able to "see" turnover.  

As the process begins and continues, the water will often take on a colored or cloudy appearance.  In some of my own local waters, turnover makes a mess.  Worse part is the fish will often simply shut down either way I've not ever had 'great fishing' during & on waters that were in the middle of the process.   Fortunately all the waters don't turnover all at the same time; usually the smaller / shallower (relative term) places start & finish first.

So I can often bounce around and find one that either hasn't started or is done.

However there's usually about a two week period where most places are a little jacked up.  I will often take that opportunity to travel abroad to fish.

A-Jay

 

[scaleface]

The lake i'm referring to starts out with about a 15 foot thermocline { thats when i first start to see it on sonar } then it slowly goes deeper as the year advances . 20 foot , then 25   and now its indistinguishable from  the bottom . So my question is does it now have the same characteristics as a lake that has turned over even though the temperature hasnt dropped to the 50 degree range ?

[Team9nine]

It should now be "turned over" and have the same characteristics. Two ways to tell. A temp probe is best to confirm, but seeing fish arches all the way down on the bottom is another. The 50 degree mark is a generalization. Different bodies of water have different temps below the thermocline. Whenever the temp is consistent throughout the entire water column, regardless of the exact temp, the lake has turned over.

[tkunk]

There's nothing magic about the 50 degree mark. The lake I normally fish turns over at least once each summer because of wind and big waves.

[scaleface]

11 minutes ago, Team9nine said:

It should now be "turned over" and have the same characteristics. Two ways to tell. A temp probe is best to confirm, but seeing fish arches all the way down on the bottom is another. The 50 degree mark is a generalization. Different bodies of water have different temps below the thermocline. Whenever the temp is consistent throughout the entire water column, regardless of the exact temp, the lake has turned over.

 Thats what I thought but had people tell me its to early .  

[Team9nine]

5 hours ago, scaleface said:

 Thats what I thought but had people tell me its to early .  

By definition, once the thermocline is gone, there is no more stratification, which means mixing can occur throughout the entire water column. This really can vary based on temp under the thermocline at the bottom of the lake.

 

As an example, the temps right now at the bottom of our 5 largest reservoirs, which are still all thermoclined, are:

 

49 deg at 100'

67 deg at 58'

65 deg at 42'

61 deg at 50'

55 deg at 55'

 

Obviously, the lakes with bottom temps in the mid to upper 60s will be "turned over" at much higher surface temps than the ones in the upper 40s and low 50s, and likely at different times.

[senile1]

4 hours ago, scaleface said:

 Thats what I thought but had people tell me its to early .  

This time of year is typically too early for any of my lakes to turnover, but in the last week we have had many inches of cooler rain.  That might possibly equalize the temperature in some of the smaller lakes and result in turnover though my larger lakes probably still have some weeks to go.  I haven't been out to check in the bad weather this past week.  I assume some of that cold rain from this side of the state came your way?

[TnRiver46]

Ah turnover. Something I never have to worry about! Our water moves pretty quick 

[BassNJake]

As I have been reading other posts it seems as if I do not understand the turnover.

 

It was my understanding that 39 degree water is the most dense and will "sink" to the bottom causing the higher temperature water from the bottom to move towards the top.

This mixes the water and cools the warmer water as it sinks.

This creates a lot of discolored water, loosened debris rises to the surface and sometimes you can detect a sulfur type of smell and the fishing becomes extremely tough.

(although the general time of the season that turnover occurs can be difficult anyways)

I have always had the belief that when this "turnover" occurs the water will mix, therefore you will not see a significant difference between the temperatures at the surface layer to those below it.

 

Some of the other posts have suggested that near 50 degrees this will happen, someone said it happens in the summer and watching BASS brackets they mentioned that the turnover could be a reason for the lack of action.

 

Could someone with a better understanding of the science behind this explain it to me/us?

 

 

 

 

[J Francho]

Turnover can also be a localized event, such as in the case of very windy conditions on a large lake.

 

[BassNJake]

42 minutes ago, J Francho said:

Turnover can also be a localized event, such as in the case of very windy conditions on a large lake.

 

Is that really a turnover then?

Isn't that just water that has been mixed in that localized area?

 

So if there is a heavy rain and the water coming into the lake from a creek has dirtied it up, thats a turnover for that localized area?

 

With a wind that creates a mudline 5 feet out from shore, thats turnover?

 

I think this is going towards the flipping vs pitching discussions.

Where most people say they are flipping, when they are actually pitching.

But that is exactly why I am asking these questions so I can get some clarity on what is being called "turnover"

 

[WRB]

It's a fact that fresh water is most dense (heavy) until  it reaches 39.4 degrees, then it gets less dense (lighter weight)  until it freezes at 32 degrees. For this reason water freezes on the surface, not on the bottom of lake.

Climates that don't freeze or get colder then 50 degrees still create thermoclines and turn over. Not all lakes develop a thermocline do to current, aeration devices or wind mixing the water column. Thermocline is basically a 4 to 5 degree warm water temperature change to colder water within a few feet and can be anywhere from a few feet below the surface to 30' or 50' deep in some lakes. Cold fall air temps slowly cool the warm surface water and the colder cooled surface water sinks until the heavier cold water out weighs the deeper warmer water and turns over sinking to the bottom. The turn over creates a mixing of the water column top to bottom. This often bring bottom decayed matter to the surface and can smell like sulfer or rotten eggs for a few days. Bass being warm water fish are affected negatively from the sudden change in water temps, usually about 2 weeks to settle down.

Tom

[J Francho]

2 minutes ago, BassNJake said:

Is that really a turnover then?

