The Susquehanna River normally supplies half of the freshwater that flows into the Chesapeake, but in August, it accounted for less than a quarter of the water in the Bay, according to the U.S. Geological Survey.
The lack of rainfall in the Susquehanna basin caused the river flow to fall to 3.6 billion gallons a day, less than half of the normal 8.2 bgd for the month.
As a result, the Susquehanna accounted for only about 23 percent of the freshwater entering the Bay.
The Potomac flow surpassed the Susquehanna for the month, contributing about 33 percent of total flow entering the Chesapeake Bay.
Normally, the Potomac accounts for only a quarter of the freshwater flow into the Bay.
The USGS said the switch reflected a combination of factors. Most of Pennsylvania has near-drought conditions, and the state Department of Environmental Protection issued a drought watch in August.
At the same time, streamflow has been near, or above, average in much of the rest of the Chesapeake watershed. In August, the USGS reported that Potomac flows were 42 percent above average for the month.
Because of the low Susquehanna flows, the overall freshwater flow into the Bay was 15.4 billion gallons a day in August, 22 percent below average.
Normally, high flows result in reduced water quality in the Bay because they wash in large amounts of sediment, which clouds the water, and nutrients, which spur algae blooms.
Extreme low flows also create problems, such as allowing saltwater to reach areas of the upper Bay and tidal tributaries that are normally dominated by freshwater, which can affect fish, underwater grasses and other species that inhabit those areas.