The river rises in several unnamed forks in the
Santa Lucia Mountains of the Coast Ranges near the
Pacific Ocean. It flows generally north, cutting into a steep gorge, receiving Lost Valley Creek and Tassajara Creek both from the left. After the confluence with Rocky Creek, the Arroyo Seco swings abruptly east, passing a pair of intermittent natural lakes simply known as "The Lakes". Here, the valley begins to widen and the stream lazes out onto a shallower, sandier bed, passing the unincorporated communities of Millers Ranch and Sycamore Flat. Another major tributary, Piney Creek, enters just downstream of the former.{{cite map Meandering east between low bluffs, the river receives Vaqueros Creek and Reliz Creek from the right, emerging from the mountains about southwest of
Greenfield. The river flows north into the
Salinas Valley, where much of its water is diverted for irrigation. Turning northwest, it begins to parallel
U.S. Highway 101 as a seasonally dry wash. It joins the Salinas just south of the city of
Soledad. The source, mouth and entire course of the river are entirely within
Monterey County. There are no dams on the Arroyo Seco, making it prone to
flash flooding. ==Watershed==