I have added shading and colors to more clearly demonstrate the process. The upper figure shows the river at the end of the 18th century. The lower figures shows the source of sediments (red arrows), the sedimentation of the river and formation of wetlands (green), and the reduced extent of navigable water. This process destroyed the port over the course of a century. As early as 1775, a visitor noted that only small craft could navigate up to Port Tobacco. These might have been lighters, delivering goods and carrying off local agricultural products to ships lying at anchor in the Potomac River.
Sadly, sedimentation continues to this day, although not as severe as in the 19th century. Nonetheless, sediments and sewage continue to find their way into this beautiful river. Join us in studying the problem from an historical perspective and consider joining the Port Tobacco River Conservancy (http://porttobaccoriver.org/) to help restore the Port Tobacco River. They are holding a fundraiser on Thursday, August 14, 7PM to 10PM, at the Port Tobacco Tiki Bar & Grill.