East Beach County Park, Mystery Bay is a saltwater beach in Jefferson County, Washington. Across 55 bacteria samples collected between 2022 and 2023 at nearby monitoring stations, it earns a Beach Safety Grade of A — Excellent.
About 2% of samples at East Beach County Park, Mystery Bay exceeded the EPA Beach Action Value of 104 CFU/100 mL for Enterococcus — the threshold at which water managers typically post a swimming advisory. In practice that means the safe-swimming limit was rarely crossed here over the monitored period. Bacteria samples almost never exceed the safe-swimming threshold.
Read this grade as directional rather than settled: it rests on only 55 samples and has no results reported since 2023. That is enough to classify how the water has behaved, but not enough to separate this beach from another a grade away — a beach graded from thousands of samples at several stations is a firmer read. As more results are reported, the grade may shift.
East Beach County Park, Mystery Bay was not under a swimming advisory in the most recent EPA snapshot. Conditions can change quickly after heavy rain or high runoff, so it's still worth checking the current advisory the day you go.