The Boulder Creek Path is part of an extensive paved bike path system in and around Boulder. It follows boulder creek all the way from the eastern reaches of Boulder to the base of Four-mile Canyon. After Eben G Fine park heading west, the path starts climbing alongside Boulder Canyon Rd. (119) and quickly turns to dirt.
The path eventually dead-ends at the edge of Boulder Canyon Rd. and you can then ride a short section of road to get to the Betasso Link Trail. In the summer, this provides a nice way to ride Betasso from town.
In the winter, the paved part of the path is maintained by the city and cleared of snow, but the dirt section is generally snow-covered. It's heavily trafficked by runners, hikers, and bikers, so it's usually hard-packed snow that's rideable with regular mountain bike tires. A fun way to get out and ride when all the other MTB trails are snowy and off-limits!