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Austin Wegner

This study in Oregon examines sediment and turbidity changes after road construction and forest harvesting in the Trask River watershed. The research outlines the hypothesis, study area, methods, results, and conclusions. Forestry practices have historically caused large sediment fluctuations, with newer methods seeking to mitigate pollution. The study finds minor increases in sediment concentrations downstream, highlighting the importance of local factors in sediment yields. Only one site supported the hypothesis of increased sediment transport due to construction and hauling activities.

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Austin Wegner

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  1. Suspended Sediment and Turbidity After Road Construction/Improvement and Forest Harvest in Streams of the Trask River Watershed Study, Oregon Austin Wegner

  2. Outline • Introduction • Hypothesis of the Study • Study Area • Methods • Results • Conclusions

  3. Introduction • Forestry practices used to be very destructive • Large episodic sediment fluctuations • Poor rehabilitation methods • Sediment drain directly into rivers • Buried organic debris • Far reaching biological impacts • Newer practices seek to mitigate sediment suspension in rivers • Variability in transport systems makes it difficult to pinpoint specific sediment contribution by management practices

  4. Hypothesis Combined effects of road construction/improvement, forest harvest, and hauling increase both turbidity and suspended sediment concentrations in adjacent streams. • If so, how often does sediment transport downstream occur?

  5. Study Area

  6. Trask Watershed Study Area • Composed of volcanic and sedimentary formations • Combination of long smooth slopes and steep dissected slopes near stream • Generally stable slopes (<20% grade) • Steeper slopes more in volcanic terrain • “Tillamook Burn”-- Series of large scale man made forest fires from 1933 to 1951

  7. Methods • 5 Road crossings studied • Sensors are placed above and below harvest site • Measurements of turbidity, SSC, and discharge are taken • Data is collected at 3 separate times in the forestry process • Data is then analyzed and compared to determine statistical relationships

  8. Results Correlation between turbidity and suspended sediment concentration given a specific discharge • Turbidity measured in NTU • SSC measured in mg/L

  9. Turbidity above and below pair (left) • Plotted against 1:1 ratio Turbidity plotted at specific intervals (right) • Dots represent 5th and 95th percentile range • Dashed line is given biological impact threshold

  10. Turbidity at sensor location below the road crossings

  11. Conclusions • Road construction and forest harvest and hauling did NOT result in significant increase in SSC or turbidity downstream • Only minor and nonsignificant increases were found • Likely had little to no biological impact • Only study site PH3 acted in a way that would support the hypothesis • Significant increases in SSC and turbidity during road improvement and hauling • Significant increases also occurred during periods of high flow • Suggests that stream adjacent processes can contribute high sediment yields • Highlights the importance of local factors

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