Boulder County is the most pro-environment area in Colorado and the most anti-mining in the state. Therefore the suggestion to head outside of the county to pan by 3xflyfisher is a good one. I am not sure what the fine currently is, but don't dip a pan anywhere in the county if you are not on private land with written permission.
In addition to the fine, equipment can/will be confiscated. From another site, "Make sure you STAY OUT of Boulder County unless you are on open public FEDERAL LAND, as even carrying a pan can get your pan confiscated. Their new ordinance does not allow for any type of prospecting. ALL PROSPECTING OR MINING OPERATIONS THAT ARE NOT ON FEDERAL LAND ARE FORBIDDEN IN BOULDER COUNTY. NOT EVEN PANNING IS ALLOWED. YOU RISK LOSING YOUR EQUIPMENT, ARREST, AND FINES IF YOU TRY IT. IF YOU PROSPECT IN BOULDER COUNTY MAKE SURE YOU ARE ON FEDERAL LAND THAT IS OPEN TO THE PUBLIC AND NOT SOMEONE'S ACTIVE CLAIM. I have not heard if these restrictions pertain to private property or to Patented claims, but if you have a simple mining claim, it could very well apply. Check with the county if you are in doubt so you are on top of their regulations."
That said, the Boulder Ranger District, west of Boulder, says you can pan there, but lots of restrictions including a possible NOI. It is easier to go to Clear Creek, the South Platte, etc. At least you can sluice there and you can even dredge parts of Clear Creek west of Golden.
