UC Merced

The work presented herein examines how and to what extent turbidity, channel bed movement and stream source water ratios change over different temporal scales in four forested mountain headwater catchments in the Sierra Nevada, California. This work focuses on the water quality topics of sediment and source waters as part of a larger study on the effects of forest fuels treatments. No effects on water quality were expected because treatments were set far back from the stream channel and relatively light (8.0% and 7.5% decrease in mean Leaf Area Index (LAI) for the northern and southern site respectively). Therefore, this work was designed to characterize sediment and source waters rather than explore treatment impacts. In doing so important insights were gained around sediment and stream water sources.

Sediment movement was found to be highly episodic and tied to low-frequency, short-duration discharge events with pronounced seasonality. Turbidity hysteresis patterns indicated localized (bed and banks) sediment sources that were mobilized quickly but became progressively depleted. Continuous measurement of channel bed elevations in the thalweg pointed to seasonal mobilization (connectedness) of sediment during winter, a gradual depletion through spring and early summer, and disconnectedness in low flow season when material builds up again at the base of banks mirroring patterns seen in the turbidity hysteresis loops. Stream water chemistry data showed higher concentrations during base flow, which was exacerbated by consecutive years of drought. The high base flow ion concentrations were tied to high ratios of groundwater. End-member mixing analysis showed that wet and dry years had similar source water ratios during high flow (winter and spring) but during the low flow season (late summer), a shift toward higher ratios of groundwater was seen for drought years. Recent tree mortality observed across the Sierra Nevada after four consecutive years of drought underscores the critical role of groundwater in maintaining baseflow and sustaining forest ecosystems. Better understanding of sediment processes, source water contributions, and drought effects in small, forested, mountain, headwater catchments provides an important foundation for sustainable land use management, more effective channel restoration design, and improved mitigation of downstream water quality.