Environmental Extremes

By Special Contributing Author: Dennis Jackson, Hydrologist

Extreme Floods

Large, infrequent floods play an important role in shaping stream channels through the erosion, transport, and deposition of bed materials. Floods with recurrence intervals of 100 years or more can result in major channel changes, and several decades may be required to re-establish an equilibrium approaching preflood conditions. Some features produced by large floods may last longer than the recurrence interval of the event, implying that large floods may be responsible for specific aspects of valley-floor formation rather than simply acting as disturbance events (FISRWG 1998).

The effectiveness of large floods in shaping channel morphology may vary depending on stream size and position in the drainage network as well as land cover. Recent changes to the land such as logging or flood control development may greatly change natural flooding processes. Floods provide many beneficial services such as depositing nutrients and sediments onto the surrounding floodplain, transporting and rearranging large woody debris within the channel, cleaning and scouring gravels in streams, recharging floodplain aquifers, and dispersing propagules of riparian vegetation. These processes are necessary for maintaining a healthy riparian ecosystem and the instream conditions required by salmonids.

Conversely, flood events can be catastrophic for salmonid populations, especially those populations that are experiencing population declines and have limited remaining habitat. Severe flows can destroy redds and prevent successful spawning of an entire reproductive year class of coho salmon (Oncorhynchus kisutch). The ability of steelhead trout (O. mykiss) to spawn over multiple years helps mitigate the drastic effects of losing an entire year class, but in depressed populations the loss of a single cohort is highly detrimental. Surveys of coho and steelhead on Gazos, Waddell and Scott Creeks from 1992–2003 document the effects of high floods (Smith 2002). The years of 1992, 1995, and 1998 experienced higher than normal flows and resulted in fewer than expected juvenile coho (Smith 1998). This effect continued through 2001 when the small 1998 cohort returned to spawn and produced few offspring even though it was not a flood year (Smith 2001). Steelhead densities were not affected, which may be explained by spawn timing (Smith 1998). Coho spawn in January and February, months that experienced the strongest storm events, while steelhead spawn later and probably avoided the devastating flows. Although the 1998 flood proved disastrous to coho populations, habitat changes, especially the addition of large wood, resulted in lasting improvements to habitat (Smith 2002). While flood events are a necessary force in maintaining habitat, salmon populations in serious decline may not be able to withstand such "natural disasters."

Fifty years of the maximum annual flood series for the stream gauge in Pescadero Creek near Pescadero have been recorded and can be graphed. The flood peaks can be presented as ratios of the mean annual flood. Scaling by the mean annual flood allows for a comparison with other locations. The maximum annual flood peak for the 15 year period from 1937 to 1951 can be estimated using the record of the San Lorenzo River near Big Trees gauge. The flood of February 1998 was the largest flood during the combined 65 years of record. That event is estimated to have a return period of about 84 years. The 1955 event was the second largest flood and is estimated to have a return period of about 31 years.

Fifty years of maximum annual flood events at the Pescadero Creek near Pescadero gauge and 15 years of estimated annual peaks are shown as ratios of the mean annual flood for the station. The 1998 event has an estimated return period of 84 years.

An analysis of United States Geological Survey (USGS) records show that the streambed at the Pescadero Creek near Pescadero gauge aggraded about 1.5 feet after the 1955 storm (Jackson 2003). The streambed lowered back to its pre-flood level in just a few years. The 1998 storm did not appear to change dramatically the channel at the gauge, even though it was the largest event on record. The effect of the 1982 storm is unknown because the records were not available.

The best regional example of a large flood that caused significant widespread changes to its channel and floodplain is the December 1964 flood on the Eel River in Humboldt County. No flood event comparable to the December 1964 Eel River flood has occurred in the Santa Cruz Mountains during the last 65 years.

Debris flows are an important mechanism for moving sediment from the hillslopes to the channel network. The frequency of debris flow occurrence increases with rainfall intensity. Widespread occurrence of debris flows are associated with very intense rainfall that occurs after the soil moisture deficit from the previous dry season has been replenished. Numerous debris flows tend to occur at the same time that extreme floods occur. Wilson and Jayko (1997) have prepared preliminary maps showing rainfall thresholds for debris activity in the San Francisco Bay region for use with geographic information system (GIS) mapping and analyses. These data (for San Mateo and Santa Cruz Counties only) are available for download from this Web site in the SRP GIS Data Inventory.

Droughts

Below-average precipitation and runoff can have significant effects on streams and watersheds. The influence of drought on watershed processes is not well documented; however, it is likely that droughts affect the input of nutrients and large woody debris to stream channels (FISRWG 1998). Within the stream channel, low flows can limit the available habitat and allow water temperatures to warm, stressing fish or creating thermal barriers that block migration. The 1977 drought is thought to have caused the demise of the already depressed population of coho in the San Lorenzo River (NOAA 2001), and today Scott Creek contains the southern most coho population.

Droughts are natural environmental events that provide ecosystem benefits. A potential benefit of drought is that it provides the opportunity for establishment of riparian vegetation within the active stream channel, which in turn can stabilize channel features, dissipate hydraulic energy, and collect sediment when flows rise again (Spence et al. 1996). However, water diversion and ground water pumping may exacerbate the effects of drought, creating drier and more deadly instream conditions than would naturally occur.

References

Federal Interagency Stream Restoration Working Group (FISRWG). 1998. "Stream Corridor Restoration: Principles, Processes, and Practices." Federal Interagency Stream Restoration Working Group (FISRWG). GPO Item No. 0120-A; SuDocs No. A 57.6/2:EN 3/PT.653. ISBN-0-934213-59-3. View on-line document.

ackson, D. 2003. "Pescadero Watershed Assessment, Analysis of the Streambed Elevation at the USGS Stream Gage." Environmental Science Associates.

NOAA National Marine Fisheries Service (NMFS). 2001. "Status Review Update for Coho Salmon (Oncorhynchus kisutch) from the Central California Coast and the California Portion of the Southern Oregon/Northern California Coasts Evolutionarily Significant Units." Southwest Fisheries Science Center, Santa Cruz Laboratory. 43 pp. View on-line document.

Smith, J.J. 1998. "Distribution and Abundance of Juvenile Coho and Steelhead in Gazos, Waddell and Scott Creeks in 1998." Department of Biological Sciences, San Jose State University. Unpublished report, 23 pp.

Smith, J.J. 2001. "Distribution and Abundance of Juvenile Coho and Steelhead in Gazos, Waddell and Scott Creeks in 2000." Department of Biological Sciences, San Jose State University. Unpublished report, 20 pp.

Smith, J.J. 2002. "Distribution and Abundance of Juvenile Coho and Steelhead in Gazos, Waddell and Scott Creeks in 2002." Department of Biological Sciences, San Jose State University. Unpublished report, 20 pp.

Spence, B.C., G.A. Lomnicky, R.M. Hughes, and R.P. Novitski. 1996. "An Ecosystem Approach to Salmonid Conservation." ManTech Environmental Research Services Corp. TR-4501-96-6057. (Available from the National Marine Fisheries Service, Portland, Oregon).

Wilson, R.C., and A.S. Jayko. 1997. "Preliminary Maps Showing Rainfall Thresholds for Debris-Flow Activity, San Francisco Bay Region, California." US Geological Survey. Open-File Report 97-745 F. View on-line source.

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