Wide-ranging changes in river flows into the Bay have been the norm not only for the past few years—or even decades—but for centuries.

In the past, such changes as widespread floods seen a century ago, were blamed mostly on the clearing of forests in the 1800s. But recent papers by researchers with the U.S. Geological Survey and others show that long-term climate patterns had actually changed during that time: the period of 1825 through 1910 had far more rainfall than previous centuries.

After several dry decades in the mid-1900s, recent decades have returned to flow patterns similar to those of a century ago.

Those conclusions come from studying fossil organisms and their shells’ chemical signatures which were found in sediment cores taken out of the upper Bay. The presence of various species is linked to changing salinity levels in the Bay, which are caused by patterns of river flow.

This information has been collaborated with long-term tree ring records, which also help to indicate climate patterns.

In fact, during the last 500 years, climate records show that 14 wet-dry cycles occurred. For instance, a multidecade “megadrought” hit about 400 years ago—around the time the Jamestown colony was established—which sharply increased salinities in the upper Bay. During some of those cycles, the salinity in parts of the Bay have oscillated as must as 10–15 parts per thousand.

All of the factors that cause those climate shifts—or the role that humans may have been playing in recent decades—are not clearly understood

Some of the natural climate cycles that affect temperature and rainfall over much of the United States include El Niño, a periodic warming of water in the tropical Pacific Ocean, which produces greater precipitation in at least part of the Bay watershed.

Its opposite, La Niña, is a cooling of the same water, which typically results in drier conditions.

Those impacts are superimposed on another climatic cycle that dominates the Atlantic, called the North Atlantic Oscillation, which appears to trigger climatic cycles that last for decades as atmospheric pressure over the Atlantic Ocean shifts.

When pressure over Iceland is low and the pressure over the Azores is high, as was the case for most of the 1980s and 1990s, strong westerly flows bring warmer conditions to northern Europe and wetter conditions to the East Coast of the United States.

When the reverse is true, drier conditions are likely along the East Coast, as was true in the 1950s and 1960s.

In addition, even longer-term global climate patterns may come into play, which are more poorly understood yet are important given concerns about human influence on future climate changes.