A spring freshet, driven by higher-than-normal snow falls followed by heavy rains, poured 5.1 trillion gallons of water into the Chesapeake Bay from the Susquehanna River during a 43-day period, according to figures from the U.S. Geological Survey.

This year's freshet lasted slightly longer than last year's 41-day high-flow event. But the peak flows were far less than those seen last year, and the total amount of fresh water flowing into the Bay was 1 trillion gallons less than the 6.1 trillion gallons that flowed downed the Susquehanna during last year's record-setting freshet, according to the USGS.

Nonetheless, this year's event resulted in the highest March flow into the Bay ever recorded. During that month, an average 144,200 cubic feet per second of water flowed out of the Susquehanna - double the 77,300 cfs recorded in March 1993. Flows were up in all the other Bay tributaries as well.

USGS crews sampled around-the-clock at the Conowingo Dam near the mouth of the Susquehanna during the peak of the flow - March 26-30 - and intermittently throughout the remaining 38 days of the storm event.

Those samples will be analyzed to determine the amount of nutrients and toxic chemicals flushed down the river and into the Bay during the event.

During last year's six-week high flow:

  • 5.3 billion pounds of sediment were washed down the Susquehanna and into the Bay; nearly twice 2.6 billion pound average for an entire year.

  • 112 million pounds of nitrogen were washed down the river, about 80 percent of the 140 million pound annual average.

  • 5 million pounds of phosphorus were washed down the river, approaching the 5.8 million pound average for an entire year.

Unlike last year's event, this year's flow never exceeded the 430,000 cfs level which is typically required to "scour" sediments stored behind the dam. Scouring significantly increases the amount of nutrients, toxics, and sediments washed downstream.

This year's event also occurred slightly earlier than last year's high flow, beginning on March 11 and ending on April 22. Last year's higher-than-normal flows began on March 25 and lasted through May 4.

Scientists will be monitoring the Bay to see what impacts the flow - and the slightly different timing - have on the Bay.

Last year's flow resulted in low dissolved oxygen levels in the deep part of the Bay, but did not result in the abnormally large algae blooms that many had expected, possibly because the flows were strong enough to flush much of the nutrient load - which spurs blooms - straight through the Bay and into the ocean.

Last year's event also did not appear to affect submerged aquatic vegetation, as many had expected, though scientists were concerned that stresses caused by those flows could show up this year.

Some credited last year's high flows with aiding a better-than-normal spawn for many fish, in part because the high flows kept water temperatures steady, which is important for spawning. At the same time, the huge flows of fresh water were blamed for killing large numbers of oysters in the Potomac and James rivers. [For more on the impacts of last year's freshet, see the January-February 1994 Bay Journal.]

While flows were higher than average in all of the Bay's major tributaries, it is the flows down the Susquehanna - which typically provides half of the fresh water to the Bay - that plays the largest role in affecting the Bay's physical dynamics.

High freshwater flows into the Bay during the spring usually cause strong stratification between the fresh water near the surface and salt water near the bottom. Strong stratification prevents oxygen exchange between those layers. As algae sinks to the bottom and is decomposed by bacteria, the oxygen in bottom areas of the Bay is depleted.

That, in turn, reduces the habitat available for the many fish and other aquatic species, forcing them to crowd into other areas. Some oxygen-sensitive species that cannot move may die, while others may suffer other stresses, such as reduced growth or reproduction.

Last year's event produced the highest storm-related flow ever sent down the Susquehanna, surpassing the 3.9 trillion gallon flow that followed Tropical Storm Agnes in 1972. The Agnes event, though, lasted only 16 days and hit a record peak flow of 1.13 million cfs.