West of U.S. Highway 101, Piner Creek flows over the Santa Rosa Plain at gradients of less than two percent. Here the predominant soil type is Zamora silty clay loam, which has been deposited as an alluvial fan as the floodplain formed. The upper soil surface of the Zamora material is grayish-brown hard, firm, sticky and plastic. Permeability is moderately slow and fertility is high for these lower reach soils, which has resulted in historic orchard use and present day vineyards.