Effects of Forest Harvesting and Different Riparian Buffer Treatments on Summer Low Flows in Headwater Streams of Coastal Northern California

Riparian areas are critical zones of terrestrial influence on aquatic systems and therefore, disturbances in these areas can have significant negative impacts. As such, policies in North America require the maintenance of vegetation along streams during industrial forest operations to protect freshwater systems. However, approaches to riparian buffer management have changed rapidly in recent decades with limited knowledge of their effectiveness. In particular, it is important to understand how different riparian vegetation management practices influence streamflow, especially during the summer low flow period, which is a critical time for many freshwater species. In our study, we assessed the effects of increasing intensity of harvesting headwater riparian areas on summer low flows and diel streamflow in Coastal Northern California. We hypothesized that increasing removal of canopy cover would reduce catchment-level transpiration, resulting in increased summer low flows and decreased diel fluctuations in daily discharge. To address our hypothesis, we quantified stream discharge in two reference catchments and three harvested catchments (area: 27.9­-40.42 ha). The harvested catchments received different riparian buffer treatments with a range of riparian canopy cover percentages based on current and past policies. One year following harvest, summer low flows increased 33-300% while diel fluctuations in streamflow increased 8-250% in the harvested catchments, after accounting for differences in precipitation between years. However, we did not observe the greatest increases in streamflow or diel variability in those catchments with the greatest proportion of riparian canopy removal. These unexpected results were likely due to confounding factors, such as percentage of catchment harvested and aspect of the harvested slopes. Our results suggest that current forest management practices of clearcut areas, harvesting on only one side of the stream, and retention of riparian vegetation may still have a significant impact on post-disturbance summer streamflow and diel variability in many headwater systems. However, the efficacy of current forest management practices at minimizing impacts on freshwater systems should be investigated more broadly.