Idaho Historical Marker: Arrowrock Dam LbNA #55436

I am doing a series of boxes about Idaho's Roadside Historical Markers.
This box is located in the pulloff before the Bridge on Highway 21 at Spring Shores Road.
From Boise, take highway 21 towards Lucky Peak Resevoir. travel for 12.75 miles.
From out of town, Take Gowen Road Exit #57 off Interstate 84 to Idaho City. Follow this road towards Lucky Peak Resevoir for 12.75 miles.
At the pulloff before the bridge, check the sign about Arrowrock Dam.
To find the letterbox, cross the concrete barrier. The letterbox is under the stone at the base of the post holding the sign.
Please use stealth and rehide well. This is a high traffic area.