I have no opinion on the subject, nor does this neccesarily apply to the Bow or southern Alberta streams, but thought I would add the following taken from this source. http://www.orvis.com/intro.asp?subject=364&bhcp=1
“Unlike fish responding to a hatch, trout can be eating terrestrials and you’ll never notice. One reason is that they might see a beetle only once or twice an hour, and the chances of you looking at exactly the right spot aren’t good. An even more important reason is that when trout eat low-floating terrestrial insects, there is hardly ever a splash. Sometimes you see a subtle ring on the water, sometimes a black snout poking above the surface, and sometimes you see a hopper just disappear into a hole in the water with no visible sign of a rise. Best places to try a terrestrial fly are where riffles deepen into a dark slot (especially near a deep bank but not necessarily), in concave impressions along a bank that form small bays, and along undercut banks, especially ones that flow through meadows.”
Other info:
http://chanriverguide.com/hoppers.htm
http://www.foxvalleytu.org/html/lets_talk_bugs_oct_05.htm
http://www.flyfishingdvd.com/Terrestrialinsects.html
http://www.westfly.com/feature/9908/feature_71.htm