An analysis of trends in baseflow recession and low-flows in rain-dominated coastal streams of the pacific coast

•We evaluate trends in dry-season flow conditions of rain-dominated watersheds.•Trends in time of baseflow recession form and annual low-flows are assessed.•46% of gages analyzed have significant trends in either low-flows or recession form.•Trends are primarily negative, indicating declining low-flows and faster recessions.•Northern California and Oregon are particularly affected by decreasing summer flows.

SummaryThe work presented here centers on the identification and calculation of indices representative of annual low-flow conditions and baseflow recession form, and the evaluation of trends in these indices with time for rain-dominated streams of the Pacific coastal mountain ranges. Two different baseflow recession analysis techniques are employed, which when combined are capable of modeling the varied dry-season flow conditions that exist over the broad range of catchments included in the study area. Results indicate that over the past 40-80 years widespread trends of increasing rates of baseflow recession and decreasing annual low-flow conditions exist throughout the region. Of streamgages analyzed, 44% were identified as having a statistically significant trend in either low-flow conditions or recession form with time. While spring flow conditions show little change over the study period, trends of decreasing late-summer flow conditions and increasing rates of recession are particularly common. Northern California and Oregon are especially impacted locations, with upwards of 60% of study gages exhibiting decreasing trends in late summer flow conditions. Detailed conceptual explanations for the connections between trends in recession form and indices of low-flow are also presented.