Fires, floods, and watersheds
2007 Southern California firestorm
Due to strong Santa Ana conditions and record low seasonal rainfall in the 2007 water year, numerous large fires burned areas of southern California in October 2007, leading to state and federal disaster declarations. In the Santa Clara River watershed approximately 65,000 acres burned, concentrated primarily in the upper portion of the watershed in Los Angeles County. For mapping and imagery of the 2007 fires, visit NASA's Earth Observatory Natural Hazards website.
The "Fire-Flood" Sequence
Wildland fires are a common seasonal occurence in California and throughout the arid American West, and have occurred for thousands of years as a result of both anthropogenic and natural factors. Many native plant communities have subsequently evolved to survive and even require fire for their persistance in the landscape. Chaparral communities depend on fire to help with seed germination; indeed a number of native California wildflower species seeds germinate only after fire has passed through the landscape, resulting in dramatic spring displays. In natural western landscapes, fire plays a critical role in maintaining habitat diversity. Native Americans used fire annually to maintain grasslands in areas that would otherwise revert to coastal sage scrub or other vegetation types in order to retain important grain resources.
Documented fire reoccurrence since 1878
While natural wildland fires play a critical role in the landscape ecology of the west, fire can also be extremely devestating, especially in California where expanding urbanization and fire suppression (which often increases the availability of highly flammable, natural fuels) have been the norm for nearly a century. Between 1919 and 1996, roughly 8.6 million acres of California burned, taking 224 lives and destroying nearly 12,000 buildings; the annual toll of structures burned has steadily risen, while average acreage burned has remained relatively constant, reflecting increasing expansion of urban areas into wildlands. Historical records indicate that much of the Santa Clara River watershed has burned at least once since the late 19th century, with many areas of the lower watershed, including South Mountain and the lower Sespe, Hopper, and Piru creek watersheds, burning up to 7 times since 1878.
Recent fires in the Santa Clara River watershed have been particularly large and intense. The September 2006 Day fire burned approximately 162,702 acres of the Santa Clara River watershed (about 16% of the total watershed area), and was the 5th largest recorded wildfire in California history. Wildfires in 2003 burned approximately 11% of the watershed.
In many cases, the immediate toll of a fire is often just a precursor to more significant, watershed-scale impacts. Hillslopes in steep, semi-arid lands typically respond during post-fire winter rains with increased runoff and accelerated erosion, resulting in debris flows, landslides, and floods—thus completing what has been dubbed the "fire-flood" sequence. As a result, fires often have significant effects on physical processes in watersheds and are a major contributor to hillslope erosion throughout the arid American West and particularly in California.
As is true elsewhere in coastal southern California, fire risk in the Santa Clara River watershed is enhanced by the seasonally hot, dry Santa Ana winds, which typically blow south-southwesterly from inland deserts between August and November when highly flammable chaparral vegetation, which pervades over much of the landscape, is at its driest. The fire season in southern California is followed closely by the winter wet season, when multi-day storms and intense rainfall are likely. Gentle rains in early winter have been shown to promote rapid growth of an erosion-inhibiting weed cover, but slopes that burn in the fall are nevertheless often prone to enhanced erosion because they are devoid of protective vegetation when the first intense rains arrive. Chaparral regrowth after fire is relatively rapid such that pre-fire fuel conditions can be re-established on landscapes quickly - often within 30 years - making frequent fire-recurrence likely. All of these factors contribute to making the erosive destructiveness of southern California's "fire-flood" sequence significant in comparison with the post-fire responses that have been observed in other wildfire-prone landscapes.
Fires can accelerate erosion in several ways:
- On steep slopes, vegetation can form organic dams, effectively retaining sediment that originates upslope; when fire incinerates this vegetation, sediment that was impounded behind it is released and can be quickly mobilized downslope by dry ravel and overland flow. Incineration of vegetation by fires can also accelerate erosion by exposing surfaces to more efficient erosion by rain impact
- Fire can accelerate erosion by causing inter-particle fusion, which makes soils coarser, and thus increases their vulnerability to raveling. Particle sizes in soils from southern California chaparral woodlands (like those that exist throughout the Santa Clara River watershed) have been shown to shift from fine, clay-rich distributions to coarser, sand-sized distributions, when soils are subjected to temperatures experienced during wildfires. This coarsening is thought to be partly responsible for increases in raveling rates after fires, but is not generally widespread, except in intense, high-temperature fires
- Fire may also accelerate erosion by changing soil permeability. In hot fires, organic compounds in burning vegetation within soils can literally vaporize and then migrate to cooler depths where the vapor condenses to form a water-repellent, or "hydrophobic," layer. Hydrophobicity is thought to be largely responsible for the characteristic post-fire development of dense networks of narrow channels, or rills, on southern California hillslopes. These rills can be formed when soils overlying the impermeable hydrophobic layer become unstable, due to saturation, and become mobilized by downslope flow. Increased runoff from the rills can help to mobilize any sediment stored in channels by raveling
Examples of the "Fire-Flood" Response from Southern California
Exceptionally dramatic post-fire responses of hillslopes have been documented during winter storms in watersheds that neighbor the Santa Clara River. After the 1985 Wheeler fire, just north of the Santa Clara River watershed, in the Ventura River basin, dry ravel contributed large volumes of sediment to the channel. The area then experienced two moderate-magnitude rainstorms. After the first post-fire rainstorm, channels aggraded (i.e. material was deposited) by 20 to 50 cm, with an estimated 90% of the channel deposits formed by post-fire sediment delivery from slopes. Roughly 90% of the deposits were then scoured away during the second post-fire rainstorm. Hence fluvial transport of the post-fire deposits was extremely effective, even though the storm flows were of only moderate magnitude.
Field studies in the San Dimas Experimental Forest of the San Gabriel Mountains indicate that fires can lead to ten- to hundred-fold increases in sediment transport rates in California chaparral. Most of these increases can be attributed to increases in dry raveling rates, both during and immediately after fires, and increases in sediment delivery along post-fire rills. The largest sediment transport events occurred in the first post-fire storms. Debris torrents in channels were surprisingly abundant, given the modest intensities of the storms that followed many of the fires. These debris torrents effectively conveyed sediment in post-fire channel deposits long distances down channel, much as stream flow did during the modest storms that followed the Wheeler fire.
Text and graphics adapted from the Santa Clara River Parkway Floodplain Restoration Feasibility Study: Assessment of Geomorphic Processes.