 Pennsylvania 's streams have frequently flooded . According to William H. Shank , the Native Americans of Pennsylvania warned white settlers that great floods occurred on the Delaware and Susquehanna rivers every 14 years . Shank tested this idea by <unk> the highest floods on record at key points throughout the state over a 200 @-@ year period and found that a major flood had occurred , on average , once every 25 years between 1784 and 1972 . Big floods recorded at Harrisburg , on the main stem of the Susquehanna about 120 miles ( 193 km ) miles downstream from Lock Haven , occurred in 1784 , 1865 , 1889 , 1894 , 1902 , 1936 , and 1972 . <unk> from the Williamsport stream gauge , 24 miles ( 39 km ) miles below Lock Haven on the West Branch of the Susquehanna , showed major flooding between 1889 and 1972 in the same years as the Harrisburg station ; in addition , a large flood occurred on the West Branch at Williamsport in 1946 . <unk> flood @-@ crest readings between 1847 and 1979 — based on data from the National Weather Service flood gauge at Lock Haven — show that flooding likely occurred in the city 19 times in 132 years . The biggest flood occurred on March 18 , 1936 , when the river crested at 32 @.@ 3 feet ( 9 @.@ 8 m ) , which was about 11 feet ( 3 @.@ 4 m ) above the flood stage of 21 feet ( 6 @.@ 4 m ) . 
