In a small saucepan, combine 1 cup water and the sugar. Warm over medium heat, stirring until the sugar fully dissolves to make a simple syrup. Remove from heat and let cool slightly.

In a blender, combine the strawberries, raspberries, acai powder, and 1/2 cup water. Blend until smooth.

Strain the berry mixture through a fine-mesh sieve into a pitcher if you prefer a smoother lemonade, pressing with a spoon to extract as much liquid as possible.

Add the lemon juice, cooled simple syrup, remaining 3 cups cold water, and ice cubes to the pitcher. Stir well to combine.

Taste and adjust sweetness or tartness if needed by adding a little more sugar or lemon juice.

Serve over ice and garnish with lemon slices and fresh berries if desired.