Figure 1. Major drainage basins in Pennsylvania and location of Cumberland, York, Dauphin, and Lancaster Counties. (Symbol designates location of U.S. Geological Survey streamflow-gaging station, number is site number in table 1.)
The floods produced by the rainfall and rapid meltdown exacted a heavy toll in Pennsylvania. Twenty lives were lost and many waterfront communities were so severely flooded that evacuations and curfews were ordered by local authorities. Sixty-nine bridges maintained by either the Pennsylvania Department of Transportation or by municipalities were destroyed or closed until inspections could verify their safety. By January 23, all 67 counties in Pennsylvania had been declared Federal disaster areas eligible for emergency relief. Preliminary estimates placed the economic loss in Pennsylvania, resulting from the blizzard and floods, at more than $1 billion.
The major drainage basins in Pennsylvania---the Ohio, Susquehanna, and Delaware (fig. 1)---all experienced major flooding during January 1996. Flooding was particularly severe in parts of the Susquehanna and Ohio River Basins. A comparison of peak stages and discharges for the January 1996 floods with peak stages and discharges for the maximum flood previously known, for key USGS streamflow-gaging stations across the Commonwealth, is presented in table 1.
Table 1. Peak stages and discharges during January 1996 and previously known for key U.S. Geological Survey streamflow-gaging stations in Pennsylvania -------------------------------------------------------------------------------------------------------------------------------------------------------------------- Site Drainage Maximum during January 1996 Maximum flood previously known number USGS station number and name area Beginning Discharge on map (square year of Date Stage (cubic feet Date Stage (cubic feet miles) record (feet) per second) (feet) per second) -------------------------------------------------------------------------------------------------------------------------------------------------------------------- 1 03079000 Casselman R. at Markleton 382 1920 1/19/96 13.06 40,500 10/15/54 14.06 50,000 2 03041000 L. Conemaugh R. at E. Conemaugh 183 1939 1/19/96 19.78 Currently unknown 7/20/77 18.85 40,000 3 03034500 L. Mahoning Cr. at McCormick 87.4 1939 1/19/96 13.68 6,790 6/23/72 13.20 6,200 4 03028000 W. Br. Clarion R. at Wilcox 63.0 1953 1/19/96 10.23 5,590 9/28/67 10.01 5,490 5 01564500 Aughwick Creek near Three Springs 205 1938 1/19/96 20.71 Currently unknown 6/22/72 19.20 23,700 6 01567000 Juniata R. at Newport 3,354 1899 1/20/96 24.69 103,000 3/19/36 34.24 190,000 7 01570500 Susquehanna R. at Harrisburg 24,100 1890 1/21/96 25.08 569,000 6/24/72 32.57 1,020,000 8 01551500 W. Br. Susquehanna R. at Williamsport 5,682 1895 1/20/96 26.71 180,000 6/23/72 34.75 279,000 9 01550000 Lycoming Creek near Trout Run 173 1913 1/19/96 22.6 Currently unknown 6/22/72 20.19 25,900 10 01532000 Towanda Creek near Monroeton 215 1914 1/19/96 20.71 Currently unknown 6/22/72 16.90 74,000 11 01534000 Tunkhannock Creek near Tunkhannock 383 1914 1/19/96 19.97 Currently unknown 3/15/86 15.77 32,200 12 01447720 Tobyhanna Creek near Blakeslee 118 1961 1/19/96 11.61 8,420 9/27/85 12.33 9,190 13 01453000 Lehigh R. at Bethlehem 1,279 1941 1/20/96 16.88 43,300 5/23/42 25.90 92,000 14 01474500 Schuylkill R. at Philadelphia 1,893 1931 1/19/96 13.36 76,800 6/23/72 14.65 103,000 15 01463500 Delaware River at Trenton, N.J. 6,780 1913 1/20/96 22.20 179,000 8/20/55 28.60 329,000 -------------------------------------------------------------------------------------------------------------------------------------------------------------------
The hydrograph, or graph of the water-surface height as a function of time, for the Susquehanna River at City Island in Harrisburg (fig. 2) shows two rapid water-level rises on January 19-20. These rapid rises resulted from the water-damming effects of ice jams downstream from City Island. On January 19, the first rise, about 16 feet in 15 hours, was followed by a 2-foot drop in water level and then by a subsequent 5-foot rise. This rate of rise is the greatest recorded for the Susquehanna River at Harrisburg since record keeping began there in 1890. On January 20, concurrent with the second peak, two spans of the Walnut Street bridge, which dates to 1888, were lifted off their piers by ice and carried downstream into the Market Street bridge. Over a 27-hour span from January 19 to January 20, 1996, floodwaters from the Susquehanna River inundated parts of Cumberland, York, Dauphin, and Lancaster Counties (fig. 1). Many communities on tributaries to the Susquehanna River also were flooded as a result of ice jams in the tributaries and in the Susquehanna River.
Figure 2. Comparison of river stages at U.S. Geological Survey streamflow-gaging station on Susquehanna River at Harrisburg, Pa., June 21-27, 1972, and January 19-23, 1996. Rapid rises on January 19 and 20 caused by downstream ice jams.
In the Lycoming Creek Basin of north-central Pennsylvania, the USGS streamflow-gaging station near Trout Run recorded a flood stage of approximately 22.6 feet, which is more than 2 feet higher than the flood stage during tropical storm Agnes. Along Lycoming Creek, the flooding claimed six lives and caused millions of dollars in damage.
Figure 3. Pathways for transmission, reception, and distribution of near real-time hydrologic data.
During normal streamflow conditions, the stations transmit stream-stage data every 4 hours; during floods, the data are transmitted on average every 15 minutes. As part of the Flood Forecast and Warning System, the National Weather Service uses the data from 38 stations in Pennsylvania to predict flood heights and timing in the Susquehanna River Basin. The Corps of Engineers uses the data to manage flood-control reservoirs. The Pennsylvania Emergency Management Agency and many county emergency-management agencies use the data to initiate evacuations and manage emergency response. Like other man-made structures located near streams, the USGS streamflow-gaging stations are susceptible to flood damage. All the stations in Pennsylvania, however, with the exception of two that were completely inundated, operated throughout the January 1996 floods and thus provided data critical to monitoring flood conditions, issuing flood forecasts, and managing the emergency conditions resulting from the floods.
--R.E. Thompson, Jr.
from U.S. Department of the Interior, U.S. Geological Survey, Fact Sheet FS-103-96