Timber Harvest and Turbidity in North Coastal California Watersheds

Turbidity regimes vary dramatically among small streams in north coastal California. In an analysis of turbidity data from 27 small streams in the region, turbidity at the 10-percent exceedence level ranged from 3 to 116 formazin nephelometric units (FNU) for the 2005 wet season, translating to 1.7 to 65 days above an oft-cited biological threshold of 25 FNU. Watersheds draining to the streams spanned disturbance categories from zero (pristine redwood forest) to intense commercial timber harvest. Grouping the sites by average annual timber harvest rate showed that the zero harvest (background) group averaged 13 FNU at the 10-percent exceedence level, while the low harvest group averaged 20 FNU and the high harvest group averaged 61 FNU, 58 percent and 369 percent, respectively—well above the “20 percent above background” regulatory limit for northern California streams. Regression analyses of turbidity on watershed natural physiographic characteristics and land use histories (timber harvest and roads) showed the rate of recent timber harvest (average annual percent of watershed area) explained the greatest amount of variability in 10-percent turbidity exceedence. Drainage area was also significant but was secondary to harvest rate. None of the other watershed variables was found to improve the regression models. Despite much improved best management practices, contemporary timber harvest can trigger serious cumulative watershed effects when too much of a watershed is harvested over too short a time period. Klein is a hydrologist, Redwood National and State Parks, Arcata, CA 95521. Email: [email protected].