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September 2, 2020

Hurricane Isaias swept over eastern Pennsylvania August 4th and 5th, dumping upwards of 8 inches of rain and resulting in wide-spread flooding. New high flow records included a measured streamflow of 6,920 cubic feet per second in the East Branch Brandywine Creek below Downingtown, Pennsylvania, the highest since the gage was installed almost 50 years ago.


Photograph of USGS streamgage during flood stage
USGS Streamgage 01480870 East Branch Brandywine Creek below Downingtown, August 2020. (Matthew Gyves, Public domain.)

Staff from Pennsylvania Water Science Center offices in Downingtown, New Cumberland, and Williamsport were in the field measuring high water flows on rivers and streams, collecting water quality and suspended sediment samples, and documenting signs of flooding in places where waters had already peaked.

Over the 2-day period, 29 flood measurements were made by Pennsylvania Hydrologic Technicians and Hydrologists at 24 streamgages in the Delaware River Basin. These flood measurements help to define or extend the upper end of the stage-discharge relationship and increase streamflow measurement accuracy of these high flows. Record flood measurements were made at 7 streamgages, many of these being newer streamgages in southeastern Pennsylvania. Storm samples for documenting water quality and suspended sediment were collected at 8 sites. These samples help to determine how the storm’s runoff affected water quality.

Graph of streamflow at East Branch Brandywine Creek, Downingtown PA

Physical signs after flooding provide information that can confirm or correct other stream stage and flow rate data. Pennsylvania staff also flagged high water marks - telltale lines of seeds, leaves, grass blades and other debris left behind on buildings, bridges, other structures and even tree trunks as floodwaters recede. When high water marks are found, they are labeled, photographed, and surveyed. The field work is highly skilled and time-sensitive, because peak water flows can quickly pass, and high water marks can be obliterated by weather and by property owners’ cleanup efforts. These marks can be used to determine the extent and depth of flooding.




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