As many know, I too am from Colorado and have had 
a ton of exposure to man-made whitewater parks. 
As far as negative side effects the land around
the park must be taken care of as many boating
"posers" are not as environmentally conscious and 
tend to leave a lot of trash around.  However,
most boaters are more careful than this and take
very good care of the land they use.  As far as
positive effects, there are many... economic
being the best evidence to use when proposing a
park to a city board.  Whitewater parks also
provide more diverse forms of recreation for
citizens in the area and can even add to the
beauty or draw of a certain stretch of river.  As 
far as I am concerned, build it and they will
come.  Jason, if you need any help planning or
anything, let me know.  I will help in any way I
can.  Brian