Spatial and Temporal Variability in Baseflow in the Mattole River Headwaters, California, USA

Abstract. Increases in human population, water use, and climate change have the potential to increase water stress and scarcity particularly in ecosystems with pronounced seasonality in precipitation, yet our understanding of the landscape features that control baseflows remains limited. Repeated synoptic measurements of streamflow in small streams (basin area <6 km2) in coastal Northern California were used to characterize variability in baseflow and correlations of streamflow with basin characteristics. A continuous streamgage was used as an index gage to calculate exceedance flows and to compare tributary flows measured on multiple dates. At 72–96 % exceedance flows tributary yields ranged from 0.23 to 0.00 mm day⁻¹. Unit-area yields varied widely, and this variation increased as flows declined at most sites. In nested basins, downstream declines in both discharge and unit-area yield were common. Basins with greater summer flow and a slower baseflow recession had steeper slopes, higher elevations, less flat ground and narrower valleys, more dissected and strongly convergent topography, and more precipitation. The difference in water yield among basins was much greater than the difference in precipitation, likely resulting from varying basin water inputs, storage capacity, and routing. The positive correlation between basin steepness and flow is attributable to the thickness of the weathered bedrock layer in water storage, and more rapid bedrock weathering in steeper basins with higher rates of uplift resulting in greater basin storage capacity. Results show that basins in a small geographic area (< 85 km²) and with fairly similar geology, vegetation, and topography may generate widely differing baseflow. Streams with naturally low baseflows are particularly susceptible to water diversion. Low-gradient streams essential for coho salmon rearing may be particularly susceptible to climate change or water diversions that reduce streamflow. Keywords: baseflow, groundwater recharge, drainage density, coho salmon, bedrock aquifer