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A modern Depression wouldn't be your grandparents' Depression for the problem of resource depletion alone. I don't think you can overestimate how important cheap, available oil is to the economy. Natural Gas (which is in worse shape than oil) is another huge problem (as much for its use in agriculture as for people's dependence on it for heat).
I don't know how bad the economy will get. It could bounce back, conceivably, riding some new wave of consumer confidence. I think blind faith that it will always come around is misplaced, however.