Discharge and Turbidity in Tributaries

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• Discharge and Turbidity in Tributaries
• to the Lower Mattole River
• 2008-2009 Winter Runoff Season
• Presentation to Mattole Technical Advisory
  Committee
• June 6, 2009

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Turbidity Lower Bound Lines Figures from
Klein, R.D., Trush, W.J., and M. Buffleben.
2008. Watershed Condition, Turbidity, and
Implications for Anadromous Salmonids in North
Coastal California Streams. A Report to the
California Northcoast Regional Water Quality
Control Board, Santa Rosa, CA. 105 p.
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Figure from Klein, R.D., Trush, W.J., and M.
Buffleben. 2008. Watershed Condition, Turbidity,
and Implications for Anadromous Salmonids in
North Coastal California Streams. A Report to the
California Northcoast Regional Water Quality
Control Board, Santa Rosa, CA. 105 p.

• Points along lower edge of plot generally from
  recessional storm flows
• Recessional stormflows longer duration than
  accessional stormflows
• Slope of lower bound line indicative of chronic
  turbidity
• Relatively insensitive to climatic variation
• Possible to define with targeted manual sampling?

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Study objectives

• Determine effects on tributary water quality
  attributable to road treatments
• Determine the need for and nature of any
  modifications to the style or rate of road
  treatments that may be warranted to reduce water
  quality impacts

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Criteria for Sampling Site Selection

• level of landowner cooperation (inventory access
  to gt80 of the basin)
• similarly small drainage areas (lt2.5 sq mi)
• ease of winter access
• range of magnitude of planned road treatments
  (especially more cy/sq mi to be treated)

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Methods

• Install staff/crest gages, survey cross-sections
• Take sufficient discharge measurements to develop
  discharge rating curve

• Targeted grab sampling for turbidity on
  recessional flows following storm events (gt.75
  rain in six hours)
• Collect and analyze at least 40 turbidity
  samples/site

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Data Analysis

• Develop discharge rating curves for each site
• Plot unit discharge (cfs/sq mi) against turbidity
  (NTRU)
• Fit lower bound lines (LBLs) to scattergrams
• Compare slope of LBLs among sites
• Compare slope of LBLs pre- and post-treatment

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Results
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Results
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Results
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Results
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Issues Affecting Data Quality

• Shaky LBL at NSEM
• Changes in cross-sections at staff gages
• Timing of sampling at lower turbidities
  significance of y-intercept

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Issues and questions

• Changes in implementation schedule and landowner
  cooperation another pre-treatment year and
  additional controls
• Many sediment treatments preventative may not
  see response until stressing event
• How many samples are necessary?
• Cut costs through changes in sampling schedule?
• Sampling by volunteers/landowners?

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Acknowledgements

• Thanks to the field crews who worked cheerily and
  intently through long wet hours on short notice
  in the rain
• Thanks to Randy Klein for data analysis and
  technical guidance
• Thanks to the SWRCB, and Janet Blake, grant
  manager, for funding and support