The weather is perfect and the water should be as well.

Post here if you're going. Here is the ride <
map>. As you can see, the route has been altered to go around the road construction on Yuba Pass, taking hwy 89 north to Gold Lake south, back to hwy 49. Once on hwy 49 at Bassetts, just follow it west. The swim spot is marked by the California State Park and bridge at map waypoint C about 25 miles west of Downieville. With the added detour expect the distance to Downieville to be 100+ miles. Nevada City, however, isn't far from the swimming hole. Query the group for when and if they'd like to head into Nevada City for a late lunch, if for no other reason than so everyone has an idea of who is going where when. The typical ride route home is through Nevada City, fuel up there and take hwys 20 & 80 home.
Park in the motorcycle parking at the base of the parking lot or wherever your bikes will fit legally between parked cars. Parking citations have never been a problem, but be courteous to other motorists so they may exit without knocking your bike over.

There's typically a uniformed Park Ranger on site distributing water for a $1 donation.
I
will not be able to join the festivities for this ride, for I am out of town this weekend. Anyone who can follow a map can lead or share leading the group there.
Riding gear: some people take leathers, but I have always gone with just swim trunks on under my blue jeans, a good set of boots and padded textile jacket. There's usually someone who stays near our gear at water's edge.
I would discourage anything less.
The morning temps over Gold Lake will make for a cool enough ride. Take sun screen along and seek shade under the bridge to lessen your exposure to sunburn. UV is most intense until after 4 PM, affecting literally most of the day you're in the water.
Pace: C or anything below B. The pavement on hwy 49 is new and you're goal is swimming. Take the pace down a notch, enjoy the scenery, the water and company.