The Hudson River flows approximately 315 miles from its source in the Adirondack Mountains southward to New York Harbor. Its path has served as a historical artery for commerce and development, notably through its connection to the Erie Canal. The lower half of the river is not a traditional river but a long, saltwater-influenced estuary. Understanding what feeds this expansive system requires looking at the vast network of land and water that supplies its flow.
Defining the Drainage Basin
The Hudson River, and all the streams that flow into it, are contained within a vast geographic area called the drainage basin or watershed. The entire watershed encompasses approximately 13,400 square miles of land, extending far beyond the immediate river banks. While roughly 93% of the watershed lies within New York State, its boundaries stretch into neighboring states.
Small portions of the basin extend into Vermont, Massachusetts, Connecticut, and New Jersey, meaning rainfall in these distant areas contributes to the Hudson’s volume. The geographical limits of this basin are defined by high-elevation features like mountains and ridges, which act as natural dividers directing precipitation toward the Hudson system.
Hydrological Inputs
The fundamental source of all water flowing into the Hudson River system is atmospheric moisture, primarily precipitation. This includes rainfall and accumulated snowmelt, which is significant during the spring thaw in the northern reaches of the watershed. The average annual precipitation across the basin ranges between 40 and 48 inches, providing a consistent input of water.
Once this water reaches the ground, it follows two main pathways to the river: surface runoff and groundwater seepage. Surface runoff collects in small streams and creeks, which then feed larger tributaries, eventually merging with the main river. The geological structure of the region allows a portion of the precipitation to soak into the ground and slowly seep into the river channel over time. Groundwater provides a steady, year-round base flow that helps maintain the river’s level even during dry periods when surface runoff is minimal.
Major River Contributors
While hundreds of small creeks contribute to the Hudson, the river’s freshwater volume is dominated by a few major tributaries. The single most significant contributor of freshwater is the Mohawk River, which joins the Hudson just north of Albany near Cohoes. The Mohawk River Basin covers approximately 3,460 square miles, accounting for about 25% of the total drainage area.
The flow from the Mohawk River is substantial, accounting for over 40% of the total freshwater discharge measured at the Federal Dam in Troy, which marks the head of the estuary. The Mohawk’s average flow is roughly 5,670 cubic feet per second, dramatically increasing the Hudson’s volume as it transitions into its lower, tidal section.
Farther south, the Rondout Creek and the Wallkill River form the largest tributary basin entering the tidal Hudson below Troy. The Wallkill River, which flows north from New Jersey, merges with the Rondout Creek near Kingston. The combined system then empties into the Hudson Estuary at River Mile 91. This joint watershed, covering about 1,190 square miles, is a significant source of freshwater input in the middle section of the river.
Tidal Influence and Saltwater Flow
The lower 153 miles of the Hudson, from New York Harbor up to the Federal Dam at Troy, functions as a tidal estuary. The Atlantic Ocean exerts a powerful influence on the river’s flow and water chemistry, even far inland. Tidal action causes the water level to rise and fall twice daily, and the flow can reverse direction, moving north during the flood tide and south during the ebb tide.
The ocean’s influence introduces salt water, which is denser than freshwater, creating the salt wedge phenomenon. The heavier saltwater pushes its way up the river channel along the bottom, while the lighter freshwater flows out over the top. This results in a horizontal salinity gradient, with salt concentrations decreasing farther north.
The location of the salt front, where the water transitions from saline to fresh, shifts considerably depending on the season and freshwater inflow. During the spring, when snowmelt creates high freshwater discharge, the salt front is typically held far south, between Yonkers and the Tappan Zee. During periods of low flow in the late summer or drought conditions, the saltwater can push significantly farther north, sometimes reaching as far as Newburgh or Poughkeepsie.