Sediment listing(s) are based on consensus opinion of the physical scientists and biologists who have worked in these basins for several years. Each of the listed streams supports a run of steelhead trout. Steelhead trout are Federally listed as threatened in central California. Coho salmon may also still be present in Pescadero and San Gregorio Creeks. Coho salmon are State-listed as endangered and Federally listed as threatened in central California. Present-day population of coho salmon in central California is estimated as less than 5 percent of historical level (NMFS, 1997); population is further depressed in streams south of the Golden Gate. The Department of Fish and Game has listed San Gregorio and Pescadero Creeks as top priority streams in its draft coho recovery plan for central California (CDFG, 1998).
Specific expression and spatial extent of stream-riparian habitat impairment is poorly documented in the above named streams. Linkage to causes has not been investigated on a watershed scale. Based upon the results of studies in physically similar basins in northern and central California27, habitat impairment likely includes: a) increase in the amount of fine sediment deposited at spawning sites and in pools; b) increase in the frequency and depth of stream-bed scour; c) coarse sediment deposition which precludes re-establishment of riparian vegetation and causes changes in channel width-to-depth ratio and streambed substrate which are unfavorable to fish production. When the imbalance between sediment production and channel transport capacity is substantial, larger scale channel changes occur that may result in excessive rates of bank erosion, inadequate flood conveyance, further loss of riparian vegetation, and a reduction in baseflow persistence.
Habitat conditions in stream channels are shaped by more than sediment load. They are shaped by the interactions of streamflow, sediment, large woody debris, and stream-side vegetation. This implies that a broader, more holistic, analytical framework is needed when the principle objective of a TMDL is salmonid recovery. Such a framework is usually is referred to as watershed analysis, as has been implemented in Washington state and Federal forest lands (Washington Forest Practices Board, 1993; Federal Ecosystem Management and Assessment Team, 1993).
Other problems which have been identified in the named streams which are thought to be associated with excess sediment production include: a) rapid loss of reservoir storage capacity; and b) inadequate flood conveyance capacity in the lower reaches of San Francisquito and Pescadero Creeks28. Actions required to restore salmonid populations should also resolve watershed problems contributing to inadequate flood conveyance and loss of reservoir storage capacity.