Last year’s near-record rainfall left the Bay awash with the greatest amount of nutrients and sediment since 1996, according to figures from the U.S. Geological Survey.
Recent figures from the USGS show that 2003 flows were 1.7 times greater than the long-term average.
Nitrogen loads were also 1.7 times higher than the average from 1990-2003, while phosphorus loads were 2.5 times greater and sediment loads 2.3 times greater.
Overall, nutrient and sediment loads from the nontidal portions of the Bay’s nine major tributaries were the second highest since the watershedwide monitoring program began in 1990.
Although higher nutrient loads are expected when flows increase, the change between 2003—the third wettest year on record—and the drier-than-average 2002 illustrates the dramatic effect weather has on today’s Bay.
River flows into the Bay in 2003 were 2.3 times higher than those the year before. The amount of nitrogen carried in the water was 2.7 times greater than 2002, while phosphorus was 5 times greater and sediment 11 times higher.
The figures highlight the difference in the way the two nutrients move down the watershed, said Scott Phillips, who oversees the USGS’s Bay-related programs.
Nitrogen easily dissolves into water, so the amount entering the Bay is usually closely correlated with fresh water flows; the more rainfall, the more nitrogen washes downstream.
Movement of sediment and phosphorus, which often attaches to sediment, depends more on the timing and duration of river flow in the watershed. Small rainfall events usually don’t generate the river discharge to move sediment and phosphorus far, if at all.
“The two primary factors are the amount and velocity of the flows,” Phillips said. “The streamflows need to get above a certain velocity to have the power to transport sediment.”
A recent USGS report on sediment transport in the watershed said the majority of sediment movement takes place during “bankful” conditions when streams approach their flood stage. That typically happens only during relatively large storms which, on average, occur once every one to two years, according to the report.
The disproportionate impact of strong flows on phosphorus and sediment were dramatically illustrated in last year’s figures for rivers south of the Potomac.
Phosphorus loads increased by seven– to eightfold in all of those rivers except the Rappahannock, which saw a 17-fold increase. Sediment loads from the Rappahannock increased by 42 times more than 2002 levels, while sediment in the James and Pamunkey rivers was up by 20 times.
That reflects the influence of several severe events, including Hurricane Isabel, which hit the Bay in late September. Winds and rain associated with the hurricane hit the southern portion of the Bay harder than areas farther north. Also, rapid rain and snowmelt in February and a series of late-spring storms sent several big surges down southern tributaries and into the Bay.
“We had several major flushing events,” said Doug Moyer of the USGS’ Richmond Office. “We had several back-to-back storms where we were routinely getting near flood stage. Certainly, Isabel added to that.”
The figures come from monitoring data collected at the head of tidal waters near the “fall line” of the major rivers, which is the geological border between the Coastal Plain and the Piedmont. It is usually marked by waterfalls or rapids where the two geological provinces meet.
Below the fall line, the back-and-forth movement of tidal waters makes it impossible to accurately count the amount of nutrients entering the Bay from rivers and creeks.
It’s generally estimated, though, that the nontidal areas of the watershed contribute about 60 percent of the nutrients reaching the Bay, while the areas below the fall line generate the rest. If that held true last year, the total amount of nitrogen from all watershed sources may have reached roughly 500 million pounds, along with 42 million pounds of phosphorus.
(By contrast, Bay Program models estimate that under “average” rainfall conditions, about 278 million pounds of nitrogen and 19.5 million pounds of phosphorus enter the Bay from the entire watershed each year.
The Bay Program’s cleanup goal is 175 million pounds of nitrogen, and 12.8 million pounds of phosphorus.)
The impact of the loads was seen in the Bay as sediment clouded the water and algae blooms from the nutrients blocked sunlight from reaching submerged aquatic vegetation, causing a record loss in underwater grass beds.
The high flows and nutrient loads also combined to create some of the worst low-oxygen conditions ever seen in the Bay. Strong river flows set up a barrier between the top and bottom layers of the Chesapeake, preventing the oxygen-rich water near the surface from mixing with water near the bottom. As excess algae sink and are decomposed by oxygen-consuming bacteria, large “dead zones” are created in deep parts of the Bay.
Weather did not always play such an important role in the health of the Bay.
Once, vast forests covered the watershed and soaked up much of the water and nutrients before they flowed off the land. As those forests were cleared, not only did nutrient loads to the Bay increase, but so did the amount of water going downstream. That’s reflected in sediment cores showing that high-salinity areas historically reached farther up the Bay than they do today because river flows were moderated by the sponge-like forests.
Nutrient reduction plans being written by jurisdictions within the watershed will help mitigate those impacts in the future, said Rich Batiuk, associate director for science with the EPA’s Bay Program Office.
Not only will those plans reduce the amount of nutrients on the landscape, but actions to replant streamside forest buffers, plant cover crops and retain stormwater runoff will help to hold water on the watershed, allowing some to be absorbed by plants or filter into the groundwater.
As a result, not only will the amount of nutrients be cut, but the peak flows that cause erosion and contribute to low-oxygen conditions will also be reduced, giving the Bay a dual benefit.
“The basic bottom line is that you are trying to build that resilience back into the watershed,” Batiuk said. “You are trying to essentially rebuild a sponge capacity to soak up the water and slow down what runs off.”