The Lake Sumac is a location found in Breath of the Wild.
The Lake can be found just northeast of Hateno Village and is adjacent to the Retsam Forest. At the south end of the lake there is a Raft that is connected with the farm that is run by Dantz and Koyin. The raft can be used to get to the small island near the center of the Lake that has a massive tree. If Link climbs the tree, he'll find a rock that has a Korok underneath it.
Though it is a fairly small lake, it is home to a variety of fish including Hyrule Bass, Armored Carp, Mighty Carp, and Staminoka Bass. Additionally, some Bright-Chested Ducks can be found swimming along the lake.
Just west of the Lake inside of the perimeter of the farm, there are two large trees at the north end of the farm. One of these trees has a secret treasure chest buried underneath it that contains a Silver Rupee. In order to find it, you'll have to befriend the Dog that is in the farm by feeding him. After doing so, he will guide you to the hidden treasure chest where Link can use Magnesis to pull it up.
Nearby Korok Seeds
Climb the tree in the middle of the lake and pick up the rock.