Iris pseudacorus (yellow flag iris) and Iris ensata (Japanese water iris)
These two species of water iris are very similar in some ways. They both like wet, boggy environments and neither plant is native to Canada's west coast. However, if you put 'yellow flag iris' into a Google Search the first place that pops up is the Invasive Species Council of British Columbia website.
This plant is native to the Mediterranean area and the Middle East. It was brought to North America as a garden ornamental but is now 'naturalized' all over the place. If you drain the pond and bring in the bulldozers, it won't make any difference. If a tiny piece of the rhizome breaks off, the plant will completely reestablish itself in a few years.
This purple Japanese water iris, on the other hand, is a real thoroughbred, with no fewer than seventeen varieties listed in the prestigious, Royal Horticultural Society's, Award of Garden Merit. Back at the Invasive Species Council, Japanese water iris is actually recommended as a noninvasive alternative to yellow flag if you're planting a water garden. That's not my worry. I live on a standard city lot and I simply don't have the space for no fancy 'cement' pond.