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Sources and pathways of ground-water flow to canyon streams as inferred from dry-season baseflow conditions, Carmel River watershed, California. Scott Brown, Mark Woyshner, and Barry Hecht. Outline. Study sites: Lower Las Garzas and Pine Creeks Proposed model of ground-water inputs

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scott brown mark woyshner and barry hecht

Sources and pathways of ground-water flow to canyon streams as inferred from dry-season baseflow conditions, Carmel River watershed, California

Scott Brown, Mark Woyshner, and Barry Hecht

outline
Outline
  • Study sites:
    • Lower Las Garzas and Pine Creeks
      • Proposed model of ground-water inputs
  • Methods of analysis:
    • Streamflow recession analysis
    • Stream surveys
    • Longitudinal measurements of specific conductance
  • Incorporating watershed disturbances
  • Implications for mitigation monitoring
stream characteristics
Stream Characteristics
  • Deep granitic canyon with broad, flat, sedimentary ridge-tops
  • Channel morphology—step-pool or canyon pool-riffle architecture
  • Intermittent stream with some perennially flowing reaches
  • Alternating reaches of thick and thin cobble/ boulder alluvium overlying granitic bedrock
  • For Las Garzas, pools are relatively free of sediment due to dam upstream
slide5

Digital elevation model—lower Las Garzas Creek

Carmel River

Lower Las Garzas Creek

Broad tributary uplands

Source: USGS National Elevation Dataset

aquifer characteristics
Aquifer Characteristics
  • Fractured-granitic, deep (regional) zone: consistent low-flow source related to permeability of discrete fractures
  • Shallow (local) zone: limited higher-discharging source responding to antecedent rainfall patterns and carry-over from wet years
  • Alluvial channel water: storage and mixing of remnant wet-season runoff and accretion flow sources
streamflow records
Streamflow Records
  • Compare baseflow recession patterns (flow persistance) from year to year
baseflow surveys
Baseflow Surveys
  • Delineate the extent of flowing reaches, isolated pools and dry segments at various times of the dry-down
  • Track the progression of stream dry-down
water quality monitoring
Water quality monitoring
  • Measure specific conductance to identify distinct ground-water sources and track trends stream profile
  • Provides a metric to use that is independent of sediment filling and easier to measure at ultra-low flow conditions
natural watershed disturbances altering baseflow
Natural watershed disturbances altering baseflow
  • Wildfire
  • Episodic high-flow events
  • Pool-sediment infilling/scour
summary
Summary
  • The shallow aquifer system, especially in the side canyons, is an important source of carry-over baseflow following wet years
    • Streamflow recession analysis and stream surveys
  • The deep aquifer supports summer flows in the deepest portion of the canyon, in the driest times of the year, and during very dry years
    • Specific conductance
conclusions
Conclusions
  • In this case, the need for careful description of hydrogeologic connection to streamflow is driven by habitat considerations, not for the more common searches for water, movement of contaminants, or locations to store contaminants.
  • Monitoring-based mitigation through multiple lines of evidence is more effective than flow-threshold or reference-year comparison-based mitigation in low-flow and intermittent streams, and more adaptive to future watershed changes