The average annual flow is about , per year. An all-time highest flow of was recorded on December 6, 1997. The United States Geological Survey has operated three stream gages on San Diego Creek and several on its tributaries. Mean, maximum and minimum flow for the period of record are given in the below table:  Before the 19th century, the San Diego Creek basin consisted of extensive grasslands and seasonal wetlands, which would flood during the winter and spring with storm runoff from the Santa Ana Mountains and San Joaquin Hills. San Diego Creek did not have a well defined channel like it does today, but rather was a series of interconnected sumps and swamps in the lower elevations of the valley. The Santa Ana River, the main waterway of Orange County, would sometimes flow into Newport Bay via what is now the lower part of the San Diego Creek channel. This was observed several times during the 1800s and geological evidence suggests this regularly occurred as early as the Pleistocene period. The area was known as the "Cienega de las Ranas" ("Marsh of the Frogs") by early Spanish settlers, for the millions of tree frogs that once inhabited the forested lowlands. Lacking a direct outlet to the sea, the water table in the area was rather high and resulted in many year round artesian springs, which supported a rich variety of native plant and animal life. Starting about 2,000 years ago, the valley was inhabited by the Gabrielino (Tongva) Native Americans, who depended on the abundant game of the area for their sustenance. A major landmark for the Gabrielino was the Red Hill, located in present-day Tustin at the northern tip of the Cienega de las Ranas. The Gabrielino called it "Katuktu", "place of refuge". According to legend, the people had climbed the hill to escape a great flood that covered the plain in ancient times. This has basis in historical events – every few decades, during an extremely wet winter, the Santa Ana River and Santiago Creek to the northwest would change their course and flood large portions of the coastal plain. During the 18th century, the first Spanish explorers traveled through the region, followed by missionaries ("padres"), soldiers and settlers. El Camino Real was established to link the chain of missions constructed in California by the Spanish invaders. The Cienega de las Ranas was a major obstacle along the Camino Real and was completely impassable during most winters. In other seasons, those who were traveling from the south by night relied on the singing of the frogs to guide them to Red Hill (called "Cerrito de las Ranas" or "Frog Hill" by the Spanish), which marked the northern edge of the swamps.