Bow Falls is a wide but relatively short waterfall (<9 meters) which, because of it's location, is often visited. When the Bow River is filled with spring thaw it really roars, but in the drier summer months it's flow is significantly reduced and the falls are less impressive.
Bow Falls is located less than a kilometer south of the Banff townsite, near the Banff Springs Hotel. You can access the falls from a parking lot immediately downriver from them or from Banff by following a trail adjoining the Bow River (there are trails on either side, but the best views are from the west side). If you continue further downriver, past Bow Falls, you'll quickly hit the junction of the Spray River as it enters the Bow.
The falls were feature's some old movies you might recognize including Marilyn Monroe's River of No Return and Robert Mitchum's Son of Lassie.