USGS refines long-term flow estimates for Chesapeake
A new look at old data has revealed a new low for the Chesapeake Bay — for waterflow.
Until recently, the lowest known annual streamflow into the Bay occurred in 1966, during a multiyear drought that spread across the watershed.
But a new analysis of older streamflow information by the U.S. Geological Survey has revealed that the amount of freshwater entering the Chesapeake in 1941 was about 10 percent less than that of 1966.
That discovery came as the USGS recently pieced together a new, longer-term look at the range of freshwater flows that have entered the Chesapeake over the past 64 years.
The review is important because the Bay’s water quality is closely tied to the amount of freshwater entering from its tributaries, which is highly variable. But water quality goals, such as nutrient and sediment reductions, are based on “average” flows.
Over the past 64 years, the USGS found that the “average” flow into the Bay was 78,695 cubic feet of water per second. But no year had that flow; the closest was 1961, when flows averaged 77,976 cfs.
So the USGS developed a statistical range for “normal” flows into the Bay, which stretches from about 64,000 cfs to 89,000 cfs.
The statistical approach, used by USGS nationwide to show hydrologic variation, involves ranking all annual streamflow data and dividing in into four “quartiles.” The highest 25 percent of the flow years are classified as “high flow” years, while the lowest 25 percent are considered “low flow” years. The middle range is classified as “normal” and more accurately represents the range of normal conditions than a single figure.
In the past 64 years, river flows into the Bay have been in the normal range 35 times, most recently last year, when the average of 64,900 barely registered as normal. During that period, there have been 14 “high flow” years that fell above that range, and 15 “low flow” years.
“This is a more qualitative measure of what is normal,” said Scott Phillips, Chesapeake Bay coordinator for the USGS. “In the past, people would say it was high flow year or a low flow year, but there really was never a way of quantifying what those are.”
More realistic assumptions about the normal range of flows can help guide management decisions, such as making more realistic assumptions about the amount of nutrients likely to enter the Bay, and the likely effectiveness of efforts to control those nutrients.
Efforts to create a long-term flow record started in the late 1960s after several drought-condition years in much of the watershed. By examining water flows in the Susquehanna, Potomac and James, which account for about 85 percent of the flow into the Bay, USGS scientist Conrad Bue in the late 1960s came up with a method for determining the total annual flow into the Chesapeake.
Bue was able to reconstruct flows into the Bay back to 1951. For years, the USGS has used that baseline for estimating long term “average” flows into the Bay.
Recently, though, the USGS was able to extend the baseline back to 1937 by analyzing other streamflow data it had available.
Although there is streamflow data for earlier years, there was not enough to estimate total flows into the Bay. “Some of the lowest flows we’ve ever seen came from 1930,” said Gary Fisher, a USGS scientist who worked on reconstructing the historic flow figures. “But we don’t have river flows from all the rivers.”
Still, the analysis was enough to discover that 1941 was the new low-flow champion. That year, average flows dipped to 47,776 cfs.
The highest year on record stayed the same, 1972, when flood waters associated with Hurricane Agnes drove the average water flow to 137,357 cfs.
The data also show that the lowest monthly flow into the Bay was September 1964, when only 8,471 cfs reached the Chesapeake. By contrast, 45 times that much entered the Bay during the wettest month on record, which was April 1993. Heavy rains, coupled with a rapid snowmelt in the mountains combined to send an average flow of 388,731 cfs into the Chesapeake that month.
The revised long-term record shows that the period from 1937 though the early 1970s was generally dominated by flows into the Bay that were at or slightly below the normal range. Since then, the area has generally gotten “wetter” — 10 of the high flow years have been since 1970.
That’s not necessarily unusual. Efforts to reconstruct past long-term climate changes in the Bay watershed show other multidecade periods with similar high-flow cycles, probably because of regional or global climate patterns. What has been unusual, Phillips noted, was that in the past decade, freshwater flow had fluctuated dramatically.
For example, 1993 and 1994 were wet years followed by a dry year in 1995, while 1996 was the second wettest year on record, only to be followed by slightly drier than normal conditions in 1997. Then, 1998 was again a “wet” year, followed by 1999, which was one of the driest years on record.
“The variability in streamflow will have impacts on water quality and living resources in the Bay,” Phillips said.
One impact of that, he said, has probably been to mask the impacts of any nutrient reduction efforts on Bay water quality.
Streamflow is a critical part of the Chesapeake ecosystem. As an estuary, where freshwater and saltwater mix, the entire Bay is impacted when changes in streamflow affect salinity levels. During years of high flow, salinity levels are reduced, with freshwater dominating the upper Bay and tidal tributaries.
But strong freshwater flows dramatically affect Bay water quality. High flows — especially when caused by strong storms — drive large amounts of nutrients and sediment from farm fields, city streets and suburban lawns into local streams.
When it reaches the Bay, sediment smothers bottom-dwelling creatures and clouds the water, preventing underwater grass beds from getting the light they need to survive.
Excess nutrients that accompany high flows spur algae blooms which block sunlight to grass beds. When the algae die, they sink to the bottom and decompose in a process that uses up oxygen.
Further, strong flows from rivers often form a barrier between the surface freshwater and bottom saltwater in the Bay. That means when oxygen is used up on the bottom, it can’t be replenished from the top, fresher water, worsening water quality conditions.
The combined result of the excess algae and strong flows can be huge, low-oxygen “dead zones” which, in bad summers, can cover a third of the Bay’s total water volume.
Extreme low flows carry their own sets of problems. When saltwater reaches areas that are normally fresh, it can kill underwater grasses, and stress local fish and other aquatic dwellers.
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