Sunsets like these are fairly common at the North San Gabriel Dam that forms Lake Georgetown.  You can approach the walk across the dam from either side – either start from the Cedar Breaks Park area or from the Lake Overlook Park side of the dam.  The walk is mostly on an old two lane road that is now closed to traffic.  You will have a nice vantage point of the lake, the San Gabriel river below the dam, and the city of Georgetown off into the distance.  There are also plenty of photo opportunities here with birds, flowers, deer, and other cool stuff.  If you have the chance to visit, I hope you enjoy the walk and the sights.  🙂