When a person drowns, water takes the place of air in the lungs. The body becomes heavier and sinks. It stays there until enough gas builds up inside the body from bacterial decomposition to make it buoyant and free it from the suction that silt and mud creates on the river bed, so that it surfaces.
How long this takes depends on the depth and temperature of the water, the amount of sunlight the corpse receives, and whether it is lying under a ledge or a bridge.
Whether the body is lying on the north or south side of a waterway can also affect the time it takes to rise, because heating of the water varies on each bank.
Obviously, the body can take longer to rise if it is underneath something like a tree, or is caught by an underwater obstruction. I once studied a body that had risen to the surface with part of a brick wall tied to it by ropes around the chest. There were five bricks covered with mortar on each side of the body and still it floated.
What's more, if a body is trapped by, say, its leg, the gases will keep forming until it is buoyant enough to detach itself from the trapped leg and rise up.
Bodies can also be washed into the side of a river and remain undiscovered, below an overhanging tree, for example.
Debris piles up on top of the body and silt accumulates. If the body lies out of the Sun's rays, which cause much more rapid decomposition, it can remain there until a flood washes away the silt and debris.
Of course, if the water is cold enough, the dead body may be held in "deep freeze" and it might never rise.
Dead bodies undergo a series of changes, biological, chemical and physical, which eventually return the body's components to the food chain.
These changes and the speed of decomposition depend on where the body is. Decomposition in air is roughly twice as fast as it is underwater and about four times as fast as underground.
A living person has almost the same density as water: when you breath in, you float, and when you breath out, you sink.
This is why dead bodies sink initially.
Then the changes begin, and one of the most important is the bacterial decomposition of sugars and proteins in the tissues, mainly coliforms, Clostridium, Psedomonas and Proteus species.
These are anaerobic bacteria that normally live inside the large intestine, though they may come from an infected wound, such as gangrene.
As they feed on the decaying flesh, the bacteria excrete gases carbon dioxide and sulphur dioxide which inflate some body parts, mainly the face, abdomen and male genitals.
Bodies usually stay underwater for between one and two weeks, although this varies widely and some bodies never come up at all. One of the factors which affects this is the temperature of the water.
Contrary to common belief, there is no significant difference between the time it takes for male and female corpses to resurface. There is one notable difference though: while most corpses rise with spine uppermost, women and obese persons may rise face-up. This is caused by gas forming in breasts and large abdomens.
Published in The Hindu on Nov 1, 2001.