Devil's Lake confirms that there are rattlesnakes at the park, specifically the Timber Rattlesnake. They say the snakes tend to shy away from people so it's not often that people see them. They are usually not aggressive.
Timber Rattlers are a Protected Wild Animal in Wisconsin, so it is illegal to take or kill the animal unless there is an immediate life-threatening situation. There have been two verified deaths in Wisconsin from rattlesnake bites since 1900.
If you are bitten by a Timber rattler you are advised to call 911 and try to remain calm.