In the early 1800s, when roads were poor or nonexistent, the postal service used riverboats to extend mail service into the expanding frontier, and in 1823, Congress designated all steamboat routes to be post roads. While riverboats and steamboats could often carry more mail than wagons and stagecoaches, they lacked the reliability of the stage services.
Ship captains found it hard to stick to the regular schedules demanded in postal contracts and occasionally diverted money from the postal coffers by carrying mail privately for a fee.
Postal funds were very important to the steamship companies seeking to make profits carrying passengers and mail along the east and west shores and linking the two coasts through Central America. The first mail contracts for this service began on October 1, 1848. But even the fastest steamships still took over a month to carry mail and passengers between the Atlantic and Pacific coasts.
Nancy A. Pope, National Postal Museum