In the spring of 1776, Lt. Colonel Jaan Bautista de Anza, Father Pedro Font, and a small group spent four days exploring the East Bay to verify an earlier expedition's report that the Rio Grande de San Francisco flowed into the Bay. They camped here and examined the shoreline, but Anza and Font began to doubt that the waters before them were a river because they saw none of the expected signs of seasonal flooding and found no current, only the rising and falling of tides that they carefully measured. Font concluded that this was not a river but a freshwater inland sea extending east to the great green tulares, the marshes that once filled much of the Central Valley. The waters were actually the northern reach of the San Francisco Bay, which also drains the Sacramento River from the north and the San Joaquin River from the south.