Identification: the side veins of leaves end at the base of leaf points, instead of point tips. Use this feature to distinguish it from similar-looking, non-toxic native wildflowers.

Tall growing plant, dark green leaves and clusters of white flowers in late July.

I have a (morbid I suppose) fascination with poisonous plants, I'm lucky(?) to have this plant grow in abundance where we live. It's common to see this growing along streams and ponds in southern Saskatchewan.

Here are some interesting quotes from wildflower reference books regarding this plant:

"Warning: all parts of water hemlock (especially the rhizomes) are extremely poisonous and   can cause sickness and death if they are eaten. ... One rhizome is enough to kill a cow, and a piece the size of a marble will kill a human. Even children making pea-shooters from the hollow stems have been poisoned."  (1)

"The thickened stem base, when cut lengthwise, clearly reveals... an evil-looking, orange-yellow resin."  (2)

"Water hemlock is the most toxic plant in North America."  (3)

"Also known as:   beaver poison,   children's bane,   poison parsnip."  (4)

Maureen has put up with me introducing all sorts of odd-looking, and off-beat plants into our garden, but she drew the line with Water Hemlock. No plans to grow this in our garden.

Took the above photos from a large patch of a dozen or so plants growing along the pedestrian / bike path which runs through our city.

Not surprisingly, seeds of this plant are listed in the Canadian government's 'Prohibited Noxious Weed Seeds' list.  

Large plants, height of 3 feet up to 5 feet, spread of up to 24 inches.

(1), (2) from the book Plants of the Western Boreal Forest & Aspen Parkland, authors Johnson, Kershaw, MacKinnon, and Pojar.
(3), (4) from the book Weeds of Canada and the Northern United States authors Royer and Dickinson.