Despite rises in drought frequency and human water demands, streamflow regime shifts from perennial to non-perennial have not been evaluated in many arid/semi-arid regions. To document shifts, we created a methodology that classifies streams as naturally perennial or non-perennial. Our classification used historical, minimally disturbed-quality USGS streamflow gages (1950-2015) across California. The number of consecutive zero flow days (>= 5 days) was used to classify 61% (96/158) and 39% (62/158) of gages as perennial and non-perennial, respectively. We developed a random forest model to predict flow regime class based on climate and watershed characteristics. To identify regime shifts, we compared the observed class of contemporary (1980-2023) minimally disturbed and disturbed gages with their modeled, natural class. For most minimally disturbed gages, the observed and natural predicted classes were the same, but 13% (7/52) of gages had a modeled perennial regime with an observed non-perennial class, indicating a drying trend. Among disturbed gages, 22% (64/290) shifted from perennial to non-perennial and 7% (21/290) from non-perennial to perennial. Trends in the minimum 7-day moving average and number of zero-flow days provided further evidence of drying at minimally disturbed streams, but no pattern at disturbed gages. Our results indicate that few minimally disturbed perennial streams have become non-perennial to date, but many streams have experienced drying from climate. Streams impacted by human activities had greater drying rates, but regulation has caused some non-perennial streams to become perennial. By quantifying expected natural streamflow regimes, this work can help monitor, manage, and conserve stream ecosystems. Our approach classified 158 historical (1950-2015) minimally disturbed gages as perennial (61%) and non-perennial (39%) in California Among 52 active minimally disturbed gages, 13% transitioned to non-perennial (1980-2022), indicating a drying trend in response to climate For 290 disturbed gages, 29% showed shifts in both directions, highlighting distinct anthropogenic effects on streamflow