Not typically; it's assumed that if you say "lake," that's it's naturally formed. That's like saying "she's a non-robotic girl." How many robot girls are walking around? Probably very few. Probably................... But this is why you say "artificial lake." Since virtually every other lake is "natural," you will need to qualify this other lake with a descriptor, in this case "artificial." I suppose if you had been talking about artificial lakes, it would make sense if you said "natural lake" to ensure that people understood you were now talking about a natural lake, but that would only make sense in the context of that particular situation.