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Notice to campers staying at Coast Campground:
The Coast Trail between the Hostel and Coast Campground is closed weekdays while salvage operators attempt to remove a wrecked boat from Santa Maria Beach. The potable water sources at Coast Campground have been shut off. More »
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2012 Harbor Seal Pupping Season Closures
From March 1 through June 30, an annual closure of Drakes Estero and certain beaches of Tomales Bay is implemented to protect harbor seals during the pupping season. Please avoid disturbing seals to ensure a successful pupping season. More »
Giacomini Wetland Restoration Project: Restoration: What's the Long-Term Future of the Restored Wetlands?: Precipitation, Run-Off, and Sedimentation
By their very nature, wetlands are dynamic interfaces between sea and land. Over geologic time, the position or location of marshes in the estuarine landscape have shifted in response to long-term changes in sea level (i.e., advancing with warming of temperatures and retreating with cooling of temperatures) and trends in sediment transport and deposition from the source watershed. Most of the terrestrially derived sediment supplied to coastal areas comes during storm events in the form of suspended sediment in run-off. Therefore, changes in precipitation and run-off may dictate to some extent how viable coastal systems remain in the face of a greatly accelerated sea level rise rate. On average, recent projections show little change in total annual precipitation in California or in the Mediterranean pattern of rainfall, with most falling during winter from north Pacific storms (California Climate Change Center 2006). However, one climate model does predict slightly wetter winters (+38 mm/yr), while another predicts slightly drier winters with a 10 to 20 percent decrease in precipitation (-157 mm/yr; Cayan et al. 2006 in Dukes and Shaw 2007; California Climate Change Center 2006). Sacramento Delta and San Francisco Bay wetlands may be subject to greater change than coastal systems due to decreases in the amount of snowpack in the Sierra Nevada Mountains and associated effects on timing and volume of stream and river flow during the winter and spring and freshwater inflows to downstream systems. Wetter winters and springs with less snowpack translates into higher, more concentrated periods of runoff that could increase erosion within the upper watersheds and downstream transport of sediment (San Francisco Bay Joint Venture 2008). The effects of wetter, warmer winters may be less magnified in coastal wetlands than in those fed by higher mountain ranges, because precipitation from source watersheds does not typically fall as snow, but prolonged wet periods could change the type of downstream vegetation communities or even wetland types (i.e., brackish versus salt marsh), although, along the coast, this increased freshwater influence would be countered by sea level rise. Based on results from recent computer modeling by KHE (2009), sea level rise in the Giacomini Wetlands would be accompanied by dramatic increases in salinity that could also change marsh dynamics by converting brackish and even freshwater marsh communities to salt marsh ones. The long-term resilience of coastal systems will be dependent to a large degree on continued sediment inputs from upstream portions of the watershed. Without these inputs, marshplains will sink below the steadily creeping rise in sea level and convert to non-vegetated types of wetlands such as intertidal mudflat or subtidal or open water habitat (San Francisco Bay Joint Venture 2008). Systems that are already sediment-starved to some degree such as the San Francisco-Sacramento Delta estuary—which has lost much of its sediment supply to upstream dams—are in greater danger from sea level rise than those in systems with either natural or even unnaturally higher rates of downstream sediment delivery (Orr et al. 2003). Somewhat ironically, the potential for higher amounts of rainfall and associated run-off could help to counter the effects of sea level rise for some coastal ecosystems, particularly those where snowpack currently mediates the intensity and timing of run-off. Within Tomales Bay, sediment delivery rates are believed to have dropped from peak levels between the late 1800s and early 1900s, but still to be higher than what occurred prior to agricultural development and lumber harvesting. The highest rates of sedimentation at the southern end of Tomales Bay occurred between 1861 and 1908 (PWA et al. 1993), but, apparently for other portions of the Bay, the largest sediment influx to the bay seems to have been in the decades between about 1930 and 1960 (Rooney and Smith 1999). Since 1957, sedimentation rates have dropped, slowing the pace of Bay infilling (Rooney and Smith 1999, PWA et al. 1993). Between 1861 and 1931, sedimentation accumulation rates within Tomales Bay averaged 94 tons per square kilometer per year, increasing to 357 tons/k2/yr between 1931 and 1957 and decreasing to 101 tons/k2/yr between 1957 and 1994 (Rooney and Smith 1999). Sedimentation resulting from erosion induced by agricultural development of the watershed is likely to have been highest first at the southern end or mouth of the Bay, with the rapidly accreting delta and construction of the Giacomini Ranch levees eventually shifting the primary area still available for sediment deposition downstream and more into the Bay itself. While watershed sediment contribution has decreased in the last 50 years, Tomales Bay continues to become shallower through sediment inputs. The present sedimentation rate in the bay, based on both bathymetric changes since 1957 and sediment yield measurements, is estimated at about 0.04 to 0.08 in/yr (Smith and Hollibaugh 1998). Without levees to restrict access of flood flows to marshplains, some portion of the sediment yield from the upper portion of the Lagunitas Creek watershed would be expected to deposit on the newly restored Giacomini Wetlands floodplains. Based on estimates of sediment loads conveyed by lower Lagunitas Creek during higher streamflow events, the Giacomini Wetlands floodplains could trap or retain as much as 19 percent of the sediment loading in Lagunitas Creek during a 2-year flow event or slightly more than 9,500 tons/day. This sediment deposition would work to counter the effects of sea level rise in the Giacomini Wetlands, particularly for the southern portion of the system, where hydraulic modeling suggests that most of the conveyed sediment would drop out of suspension (KHE 2006). This continued sediment input would continue to build elevations in the southern portion of the Giacomini Wetlands, countering sea level rise pressures, at least in this area, but the northern portion would be dependent on sediment subsidies from smaller drainages such as Tomasini and Fish Hatchery Creeks. This could potentially lead to an exacerbation in the sharply deltaic or wedge-shaped topography of the wetlands: southernmost elevations currently range between 10 and 11 feet NAVD88, while northernmost elevations range only between 3 and 5 feet NAVD88. Another factor that may increase resilience of coastal marshes to sea level rise, particularly marshes in sediment-starved systems, is increase in surface elevation associated with organic, rather than mineral, matter. A recently published study by Langley et al. (2009) found that the elevation of salt marshes increased in areas simply through an increase in root biomass, even when where the contribution of mineral sediment deposition was negligible. Not only is surface elevation increased through increases in root biomass, but also through accumulation of undecomposed plant matter or peat, and systems exposed to frequent sedimentation or flooding events are likely to develop more layers or strata of peat that contribute to increases in marsh surface elevation. In one study of 55 wetlands in the Gulf of Mexico and three (3) wetlands in Rhode Island, Turner and colleagues found that organic matter accumulation represented the "dominant influence" on vertical accretion in salt marshes (Turner et al. 2000), which supported earlier findings published by Callaway and colleagues demonstrated a strong statistical relationship between organic matter accumulation and vertical accretion rates in 5 Gulf of Mexico wetlands (Callaway et al. 1997).
What's the Long-Term Future of the Restored Wetlands? restoration includes: |
Did You Know?
Northern elephant seals (Mirounga angustirostris) began breeding at Point Reyes in 1981 after being absent for over 150 years. The population breeds at terrestrial haul out sites at Point Reyes Headland, one of only eleven mainland breeding areas for northern elephant seals in the world. More...