According to what we were taught in college (many, many years ago, lol) yes. Anything that results in the disruption of a thermocline, and homogenizing the water temp would be a turnover event.  Water temps can fluctuate greatly due to weather events.  A north win, beating the south shore of Lake Ontario can result in the arrival of many small mouth at the outlets and mouths of small bays.  Warm water gets stacked up, and the commotion stirs up invertebrates, which brings bait fish, and likewise, predators.  I've used this to my advantage to place well in many tournaments on these bays, when the Lake was off limits due to weather.  The opposite can be true when a typical westerly wind (like shown in the picture) once it calms down, water temps are way down, due to the mixing, and fishing can be super tough, if not impossible due to all the debris in the water.  Like Tom noted, water doesn't have to 39° for a thermocline to develop, or collapse.  There just needs to be enough depth and stratification.

[BassNJake]

2 minutes ago, J Francho said:

According to what we were taught in college (many, many years ago, lol) yes. Anything that results in the disruption of a thermocline, and homogenizing the water temp would be a turnover event.  Water temps can fluctuate greatly due to weather events.  A north win, beating the south shore of Lake Ontario can result in the arrival of many small mouth at the outlets and mouths of small bays.  Warm water gets stacked up, and the commotion stirs up invertebrates, which brings bait fish, and likewise, predators.  I've used this to my advantage to place well in many tournaments on these bays, when the Lake was off limits due to weather.  The opposite can be true when a typical westerly wind (like shown in the picture) once it calms down, water temps are way down, due to the mixing, and fishing can be super tough, if not impossible due to all the debris in the water.  Like Tom noted, water doesn't have to 39° for a thermocline to develop, or collapse.  There just needs to be enough depth and stratification.

Ok, that makes a lot more sense now.

I am familiar with fishing the south shore of Erie when there is a north wind.

(I was actually given the nickname of north wind as everytime we tried to fish Erie, we had a north wind regarless of what the forecast called for)

 

I was thinking the turnover was a specific event that happened lake wide when the surface water temp reached 39 and then would begin the turnover event.

 

Thank you for the clarification.

[Team9nine]

2 hours ago, BassNJake said:

As I have been reading other posts it seems as if I do not understand the turnover.

 

It was my understanding that 39 degree water is the most dense and will "sink" to the bottom causing the higher temperature water from the bottom to move towards the top.

This mixes the water and cools the warmer water as it sinks.

This creates a lot of discolored water, loosened debris rises to the surface and sometimes you can detect a sulfur type of smell and the fishing becomes extremely tough.

(although the general time of the season that turnover occurs can be difficult anyways)

I have always had the belief that when this "turnover" occurs the water will mix, therefore you will not see a significant difference between the temperatures at the surface layer to those below it.

 

Some of the other posts have suggested that near 50 degrees this will happen, someone said it happens in the summer and watching BASS brackets they mentioned that the turnover could be a reason for the lack of action.

 

Could someone with a better understanding of the science behind this explain it to me/us?

 

 

 

You are getting into a bit of a semantics confusion based on how the term gets used. There is the process of turnover whereby the mixing is occurring and the thermocline is dropping, but technically, turnover hasn't completely happened until the water temps (and frequently oxygen content) are close to equalized from the surface all the way down to the bottom. Then turnover is complete.

 

The 50 degree mark is just a generalization. As I posted earlier in this thread, if you look at my local lakes, the water at the bottom of them ranges from 49 deg. to 67 deg. Once surface waters cool to match those temperatures at the bottom of the lake, regardless of what that exact temperature happens to be, that lake will have completed turning over. Some lakes turn faster than others, just like some sections of large lakes will turn faster than others. There is no one magic temperature.

 

While a lake is in the process of turning over, it is very unstable environment, and fishing typically suffers a bit. Once turnover is complete, the lake column stabilizes considerably, and fishing usually picks back up.

 

 

19 minutes ago, BassNJake said:

Is that really a turnover then?

Isn't that just water that has been mixed in that localized area?

 

So if there is a heavy rain and the water coming into the lake from a creek has dirtied it up, thats a turnover for that localized area?

 

With a wind that creates a mudline 5 feet out from shore, thats turnover?

 

I think this is going towards the flipping vs pitching discussions.

Where most people say they are flipping, when they are actually pitching.

But that is exactly why I am asking these questions so I can get some clarity on what is being called "turnover"

 

Looks like you just got your answer, but it is very common for big storm events that have high winds to temporarily (or permanently) turn an entire lake, or section of a lake, over due to the extreme mixing forces. Shallower sections of large lakes can be completely turned over while deeper main basins have a ways to go. This is usually correlated to depth, and specific wind direction can definitely play a part as @J Francho mentions.

[WRB]

Most fresh water lakes have 3 thermal layers. We generally fish the upper layer (epilimniom) down to the middle layer (metalimnion) we call the thermocline and the bottom layer (hypolimnion). 

I think we agree a turnover mixes the 3 layers into 1. I am not sure if the Great Lakes ever turn over do to size and depth, except in bays? The lakes I fish don't freeze and will turnover in the fall, but the deeper water doesn't turnover, just water 80' or less tends to turnover. There is a aeration system near the deepest water at the Casitas dam to mix the water column year around, but not at Castaic, the dam pulls water from the bottom and mixes it.

Lots of variables within each lake.

Tom

 

[J Francho]

15 hours ago, WRB said:

I am not sure if the Great Lakes ever turn over do to size and depth, except in bays?

They definitely do turnover.  Surface temps on Ontario can range from 75° down to frozen, though it never totally freezes over, like Erie.