Rick...
I got a lot more info., too,on the sinking of ships in the ocean. A great deal of research information is available on this. These variables include ocean currents, the changes in salinity and solute concentration at various depths as well as great temperature changes, water pressure compressing object as they sink, thus increasing their mass/volume (density), release of air , wood and other materials of lower density as the ship sinks, etc,etc.
Even more information from the commercial fishing literature on the sink rates of nets and other gear in waters of different temperatures..........
Then I got to thinking that in the event that the angler was casting with warm air temperatures, and the sinking fly line landed in much colder water, that it would increase in density to a slight degree.....but probably not enough to make a real difference in its sink rate.
The sink rate, too, is partly determined by the tension of the line in the water against the current and against the resistance of the rod tip. That, however, wouldn't change appreciably with changes in water temperature, though, theoretically, colder/denser water would exert more force per unit area on the fly line.
Then there is the fact that that fly line is likely to become stiffer in colder water. Whether THAT makes any difference or not, I don't know.
An example of the use of tension to control the depth of a fly line, is the use of an intermediate line when the angler wants it to stay as close to the surface as possible. Here the trick is to maintain tension without interruption by making the STOP early using a high forward cast line plane and actually starting the retrieve before the line/leader/fly have hit the water. It's amazing how you can control the sink rate of even a full sink line by this method.
When one considers all the variables, it makes one reach for the asperin bottle !
We talked about dead-drifting a fly for salmon, and a method of actually making the fly swing in both directions across a river......or even swing downstream and then upstream. Check out pp 203-209 of Hugh Falkus', SPEYCASTING a new technique. (Chapter XIV, " Two way fishing".)
Gordy