Despite the prevalence of natural stream drying and potential increases in intermittency from human alteration and climate change, evidence of flow regime shifts has not been assessed in California, USA. To understand if and how regimes have changed, we developed a natural intermittent stream classification based on 201 U.S. Geological Survey gages with minimally modified flows (reference) from 1950 to 2105. We used the number of zero flow days to distinguish perennial from intermittent streams. We then developed a random forest model to assign all reference gages to an intermittent class based on climate and watershed characteristics and assessed model performance using a cross-validation leave-one-out approach. To assess recent shifts in flow regimes (1980-2020), we compared the observed stream class at reference and nonreference gages with the expected, natural intermittent stream class. We also calculated trends in the minimum 7-day moving average (Q7) and number of zero flow days metrics using the Mann-Kendall trend test. Our results indicated that the majority of reference gages (140; 60%) were on perennial streams. Among the reference stream gages, 5 (11%) showed shifts from intermittent to perennial but none shifted from perennial to intermittent. However, trends in low flow metrics within both stream classes resulted in widespread increases in drying. Shifts in stream class for nonreference gages were more common. Nearly 18% (62) of nonreference gages changed from perennial to intermittent, whereas 7% shifted from intermittent to perennial. Trend analysis in the low flow metrics for nonreference gages revealed both increasing and decreasing trends with no apparent spatial pattern, indicating that human activities have varied and localized effects on low flow dynamics. By quantifying expected natural drying regimes and the degree to which they were impacted by anthropogenic activities, this work can inform environmental flows and guide efforts to manage and conserve freshwater ecosystems.