In the winter, the air temperature is typically below freezing with frequent snow and ice storms. Water temperatures can drop from the 50s to the 30s by December.

The Delaware River undergoes extreme temperature shifts throughout the year, with winter temperatures often dropping to near or below freezing (32°F / 0°C). In the Upper Delaware regions, it is common for large sections of the river to freeze over completely or develop "frazil ice" by late December and January. For anyone planning to be on the water, the National Park Service notes that water temperatures below 50°F (10°C) are extremely hazardous, as they can cause "cold shock" and hypothermia within minutes if a person falls in. Because of this, life jackets are legally required to be worn by all boaters from November 1st through April 1st. In contrast, by late July and August, the river can warm up significantly, reaching comfortable swimming temperatures of 70°F to 80°F (21°C–27°C). This dramatic range makes the river a versatile recreation spot, but it requires seasonal awareness—the same water that is perfect for tubing in August can be life-threateningly cold just four months later.