It's a bit on that cold side, but even though the days are a lot shorter and the waters whole lot colder, it is still the best time of year to spend a day in the mountains. This time of year you don't have to worry about mosquitoes and ticks. As far as the rattlesnakes go, if their out, they will be moving very slowly and not much of a danger at all. It is definitely a lot more comfortable hiking backup to the truck in 60° weather rather than 100° weather.
As long as the water stays low, its a great time a year to be working down on the creek or up in the high gold fields. It won't be long before the snow runs us out of the high gold fields and the rains run us up the bank to where we can only find flood gold.
When heading to the mountains and this time a year be sure you have several layers of clothing. The mornings can be very cold and the afternoons can be comfortably warm. If you take just a heavy warm jacket, you may be too warm in a morning and too cold and afternoon.
Even for a short hike, a trip to the mountains this time a year is well worth it just to see the mountains changing color. So pack a lunch and head to the hills.