Complex sentence: Although the U.S. Congress authorized the National Road in 1806, that bridge was not completed--although the road formally reached Wheeling on August 1,1818, and in 1820 Congress authorized the road's extension to St. Louis, Missouri.
Split into simpler sentences: Although the U.S. Congress authorized the National Road in 1806, that bridge was not completed. In 1820 Congress authorized the National Road's extension to St. Louis, Missouri.