The Susquehanna River is one of the longest rivers on the East Coast of the United States, flowing over 440 miles from New York through Pennsylvania and into Maryland, where it empties into the Chesapeake Bay. Near Northumberland, Pennsylvania, the river’s East and West Branches meet at a wide confluence, marking the point where its two main tributaries unite to form the main stem of the Susquehanna. This junction has long been a strategic and cultural focal point, both geographically and historically.
At river confluences, several distinct geologic and hydrodynamic features can be observed. Flow stagnation zones often form near the meeting point, where the velocity of incoming tributaries decreases sharply, which leads to localized sediment deposition. A shear layer typically develops between the two merging flows, which creates a mixing interface where water properties such as temperature, sediment load, and chemistry begin to mix. This boundary can cause turbulence, erosion of the riverbed, and the formation of scour holes or bar deposits downstream.
These processes are important because they influence long term river morphology, sediment budgets, and floodplain development. The way water and sediment interact at confluences can reveal upstream watershed conditions, sediment transport efficiency, and potential zones of erosion or deposition. These features can also influence how floodwaters behave since confluences may create hydraulic bottlenecks or zones of flow convergence that raise water levels and increase flood risk, especially during high discharge events.
It is also worth mentioning that confluences can be classified as symmetrical or asymmetrical based on the relative size and flow of the joining channels. In a symmetrical confluence (on the left in image), the two rivers are similar in width, discharge, and sediment load, resulting in a more balanced mixing zone and a central shear layer. In contrast, an asymmetrical confluence (on the right in image) occurs when one tributary dominates in size or flow, often causing the smaller stream to be deflected along the bank and creating uneven erosion and deposition patterns.
Logging Requirements:
- Are you able to see any features associated with river confluences (such as the shear layer/mixing interface)? If so, which branch do you think has a higher flow? Next estimate the width of the river. Would you classify this confluence as symmewtrical or assymmetrical?
- Looking downstream, you’ll see Packers Island near the confluence of the East and West Branches. How do you think the presence of the island affects the river’s flow patterns at this junction? Do you think flooding impacts here are greater or less than those experienced upstream or downstream? Explain.
- Upload a photo of either yourself or a personal object at the listed coordinates with the river in the background.