The speed of the Hudson River is highly variable because it is a tidal estuary, meaning its flow is influenced more by the Atlantic Ocean's tides than by simple gravity. In the "Lower Hudson" (from New York City to Troy), the river actually flows in two directions: south toward the ocean and north toward Albany, depending on whether the tide is ebbing or flooding. During a peak ebb tide, the current can move at approximately 2 to 3 knots (about 2.3 to 3.5 mph). However, the "freshwater" flow from the north is much slower, often averaging only 0.5 to 1 mph. During periods of heavy rain or spring snowmelt (known as "freshets"), the southward flow can speed up significantly. Interestingly, because of these competing forces, a piece of driftwood dropped in the river near Albany might take weeks to actually reach the ocean, as it is pushed back and forth by the tides twice every day, a phenomenon native peoples described as "the river that flows both ways."