As long as you have not caught and kept the daily limit for the water you are currently fishing, you are legal. You can catch 2 in a river with a limit of 2 and then find a river with a limit of 4 and keep 2 more. Just don't do it in reverse.

This is not a perfect example, but it comes straight out of the regs and can be carried over to your question. It reads: "The combined daily limit for an angler that fishes in both rivers and lakes on the same day is 5 TROUT. In the combined daily limit only 2 TROUT may be from rivers."
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Mike Gilchrist