Yellow Flag, an introduced and possibly invasive species, has been added to the USWildflowers database (11/27/2012.) Scientific name is Iris pseudacorus.  It  is also known by the common names Paleyellow Iris, Yellow Iris, and Water Flag.  Photo below was taken along an irrigation canal in Boise, Ada County, ID, on June 8, 2010.   Go to the Yellow Flag detail page for more photos and information.

Yellow Flag Iris, Paleyellow Iris, Yellow Iris, Water Flag – Iris pseudacorus