December 31, 2008
As I tap away at my computer keys, the sun outside once again filters through the trees. Weather forecasts predict rain this coming weekend, but to date (nearly the end of October) we have had at most one inch of precipitation in the Santa Cruz Mountains. That first rain event led to Reverse 911 calls to downstream residents of the 4,000 acre Summit Fire, warning of potential flooding, debris flows and torrents, and hazardous road conditions. Of course, once the rains begin in earnest and the bare hillsides mobilize, hazards to our threatened steelhead and endangered coho salmon populations will be imminent as well.
But just what will those adverse impacts to fish be, and will they be long-term as well as short-term? Fish have evolved with fire, but at a time when human-scale land use activities were at a minimum. And fire regimes have altered with decades of fire suppression, logging, and grazing, which have removed fire-adapted species or specimens. Science is attempting to understand the extent of modern fire impacts to fish and forest health as well as its effects on global warming. I will attempt to keep the interests of fish as well as forest and watershed health in focus as I write this and future columns on fire for this newsletter.
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Hydrophobicity will be a factor in overall erosion and sediment input into Soquel and Corralitos Creeks. You may scratch your head remembering that hydrophobia or `fear of water' is a symptom of rabies. Well, fires can cause the formation of soils that repel water, known as hydrophobic soils. According to the USDA Natural Resource Conservation Service, (www.statlab.iastate.edu/survey/SQI/) "A thin layer of soil at or below the mineral soil surface can become hydrophobic after intense heating. The hydrophobic layer is the result of a waxy substance that is derived from plant material burned during a hot fire. The waxy substance penetrates into the soil as a gas and solidifies after it cools, forming a waxy coating around soil particles."
The significance of such a layer is that it repels water, reducing the amount of infiltration from rainfall and runoff. With increased surface flows comes increased erosion and sediment input into streams. Sometimes this hydrophobic layer can persist for years, especially if it is relatively thick. Much of the fertile topsoil layer may be `lost', but this loss may actually bring added fertility to stream systems. However, while surface flows are increased, the reduced water infiltration lessens the likelihood of debris slides on denuded steep slopes. Disturbing this layer could actually increase sediment loads in streams. Over time, as plants regenerate and growth resumes, plant roots, soil microorganisms, and soil fauna will break down the hydrophobic layer.
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To check for a hydrophobic layer, scrape away the ash layer and expose the mineral soil surface. Place a drop of water on the air-dry soil and wait one minute. If the water beads, then the soil layer is hydrophobic. Because the upper few inches of soil are often not hydrophobic, it may be necessary to scrape away another one-half to one inch of soil and test again for the upper level of water-repellency. If you continue to scrape away the soil layers and repeat the water drop test, you can discern the depth of the hydrophobic layer.
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While everyone is certain that significant rainfall will lead to substantially increased sediment loads in local streams, the impacts on salmonid fish populations are not so clear. In the short term, spawning habitat is likely to be inundated with heavy sediment layers, while large wood input will most likely increase. Given the very fragile existence of our few remaining endangered coho, buried spawning habitat could spell doom to fish in those watersheds. Just this summer, juvenile coho were found in Soquel Creek, where they had been conspicuously absent in recent years. Will they survive, will they return? And what about the more hardy steelhead trout? Only time will tell. For now, everyone is waiting to see how these denuded mountain slopes will fare this winter, if and when the rains do come.
For more info: jodifredi@aol.com
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TOC for Forest & River News, Winter 2008