Fifty three new heat flow measurements together with 16 published data indicate that the heat flow (average of best values equals 1.74 μcal/cm2s) through the basins of the southern California borderland is intermediate between heat flows through the Sierra Nevada and the Basin and Range provinces. In general, geothermal gradients in the borderland decrease with depth in the sediments, corresponding to an increase in thermal conductivity of 2--3% per meter. Owing to rapid sedimentation, surface heat fluxes may be locally masked from their steady state values by as much as 30%. Topographic corrections are usually less than 5%. Seasonal bottom water temperature variations are insignificant, but long-term climatic change in the last 37.000 yr may have reduced the gradient by 2--3%. Anomalous values were found in close proximity to turbidity current channels. The heat flow distribution shows two significant geographic trends. First, heat flows increase systematically southeastward in response to a late Cenozoic southeastward-migrating triple junction. Second, heat flows increase slightly toward the continent, suggesting that the offshore basisns were formed progressively landward. In specific basins the measured values are uniform (standard deviation ?0.1 HFU, where 1 HFU=1 μcal cm2 s1), but in other basins, only the corrected values are uniform. In the Santa Cruz Basin the corrected values are dominated by a north-south linear trend.