Tropical Storm Agnes in June of 1972 was the first and only time Indian Rock Dam has seen spillway flow. What that means is that the area behind the dam filled to its capacity - holding back 9.1 billion gallons of water from downstream communities - and excess water flowed past the dam through its concrete spillway. All elements functioned as designed. While Indian Rock Dam did not prevent all flooding in York, it did significantly reduce what could have been much worse flooding. Some have estimated that water could have been 13 feet higher than it was in York during Tropical Storm Agnes if Indian Rock Dam had not existed.