I've gone fishing here at 12am and 3am I have never seen them lock the gate its mainly for a safety issue due to wild life and risk of drowning at night in the willamette. There is also a chance to get lost I have walked with my cousin along the river that we weren't in the park anymore and had to climb through the wild BlackBerry bushes.
According to the website it says 6:00 a.m. to 9:00 p.m. But the sign as you enter the park says open from dawn to dusk.
I believe it is 5am - dusk
When it's dark usually
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