It looks like purple loosestrife.
It was brought here with settlers. It is considered an invasive weed. It
is known to choke out other native plants. Personally I love it for a
garden but I realize there could be pitfalls if one plant species takes
over a wetland area. Some communities introduce beetles that eat the
loosestrife to control it.

Here's some links about the plant

http://www.npwrc.usgs.gov/resource/1999/loosstrf/loosstrf.htm
http://www.msue.msu.edu/seagrant/pp/
http://www.seagrant.umn.edu/exotics/purple.html

jayinytown wrote:
> These plants with purple flowers have been seen along Lake Glacier
> and many other shoreline areas of Mill Creek Park. Click on the
> photo file link below to view the picture.
> http://groups.yahoo.com/group/millcreekpark/files/lgplant.JPG
> I've been told that the plant is not native to the watershed and
> will eventually take over the entire shoreline. Is this true?
>
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