Depends on what time of year. The Big T runs through town. There isn't a park but behind the Dairy Queen is a decent hole to play in. There is really only enough water though during run-off.
The Big T below in the canyon has some water, but it is all dam controlled and rarely ever has enough water to float down. Typically only after a large rain storm. Probably better off heading 25 minutes south to Lyons or an hour north up to the Poudre outside of Ft. Collins.