Kaskaskia historic site lying underwater in the Mississippi River near Chester, Illinois. In 1703 French Jesuits founded a mission for the Kaskaskia Indians near the mouth of the Kaskaskia River. A French settlement grew up, and Fort Kaskaskia was built on a bluff overlooking the settlement. In 1763 the region passed to the British, and in 1778 it was captured for the United States by George Rogers Clark. Kaskaskia became the capital of Illinois Territory in 1809. It declined after 1820 when the capital was moved to Vandalia. Late in the 19th century the river changed its course and covered the old town site. Fort Kaskaskia, however, remained, and Fort Kaskaskia State Park was created in 1927.