The interaction between the remnants of tropical depression Ivan and a frontal boundary in the upper Delaware River basin on September 18-19, 2004, produced 4 to more than 6 inches of rainfall over a 5-county area within a 24-hour period. Significant flooding occurred on the East Branch Delaware River and its tributaries, and the main stem of the Delaware River. The resultant flooding damaged more than 100 homes and displaced more than 1,000 people. All of the counties within the basin were declared Federal disaster areas, but flood damage in New York was most pronounced in Delaware, Orange, and Sullivan Counties. Flood damage totaled more than $10 million. Peak water-surface elevations at some study sites in the basin exceeded the 500-year flood elevation as documented in flood-insurance studies by the Federal Emergency Management Agency. Flood peaks at some long-term U.S. Geological Survey (USGS) streamflow-gaging stations were the highest ever recorded.