One of the most toxic pants in North America also really pretty. It's not what you want to see along the ditch that runs through your hay field, though. Water hemlock, cicuta, (c. maculata, above) is a member of the carrot family (Apiaceae). The white clusters of flowers look similar to Queen Anne's lace and cow parsnip. It grows in wet meadows, and along streams and lakes.
Queen Anne's Lace (Daucus carota) on the left, and water hemlock on the right.
Stem and leaves. The stem is hollow and smooth. For as poisonous as it is, water hemlock is surprisingly common in Colorado. For more info, a great resource is Field Guide to Plants Poisonous to Livestock : Western U.S. by Shirley Weathers, available from amazon.