We present water quality monitoring data collected between 2000 and 2005, to investigate how seasonality and land use influence soluble reactive phosphorus (SRP) concentrations in streams and rivers that drain into the Monterey Bay, California, USA. In several waterways SRP concentrations increased late in the summer, between September and the first winter storms. This pattern occurred in San Juan Creek, the Pajaro River, and Corn Cob Canyon Creek. In contrast, the highest SRP concentrations in Watsonville Slough occurred from the fall through spring, with concentrations declining to an annual low point in mid summer. In Llagas and Uvas Creek concentrations were generally highest in the winter time during high water flows. By combining other sources of data, we found no long-term trend in SRP concentrations since the 1950s in the Pajaro River. SRP concentrations varied across the landscape. Llagas and Uvas Creeks had low SRP concentrations at their source reservoirs (upstream), in urban areas (midstream), and in agricultural areas (downstream). San Benito Creek and Miller's Canal, which were both sampled near agricultural fields, also had low median ortho-P concentrations. In contrast, the Pajaro River, Corralitos Creek, and Carneros Creek, located west of the Santa Cruz Mountains, had relatively high levels of SRP in all types of land uses. Geological differences on the western and eastern sides of the Santa Cruz Mountains play a role in SRP concentrations. Land use played an important role in SRP concentrations but our data suggest other criteria such as local geomorphological conditions play a key role in SRP concentrations in surface waters.