Approximately 10,000 years ago the last of 3 glacial ages finished gouging out the lake bed. Terminal and lateral moraines along with sheer granite walls contain the 8 square miles of surface area. Fremont Lake is approximately 12 miles long and a little over 1 mile wide. Average depth (click for depth map) is 269 feet with a depression of 625 feet mid lake west of the campground.

Because of the granite in the 76.5 square mile drainage and the granite lake bed, Fremont Lake has the 2nd lowest dissolved solids content of all U.S. lakes, making Fremont Lake one of the nation's cleanest.