Temperature And Depth

[Thad]

I thought this was pretty interesting. I always knew temperature changed as water depth increased but I never knew how much. This is a graph of a temperature reading on Smith Mountain Lake. For those that don't know it's a deep mountain lake in Virginia that is usually very clear.

[Siebert Outdoors]

Here is a dive profile from last weekend

Temperature: 82°F

Visibility: 24' - 30'

At 15 foot Depth:

Temperature: 82°F

Visibility: 24' - 35'

At 20 foot

Temperature: 72°F

Visibility: 20' - 30'

At 45 foot Depth:

Temperature: 51°F

Visibility: 25' - 35'

At 55 foot Depth

Temperature: 49°F

Visibility: 25' +

At 85 foot

Temperature: 42°F

Visibility: 25' +

At Bottom: 114ft

Temperature: 42°F

[Wayne P.]

That is quite common in the late Summer on lots of lakes with a low current flow. That is why the thermocline pattern works so well.

This was late last summer at a small lake in Va near Fredericksburg with a similar profile. No thermometer was needed to see the temperature changes.

[waskeyc]

So what does it look like beyond 100'?

I've often wondered if deep lakes (definition of deep unknown, but SML should meet the criteria) maintain a bottom temperature of 39F year round. The depth of SML in the main basin is over 200', and extrapolating the temperatures from the surface (~77F) to 100' (~58F) for another 100' would bring the temperature close to 39F. (Note that 39F is the most dense water gets, below that temperature it begins to expand again, which is why ice floats.) If so, then on a hot summer day, provided oxygen is present in the depths, a fish can find any temperature water it wants, from hot to 39F.

Also, at what surface temperature does fall turnover begin? Is it the temperature at or near the thermocline (~68F in the SML example)?

[WRB]

Within bass natural range the coldest the water will be near a lake bottom is about 60 degrees F, the same temperature as the earth surface that holds the water.

The surface water is affected directly by the air temperature, water is thermally conductive.

If the ambient water temperature is about 60F and surface water is warmer or colder, then the water column temperature will form layers; warmer less dense or lighter weight water suspended on colder more dense water. Wind or current tends to mix the water layers.

The mixing of water temperatures doesn't happen quickly, it's a gradual change unless severe weather occurs to speed up the process.

Lots of things are going on at the same time; longer days warm the surface water, aquatic organisms grow and die out, sinking to the lake bottom. The dissolve oxygen mixed into the water is a higher percentage in colder water and less in water above 80 degrees. As organic materials decay on the lake bottom, the process uses up the DO and drops below 3 mg/L and without current to mix the deep water, the deep water becomes void of most game fish.

Most lake will layer during the summer and create a thermocline where the water changes rapidly within a few feet. Bass preferring good DO levels and 65-75 degree F water tend to suspend just above the thermocline depth. Bass can easily move up and down within 35 feet, this becomes the water zone where most activity is during the summer.

The fall brings colder weather and shorter day light periods cooling the water surface every day. The fall turn over happens when more colder water out weighs the warmer water below it and the lake turns over. The turn over brings the bottom decaying material to the surface. Wind and cold weather mixes the water column.

The bass during the cold water seek the warmest water they can find.

Tom

[waskeyc]

Within bass natural range the coldest the water will be near a lake bottom is about 60 degrees F, the same temperature as the earth surface that holds the water.

Although at first glance I would suspect that the water in the depths should be near the underground mean temperature (~57F in VA I think), the temperature profile at SML shows the temp at 100' near 57F and still dropping steadily. Also, looking at the measurements from Missouri, the minimum temperature is 42F, which is getting quite close to 39F, and probably below the subterranean temperature.

Has anyone been able to measure the temperature all the way to the bottom in very deep lakes during the summer? What did you find?

Of course, if there isn't enough disolved oxygen below the thermocline, then it doesn't make much differences for the purposes of fishing, but some clear lakes with high water clarity may have adequate disolved oxygen, even in the depths.

[WRB]

Although at first glance I would suspect that the water in the depths should be near the underground mean temperature (~57F in VA I think), the temperature profile at SML shows the temp at 100' near 57F and still dropping steadily. Also, looking at the measurements from Missouri, the minimum temperature is 42F, which is getting quite close to 39F, and probably below the subterranean temperature.

Has anyone been able to measure the temperature all the way to the bottom in very deep lakes during the summer? What did you find?

Of course, if there isn't enough disolved oxygen below the thermocline, then it doesn't make much differences for the purposes of fishing, but some clear lakes with high water clarity may have adequate disolved oxygen, even in the depths.

You could measure the tail race water, or use temperature probe with a long enough wire.

It's possible for lakes that freeze or lakes with very cold water during the winter to have that cold water sink during ice out or late winter during the initial warming period. Water is most dense at 39.4 degrees F, so in theory 39 degree water could be trapped deep.

In Canada it is common for lake trout to hold at the 50 degree F thermocline during the summer on clear lakes usually about 100' with depths over 300 feet.

Lake Casitas where I bass fish most of the time develops 2 thermoclines; 35' and 60', the 35' is around 70 degrees, the 60' is about 60, both have good DO levels.

Casitas does't have any water drawn to generate power, so the lake layers tend to be well established.

[Siebert Outdoors]

waskeyc just to clarify that temp is from a spring fed quarry. Its that temp year round down there.

Down 100 ft plus from what I've seen WRB is right. There is nothing. I've never seen fish that deep. Everything is dead and all you see is structure and the bottom.