http://www.dundee.ac.uk/forensicmedi...otes/water.pdf
Department of Forensic Medicine, University of Dundee
Lecture Notes
Bodies from Water
(There is a link under the title on the first page that will bring you to the section header "Effects of Immersion" so that you don't have to scroll to the last portion.)
EFFECTS OF IMMERSION
Sinking, putrefaction and refloating
A body in water will usually sink but because the specific gravity of a body is
very close to that of water then small variations e.g. air trapped in clothing have a considerable effect on buoyancy. Having sunk to the bottom the body will remain there until putrefactive gas formation decreases the specific gravity of the body and creates sufficient buoyancy to allow it to rise to the surface and float. Heavy clothing and weights attached to the body may delay but will not usually prevent the body rising. Putrefaction proceeds at a slower rate in water than in air, in sea water than in fresh water and in running water than in stagnant water.
The principal determinant is the temperature of the water so that
in deep very cold water e.g. the North American Great Lakes or the ocean the body may never resurface.
For the Thames, Simpson offers the following guidelines for resurfacing times:
June to August: 2 days; April, May, September and October: 3-5 days;
November, December: 10-14 days;
January, February; possibly no resurfacing.
At water temperatures persistently below 45°F there may be no appreciable
decomposition after several weeks.
Please read more at the above link.
BBM
ETA: I posted this information on one of the threads for DR a while back, although I cannot find the post in looking through my history. If I find the post I specifically wrote in regard to DR's case, I will add the link here... [Update: I did an advanced search, and I have numerous posts discussing the effect of the cold water temperatures on decomposition in the last 3 threads regarding this case. One with a link to the page I submitted above. I will not link to them all, but they will show up under a search for "decomposition", along with my user name, and then searching the threads themselves for those same values.
ETA II:
Please note that the current estimated average water temperature in Lake Vallecito is 37 degrees as reported on this website:
http://www.fishingnotes.com/lakeinfo.php?id=3115
This page includes a trending chart for the last week. I have not been able to find a long-term water temp. chart as of yet.
http://www.fishingnotes.com/laketrends.php?id=3115&v=3
I found a website that does keep historical records in regard to the water temperature, but it requires a premium subscription:
http://www.fishexplorer.com/co/lakedet.asp?lid=2143
ETA III: To tie into the information posted from the forensics page, I researched the average depth of the Great Lakes. I was surprised to find that with the exception of Lake Superior the average depths of the water are less than 300 ft. Of course, the maximum depths recorded are much deeper. Keep in mind that at its deepest part, near the dam, Vallecito Reservoir is approximately 270-280 feet (iirc).
The following is from:
http://www.epa.gov/greatlakes/factsheet.html
Average Water Depth (in feet):
Superior: 483
Michigan: 279
Huron: 195
Erie: 62
Ontario: 283