While model estimates show a steady, if slow, decline in the amount of nitrogen, phosphorus and sediment entering the Bay, water quality monitoring in rivers shows a more complex story.

The actual amount of nutrients and sediments—or “loads”—has shown little significant trend since a coordinated monitoring program began in 1985.

Scientists have cited a number of reasons that contribute to the lack of change:

  • Lag Time: It often takes years or decades for the full impact of actions on the land to show up in the Bay. Some nitrogen travels through slow-moving groundwater, and much of the phosphorus is bound to slow-moving particles of sediment. Also, nutrient control practices, such as tree plantings, often take years to become fully effective.
  • Location of Monitoring Sites: Most of the river monitoring takes place at or above the “fall line” which is near the demarcation between free-flowing nontidal rivers and tidal waters. Monitoring nutrient loads in tidal waters, which move back and forth, is difficult. But much of the population lives in large urban areas at or below the fall line, and a disproportionate amount of wastewater treatment plant upgrades have taken place in those areas.
  • Lack of Effectiveness: Many nutrient reduction practices implemented on the ground may not be as effective as thought, or properly maintained. The effectiveness assigned to best management practices is often based on site-specific research projects and may not achieve the same benefits in all situations. Also, the Bay Program assumes that all plans, such as nutrient reduction plans on farms, are fully implemented.
  • River Flows: The amount of nutrients sent into the rivers is greatly influenced by rainfall. More rain washes more nutrients off the land and flushes them into the Bay. A number of unusually wet years the last two decades have sent huge slugs of nutrients into the Chesapeake. Those spikes had the effect of canceling out any downward trend in loads that existed.

Of all those factors, river flows is the most important in affecting year-to-year nutrient and sediment loads entering the Chesapeake Bay.

In fact, when scientists adjust water quality samples to adjust for changes in river flows, downward trends do occur in many locations

While flow-adjusted figures are important because they offer evidence of whether management actions are having an effect, the actual water quality in the Bay is affected by the amount of nutrients flowing in, and when those nutrients arrive. (Winter and early spring inputs are often worse for dissolved oxygen, while spring and early summer can be worse for underwater grasses.)

The ultimate goal of the Bay cleanup effort is to achieve enough nutrient and sediment reductions to maintain water quality goals under real-world conditions. If the region’s climate changes in ways that result in a pattern of increased precipitation, greater nutrient reduction efforts may be needed in the future to achieve those goals.