The lake itself is open 24/7 but certain spots like the marina and the swimming areas and the visitor center, the last being where you purchase day or longer passes, open 10 am and close 4 pm as refered to by another reviewer answering your question.
There are no gates so you can get access anytime. You will need to get a receipt from the Visitor Center to display on your windshield. Get the receipt as soon as the VC opens.
Open 10:00 a.m. Close 4:00 p.m.
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