The tribe closed the Olympic Peninsula lake last month to swimming and other non-tribal activities, citing pollution problems, unsafe activities, invasive species issues and diminished fish runs.
"You have tourists and you have the general public come to an area where they have no idea where these high pollutants are, and they could go out and swim right in the middle of a bunch of E Coli," said tribe president Fawn Sharp.
As late as Tuesday afternoon, swimmers were arriving at the lake only to find signs saying they weren't allowed in.
"We drove all the way up here and take the little ones wading," said visitor Justin Calderwood. "And yes, it's a long drive to look at an empty lake."
Non-tribal boating and other recreational activities remain banned at the tribe-owned lake.
Sharp said Tuesday in a statement that the tribe intends "to manage our sacred lake in a way that preserves its natural integrity for generations to come."
Sharp has said tribal leaders are concerned that leaky septic tanks owned by non-tribal residents in the area may have caused untreated sewage to get into the lake. She also says boats from outside the area can bring invasive mussel species, which she calls "lake killers."