There is a lack of quantitative information describing the impact of farming on water quality at the watershed scale. This study documents the surface water quality of Walnut Creek—a 5130‐ha watershed with about 86% of the land used for crop production. Starting in 1990, flow and concentrations of NO3‐N and four herbicides—atrazine [6‐chloro‐N‐ethyl‐N′‐(1‐methylethyl)‐1,3,5‐triazine‐2,4‐diamine], alachlor [2‐chloro‐N‐(2,6‐diethylphenyl)‐N‐(methoxymethyl)acetamide], metribuzin [4‐amino‐6‐(1,1‐dimethylethyl)‐3‐(methylthio)‐1,2,4‐triazin‐5(4N)‐one], and metolachlor [2‐chloro‐N‐(2‐ethyl‐6‐methylphenyl) ‐ N ‐ (2 ‐methoxy ‐1‐methylethyl)acetamide]—were measured at eight locations. Nitrate‐N concentrations often exceeded 10 mg L−1 during May, June, and July. Total losses from the watershed ranged from 4 to 66 kg ha−1 yr−1 and represented 6 to 115% of the N applied as fertilizer in any year. Atrazine and metolachlor were detected at concentrations >0.2 µg L−1 in about half of all water samples, while alachlor and metribuzin were seldom detected. Median concentrations for atrazine and metolachlor were below 1 µg L−1 for all locations within the watershed. During runoff events, herbicide concentrations in the stream increased while NO3‐N concentrations decreased. Yearly losses from the watershed ranged from 0.2 to 7.5 g ha−1 for atrazine and from 03 to 6.7 g ha−1 for metolachlor. These losses represent 0.18 to 5.6% of the atrazine and 0.047 to 1.6% of the metolachlor applied in any year.