Lake Travis conditions 1/10/05

[archman]

Six days after observing 35+' vis, we dove the same site again and it had deteriorated to ~8-10'.

HID's have a much higher penetrating power than most other lights, so I'm assuming the reduced vis was particulate in nature and not from staining. Assuming no one "messy" was diving there earlier, that's odd for a deep lake bottom. Might be a runoff-induced bottom current doing that, but it's hard for divers to measure those properly, or even notice them. They're typically slow as molasses.

The riverbed is, of course, the natural feature that held the Colorado River prior to inundation by the creation of Mansfield dam. The bed is heavily silted, the actual composition is unknown to me as great care is used to not make contact with it. I have, however, inserted my hand/arm to extract 'treasure' and did not detect a distinct substrate 18-20" below the top of the silt layer.

That's what I thought. If it's that silty, it would be easy as pie for any minor "event" to resuspend that junk and it'll stay suspended for hours-days. That's what regularly happens at the outer edge of the pecan groves, at least at Windy Point. It would be interesting to find out if there are other density layers below the pecan grove. If so, they'll have different turbidity characteristics. As an aside, is there any good "treasure" down there?

Temps were 52-54F at >160'.

This would indicate a deeper layer(s). However accuracy on recreational thermometers is absurdly variable, so a same-dive shallower value from the pecan grove would be needed to compare. Too bad the Lake Travis thermal profiler doesn't go this deep.

Using absolutes such as 'always/never' to describe dynamic systems will usually turn out to be erroneous.

Ah, but ARE the deep water masses in Lake Travis dynamic? That's the question. And the fun.

 

[sdp]

Answer these questions, and I'll consult my Lake Travis voodoo cards!

1. Is the "riverbed" an actual bottom feature, and if so, what's it depth range. Or what's the depth difference between the edge of the pecan grove and where the riverbed starts.

2. Describe the sediment and structural features of the riverbed. If its rocky, are they heavily silted over. What about preserved tree stumps or other woody vegetation present.

3. Is clearer vis in the riverbed area seasonal, constant, or variable. If intermittent/seasonal, what times of year is it the clearest.

4. Any discernible temperature differences between the pecan grove and the riverbed.

5. Is the clarity change below the pecan grove at a fairly static depth.

6. What type of dive lights are routinely used by the folks who make these reports.

Man! This guy asks a lot of questions.

I suspect that the lake is in the process of turning over and viz will probably get worse before it gets better. I did a dive to 114' last summer at Windy point and had 25' viz at depth and didn't need to use a light. As far as being butt cold, (from an earlier post) that is pretty close to normal California diving temps. Go and dive the great lakes sometime in the winter, gives you a new appreciation for the cold. (or in the polar regions where cold takes on a whole new meaning. (28degree water in Antarctica))

My experience with lake systems is that temp/vis/and themoclines vary with the seasons. This is much more pronounced in northern lakes that freeze over, they usually go through a thermal inversion in the spring and the fall. Refered to as 'turning over' Viz is usually good very early or very late in the year just before or after the ice goes in or comes out. Spring is especially good before the plant life and algae bloom up. Late summer (August) is usually pretty poor viz, cause the water is heated up and starts to turn green from the algae.

Randy Cook
Grand Prairie, TX

 

[archman]

unfortunately...

Lake Travis and others that never ice over are incapable of spring turnover. The surface waters stay warmer or similar to the deeper layers. Most of the mixing would be coming from winds or runoff, which tend to both be stronger in winter/spring. That'll disrupt the surface layer stratifications, but as you get deeper and deeper the effects attenuate. Thus the deep water masses should be relatively unchanging in physical characteristics. This can be crudely measured by the presence of "permanent thermoclines", which aren't found in northern lakes (except super deep ones).

As far as I've heard, Travis maintains the same 'ol thermocline right around the pecan grove, and if cyklon's thermometer isn't off, there's at least another one further down. If those layers persist through to May, that would indicate a lack of turnover activity, and put a big question mark on why there's entrained particulates way down there.

If I didn't limit my max depth to 120 feet, this would be a great limnology project. Heck, I don't even like going below 80. That's only 20 feet deeper than the automated station takes measurements.

 

[Slipstrm]

I've routinely seen 40'+ of viz at 120'+ .
Ambient light was good enough that I never had to fire up my MLS.
Temps at that depth and sometimes deeper are also in the high 70's later in the year usually around August & September.
I dive mostly in the area around Mansfield Dam and off the Point north of the swim cove.

 

[cyklon_300]

to 170' were made during a period of no precipitation, so runoff isn't likely to be responsible for the deterioration in vis. And it's somewhat doubtful that particular area was visited by other divers in the interim period.

There was a noticeable increase in the amount of suspended particulates between dives. Three different Suunto Vypers recorded temps between 52 and 54F on the two dives.

IIRC, at that time, descending from the pecan grove, there was a thermocline at ~140'...

 

[archman]

Temps at that depth and sometimes deeper are also in the high 70's later in the year usually around August & September.

Really? That's pretty dang weird. The few "dam" dives I've done (for September) registered steep temperature drops well below the 70's. High temps deep down don't make much sense... there's no good physical explanantion for it.

Now that little cove will warm up quite a bit in relation to the main lake water, even at depth. There's a term for this phenomenon in lakes, I can picture the diagram in my mind but not the name or description. "Thermal wedge" or something.

How are you ultra-deep folks reading vis between 100-120' (outer edge of pecan grove)? These are the "deep" areas for the vast bulk of divers, and where my "library" of poor clarity values comes from. I've got only a handful of 10'+ reports among a few dozen recreational divers' logs over the last several years. I don't think anyone had a hi-powered light, or could differentiate clarity between particulates/staining/lighting problems. Few divers can, of course.

 

[carlislere]

I dove the dam to 125 in October and we had a good 15 foot visability with temps in the low 70s. I also dove the cove off the nearside point to 126 earlier in October and it was about the same vis and temp.

 

[Slipstrm]

We were actually quite far out of the Cove into the river channel with high temps and great viz.
I have at times noticed steep temperature variations around the Dam.
I am using a Uwatec Smart Pro and a DR Nitek for temps and they were within a degree or two of each other.

 

[archman]

Where are the intakes for the dam located? Are they low down?

 

[Slipstrm]

They are at 130'