Here’s the Arvada Municipal Code for their park system. It does say no swimming or wading except where posted. https://library.municode.com/co/arvada/codes/code_of_ordinances?nodeId=PTIICOOR_CH66PARE_ARTIINGE

I suppose some individuals would think that bans prospecting?? Anyway, read it for yourself and decide. Just remember, in the US everything is allowed except that which is forbidden. I don’t have to prove something is legal in my view, the city has to show what makes it illegal. Hope this helps.

All that said, I strongly urge you to stay out of manicured parks and limit your digging to undeveloped open space greenways. That is less likely to make city staff nervous. Obviously you should also clean up after yourself so it is hard to tell you were there!