The January floods that sent the greatest surge of fresh water into the Bay since Tropical Storm Agnes also delivered about half as much nitrogen to the Chesapeake as the Bay typically gets in an entire year.
Despite that, signs of the flood had largely vanished from the Bay by early spring, except for the huge amount of logs and debris in the water and along the shores.
In terms of water quality, right now "It's almost like there wasn't much of a flood out there," said Sherm Garrison, an environmental specialist with the Maryland Department of Natural Resources' Tidewater Ecosystem Assessment Division.
In fact, scientists almost took the finding of an algae bloom near the Bay Bridge in April as good news: The bloom, which usually appears during the spring, was not much larger than normal, nor did it last for an abnormally long period.
That's a sign that rather than sticking around to fuel large algae blooms, much of the nutrients carried into the Bay may have been flushed straight through the system and into the Atlantic Ocean by the powerful January flows. Or, they settled to the bottom with the sediment.
The January flood carried 20 times as much nitrogen and 5 times as much phosphorus into the Bay as is normal for the month. Nitrogen and phosphorus are targets of a Baywide cleanup effort because they can fuel abnormally large algae blooms that dramatically reduce the Chesapeake's water quality.
The full impact of the flooding on the Bay won't be known for months. Scientists will continue to monitor water quality, phytoplankton (algae) blooms, fish and shellfish reproduction and the extent of grass beds in the Bay for signs of stress from the flooding.
But the Chesapeake Bay Monitoring Program found that by spring, salinity levels in the Bay were only slightly below the long-term average. That would be good news for some species, such as oysters, which can be killed by long spells of low-salinity water.
Also, water clarity returned to normal - an important factor for Bay grasses, which begin growing in the spring. Grasses provide important food and habitat for many Bay species, but require clear water to receive sunlight.
This is a far different picture from the aftermath of Agnes in 1972 - a flood of similar size that had devastating effects on the Bay: burying shellfish and grass beds in sediment and spurring huge algae blooms that clouded the water.
While exact comparisons are not possible because there was no comprehensive water-quality monitoring program in 1972, estimates made by Linda Zynjuk of the U.S. Geological Survey indicate that this year's storm, which flushed 4.9 trillion gallons of water into the Bay, may have carried slightly more nitrogen - but dramatically less phosphorus and sediment - than Agnes.
The reason for the difference is timing. The 63 million pounds of nitrogen carried into the Bay this January was probably slightly more than was brought by Agnes, according to Zynjuk's estimates. Nitrogen dissolves easily in water, and the amount entering the Bay is closely tied to water flow. But Agnes hit in June, when nitrogen-absorbing trees and plants were active, and likely made use of some of the nutrients.
Plants were dormant during this Year's storm, so there was nothing to take up the nitrogen. In addition, much of the water in January resulted from snowmelt. That snow had filtered nitrogen oxides from pollution out of the air as it fell, and stored them until being washed into the Bay.
That difference in timing also means that Agnes may have carried 5 to 10 times more sediment and phosphorus into the Bay, according to Zynjuk's estimates. Sediment and phosphorus loads are often related because phosphorus tends to bind with sediment particles. With Agnes, plowed fields and soft ground were subjected to two days of pounding, erosive rainstorms which flushed large amounts of soil - and the accompanying phosphorus - into rivers and streams.
Much of this Year's runoff resulted from snowmelt, not driving rain. The ground was further protected because, being winter, it was frozen and less subject to erosion. "The snow and frozen ground prevented a lot of land runoff," Zynjuk said. Much of the erosion that took place occurred in stream channels carrying huge amounts of water.
Indeed, the upper Bay had only a thin layer of sediment deposited as a result of the January flood. After Agnes, the same area had about 8 inches or more of sediment deposited.
The timing of the flood had another benefit. Frequently, floods force the closing of shellfish beds as wastes from flooded sewage treatment plants, failed septic systems and animal feedlots are flushed into the water.
But, because the bacteria associated with those wastes cannot survive long in winter temperatures, and because shellfish filter water at a slow rate during cold months, there were no flood-related closures of shellfish beds.
One of the most noticeable impacts on the Bay was not from nutrients, but from debris. By some estimates, the floods sent as much as 150,000 tons of debris down the rivers and into the Bay. Some of the material, such as logs, may provide habitat as they accumulate near the shoreline. But in the open Bay, trees can pose a threat to boaters. And, much of the debris was trash.
"People have reported seeing 275-gallon heating oil tank containers washing ashore, along with propane tanks washing ashore," Garrison said. As these tanks deteriorate, the "Propane will be released to the air and pose more of safety problem than a pollution threat, but fuel oil can contaminate a local area. Hopefully, as people see the things, they will alert local authorities to get them removed."