VERY good question. I was just discussing this with someone earlier today.
My advice then as it is now - take the dog in for a blood panel and liver function this week. There are some toxins that present with initial symptoms, often of vomiting, lethargy, excessive water intake, drooling or any number of other behaviors that raise a sense of urgency - and then, in a day or two, seem to resolve. Owners think everything is fine - the dog acts perfectly normal. Then, anywhere from a week to even a couple months later - symptoms present that don't alleviate and by the time the diagnosis is made, there's liver damage or even failure.
I'm not at all saying this is what is happening with your companion and I really don't want to scare you, but I truly believe in erring on the side of caution.
If your pet was exposed to anything toxic or a bad bacteria, etc., I'd expect much more reaction than what you describe (ie: vomiting probably the most common).
Since you didn't see any algae ( pond scum) - it's another good sign. Also, as long as the water was moving and not stagnant, another very, very good sign.
I do not know of any dogs that have succumbed to lake water immediately without a very public announcement on t.v., radio and postings .
Just watch her for now and keep the vet's number handy in case of complications. Without symptoms, call to schedule an appointment on Monday for this upcoming week.
Is she resting comfortably?