Cranberry Orange Sparkling Punch

1 pound fresh or frozen cranberries (4 cups; thawed if frozen)
5 cups water
2 1/2 cups sugar
3/4 cup fresh lemon juice
1/3 cup fresh orange juice

Seltzer

Boil cranberries, 2 cups water, and 1/2 cup sugar in a 5- to 6-quart heavy pot, stirring occasionally, until cranberries have burst, about 15 minutes.

Purée cranberry mixture in batches in a blender until as smooth as possible (use caution when blending hot liquids). Force through a medium-mesh sieve into a bowl, discarding solids.

Bring remaining 3 cups water and remaining 2 cups sugar to a boil in a 2-quart saucepan, stirring until sugar is dissolved, then remove from heat and cool syrup.

Stir together cranberry purée, sugar syrup, and citrus juices, and cool.  Mix with seltzer.