Is there such a thing as chance?

Well... right now, modern physics seem to imply there really is such a thing as chance.
"Something that happens unpredictably without discernible human intention or observable cause"? Radioactive decay seems to fit the bill here. As it is right now, we simply cannot predict when a nucleus will decay. We can calculate the probability that it will within a given time interval, but that's it.
Maybe this is just a result of our limited knowledge of the laws (and I use the word losely) of nature, but this only means we'll have to accept the concept of chance tentatively.
As long as the explanation of chance is the best one available for the phenomena we observe (in terms of accurately describing reality), it's the one we'll use.Phenomena on a quantum level are the only ones I know of that currently can be seen as the product of "true" chance. In all other cases I can think of chance is simply a model we use because it's convenient. Take your lake example for instance. We could, theoretically, sit down and calculate the motion of every single particle in this lake. It would however be an incredibly complex system (with a lot of initial values that would be extremely hard (or impossible) to find) that would be taxing even for our fastest computers to solve in any reasonable ammount of time.
Instead, we create a "fluid model" which makes some observations about the way large collections of particles behave on a macroscopic level, these observations are largely statistical, and thus use the concept of chance.
So no, your example doesn't really involve "true" chance, as we understand it right now, but chance is the best model to use, because it gives us a great degree of accuracy as well as a pretty short time for solving problems.Whether true chance really exists though... we really can't say, and the tentative nature of science tells us we never will.

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"tellement loin de ce monde..."

We carbon date (or date using radiometric methods in general), not by knowing exactly when a particular nucleus will decay, but by knowing how many nuclei will decay, on avarage, over a large ammount of time.ADDED BY EDIT:
Or, like JonF mentions, by using an avarage over a lot of particles, which actually makes more sense. This message has been edited by Maxwell's Demon, 01-11-2005 09:03 AM