I do know of people who have fished up the river, above the lake boundary, caught and kept fish that would be legal there (under 9 inch crappie specifically) and then run back down below the lake boundary to the boat ramp, quite a ways below, and then got checked at the ramp and issued a ticket for under under length fish. Once you cross back into "impounded waters" whatever you have must meet those rules, period. I do think that marking, with a sign the lake boundary is a good thing, however it is difficult on a lake that can fluctuate several feet. Easy to identify locations like bridges are ideal, but seldom available in anywhere near the correct locations. Signs placed high enough not to go under water are difficult to install and maintain. Most agents are pretty flexible unless the fishermen are in blatant violation.