Tmdl schedule and workplans status july 1999


Numeric Targets: The Desired Future Condition



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Numeric Targets: The Desired Future Condition

The desired future condition is adequate spatial and temporal distribution of suitable spawning, rearing, and refuge habitats as necessary to support "Fish in Good Condition" at the individual, population, and species assemblage levels (Moyle et al., 1998).


Status:

In-stream habitat attributes which serve as the indices of the quality of spawning, rearing, and refuge habitat are needed. These may include: pool volume, baseflow persistence, spawning gravel permeability, large woody debris loading, and overstory canopy closure. Pool volume and gravel permeability would pertain to sediment load; over-story canopy closure and large woody debris loading would pertain to riparian management.


Work Needed:

Development of numeric targets for sediment (and other causes of impairment) may require extensive regionwide technical studies. For example, sediment targets should be set to specify allowable sediment discharge rates necessary to restore desired habitat quality obtainable within specific sub-basin29 and stream reach types30. For sediment, there are two types of targets needed: 1) allowable rates of sediment input to stream channels (sediment production targets); and 2) stream habitat quality. Sediment production is the focus of measurement and compliance targets because it is the direct expression of what is coming off the land. Measurement of sediment load in streams is much less useful than sediment production because: a) stream sediment load does not lead to distinction of natural versus management-related contributions b) load will vary dramatically between years independent of management activities (see discussion below); and c) there are temporal lag times of a few-to-hundreds of years between delivery and discharge of sediment stored in channels.


Sediment production targets should be based on a ratio of anthropogenic to total sediment production measured on a 1-to-5 year basis per the following rationale:
a) Inter-annual variation in sediment production rates is extreme in California and governed primarily by the substantial variation in character of a given wet season. A ratio between anthropogenic and total production is superior, therefore, because anthropogenic contributions are always placed within the context of the hydrologic driving forces.
b) Sediment production rates can be measured at least as accurately as sediment load in streams. Costs are similar, and professional practices for distinguishing whether causation is natural and management related are well established.
c) Measurement frequency is suggested as once in one-to-five years to aid adaptive management decisions and evaluate effectiveness of management practices and restoration projects.
Numeric targets for stream habitat attributes which are affected by sediment load (and other causes of impairment) need to be biologically meaningful. In-stream habitat attributes which serve as the indices of the quality of spawning, rearing, and refuge habitat are needed. These may include: pool volume, baseflow persistence, spawning gravel permeability, large woody debris loading, and overstory canopy closure. Loss of pool volume by fine sedimentation (Vstar method) and stream bed permeability at spawning sites pertain to sediment load. Pool volume and gravel permeability would pertain to sediment load; over-story canopy closure and large woody debris loading would pertain to riparian management.
Once the regionwide effort to establish targets is completed, sediment TMDL numeric target development in San Mateo Creeks should be reduced to adaption of the regionwide targets and the development of rapid sediment budgets as decribed in the Source Analysis section below.
Resource Needs:



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