Digital SAT Reading and Writing Practice Question #341

Researchers have long hypothesized that the abrupt onset of the Younger Dryas cooling period roughly 12,900 years ago was triggered by a massive influx of freshwater from the glacial Lake Agassiz into the North Atlantic Ocean. According to this model, this sudden flood diluted the ocean's salinity, which in turn disrupted the thermohaline circulation—the global system of ocean currents driven by temperature and salinity differences—thereby plunging the Northern Hemisphere back into near-glacial conditions. This prevailing hypothesis fundamentally relies on the assumption that the geological routing of Lake Agassiz's freshwater shifted from the Mississippi River drainage system to the St. Lawrence River valley immediately preceding the climatic anomaly. Which finding, if true, would most directly undermine the researchers' hypothesis?