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Notes on Regional Baseline Values for Fine Sediment

Relative to McNeil samples, the 14% threshold for fines (<0.85 mm) advanced as part of TMDL in northwestern California is conservative relative to  the National Marine Fisheries Service (1996) guidelines for properly functioning salmon habitat, which set a  limit of 12% fine sediment less than 0.85 mm. McHenry et al. (1994) found that fines less than 0.85 mm were nearly 100% lethal to coho and steelhead eggs when they exceeded 13% within the redd. Fine-gravel sized particles <6.4mm may intrude into the stream bed, forming a layer in the stream gravels which may inhibit the emergence of fry (Chapman 1988). Barnard (1992) used freeze core sampling in Freshwater Creek, a Humboldt Bay tributary, and  found that sediment less than 1 mm comprised  less than 10% of the samples at most sites. 

Barnard (1992) measured fine sediment inside and outside coho salmon redds in Freshwater Creek, after the watershed had 40-60 years of rest from logging activities. At ten sampling sites, he found the average fine sediment levels (<1.0 mm) outside coho redds to be 13%, with fine sediment levels less than 10% at 8 of 10 sites (see chart). This study is useful because the geology in the Freshwater Creek basin is especially erodible. Two thirds of the Freshwater Creek basin is in Franciscan bedrock geology, similar to the western Gualala River watershed. The remaining third of the Freshwater basin is in Wildcat terrain, which is less cohesive than many rock types in the Gualala. Barnard's choice of 1.0 mm instead of 0.85mm for fine sediment and his use of the freeze core sampling method, which is highly effective for collecting fine sediment, most likely biased his samples toward higher values of fines. Therefore, control values for fine sediment for northern California streams is well under the lowest values reported by Burns (1972).

Burns (1972) finding of 20% fine sediment (<0.8 mm) in 1966 before logging resumed in the Little North Fork basin probably does not represent baseline conditions (pre-disturbance). The Little North Fork had been railroad logged in a prior era and the roadbed still extended along the active stream channel, providing a source of sediment. Caspar Creek, another Burns (1972) study site, had been completely railroad logged as well. The North Coast Regional Water Quality Control Board (1998) pointed out that Burns (1972) samples also followed the 1964 storm  which would have elevated sediment levels.