Floods along the three rivers in Pittsburgh are a seasonal occurence. Every year, at some point in time, the water levels along the Monongahela, Allegheny and Ohio Rivers creep towards flood level as a result of extended rainfall or melting snow. Occasionally the water level will reach the flood stage of twenty-five feet, causing minor flooding along the river banks and low-lying areas near The Point. This type of yearly flooding is common for cities like Pittsburgh that are situated along inland waterways. Then there are the larger floods that occur every decade or so that push the river levels to the major 100-year (20+ foot above normal) and catastrophic 500-year heights (30+ feet above normal). Major flooding occured during Hurricane Agnes on June 24, 1972. Eleven inches of rain over a three-day period caused river levels in Pittsburgh to rise almost twenty-one feet above the normal pool level of fifteen.

The Point is submerged under several feet of water as levels approached the decks of the three Sister Bridges. It was the worst flooding event in the city since 1942. Without the benefit of recently completed flood control measures, the severity of the flooding could have eclipsed the Saint Patrick's Day Flood of 1936 as the worst in Pittsburgh history.

At the height of the flooding, Pittsburgh's Point lies submerged under several feet of murky water. People in the Golden Triangle of downtown Pittsburgh watched with increasing apprehension as the flood waters crept up the Point and into the Fort Pitt Museum. Mayor Pete Flaherty asked businesses to release their employees so they could evacuate the downtown area. Around the city, steel flood doors over entrances to underground garages were bolted shut, low level vents were closed and truckloads of sandbags were shipped in. Everything that could be moved was relocated to higher floors. Pittsburghers prepared for the worst.

Water surrounded Three Rivers Stadium on June 24, 1972. The Agnes flood crested at 35.85 feet in downtown Pittsburgh, eleven feet above flood stage. The Point was submerged to the Portal Bridge and beyond. The Fort Pitt Museum was deluged with four feet of water. Homes and factories all along the river basins were damaged. The flood wave continued down the Ohio, heavily damaging McKees Rocks, Coraopolis and unprotected towns all along the upper Ohio River.

Water rose to four feet at the entrance to the Fort Pitt Museum. By Sunday, July 2, the rivers had returned to their banks, the summer sun began to dry things out, and Pittsburghers moved on to the tedious chore of digging out from under the accumulated mud and debris. Damages in the city were estimated at $45 million, but would have been much higher had it not been for the effective system of dams and reservoirs constructed by the United States Army Corps of Engineers. One such improvement was the Kinzua Dam, built along the upper Allegheny River in 1965. Kinzua was one of the Pittsburgh district's dams authorized by the federal government in 1938. During Hurricane Agnes, these dams stored immense water volumes. East Branch was more than full. Water was within three feet of the top of Kinzua Dam. Tygart Reservoir was 85% full and other dams had stored water to 90% of their capacity.

The Yellow plaque shows the June 1972 high water mark on the Kinzua Dam. Water levels rose over thirty-two feet. The deluge of rain falling over uncontrolled streams alone was enough to push the rivers in Pittsburgh above flood stage. Without the flood control measures, it is estimated that flood levels would have topped forty-seven feet or more in the city. The system of dams and reservoirs along the rivers prevented $850 million in damage in the state of Pennsylvania. The Kinzua Dam alone, built at a cost of $108 million, was responsible for $247 million of that savings. The sense of relief felt within the city of Pittsburgh did not go unacknowledged. The headline in the June 25 Pittsburgh Press read "The Engineers Were Right." Hurricane Agnes was responsible for 122 deaths and caused catastrophic damages, especially in eastern and central Pennsylvania. In Harrisburg, Governor Milton Shapp was forced to evacuate the executive mansion. Here in Pittsburgh, not a single person died as a result of what could have been the greatest flood of record. The next time Pittsburgh experienced flooding of this magnitude was during the winter of 1996 , when a blizzard, followed by rain and warm temperatures, caused the city to be inundated with a flood of ice and water reaching 34.6 feet. Once again, the Army Corps of Engineer's system of dams and reservoirs helped avert a potential catastrophe.