There is no water in Castle Rock,but you are about 30-45 minutes from the So.Platte via Sedalia;Chutes run II[III],Foxton on theN. Fk. III low to IV-high,Waterton III+ to IV w/play and long season.It will take you 1/2 hour to get into Denver +15 minutes to get to play parks at Union,Confluence,and Golden. Apx. 2 hours to Pueblo play or the lower Arkansas;Parkdale III and Royal Gorge III-IV. If you're looking for more advanced stuff there are many creeks within an hour or two of Denver.
Highly recomended that you buy 'Colorado Rivers and Creeks II" can be hard to find. Go to the playparks or Waterton on a weekend you'll meet some boaters.