Episodic Suspended Sediment Transport and Elevated Polycyclic Aromatic Hydrocarbon Concentrations in a Small, Mountainous River in Coastal California

Abstract

Suspended sediment and organic contaminants were measured during a period of 2 years in the San Lorenzo River, central California, which discharges into the Pacific Ocean within the Monterey Bay National Marine Sanctuary, in an effort to quantify the potential environmental effects of storm events from a steep, mountainous coastal urban watershed. Most suspended sediment transport occurred during flooding caused by winter storms; 56% of the total sediment load for the 2-year study was transported by the river during one storm event in January 2010. Concentrations of polycyclic aromatic hydrocarbons can exceed regulatory criteria during high-flow events in the San Lorenzo River, and total annual polycyclic aromatic hydrocarbon load was on the order of 10 kg in water year 2010. These results highlight the importance of episodic sediment and contaminant transport in steep, mountainous coastal watersheds and emphasize the importance of understanding physical processes and quantifying chemical constituents in discharge from coastal watersheds on event-scale terms. Published 2012. This article is a U.S. Government work and is in the public domain in the USA.

Sediment yields from small, steep coastal watersheds of California

“…14). These results are consistent with longer records from larger rivers in the southern California (Warrick and Milliman, 2003) and other small watersheds in semi-arid regions throughout the world (e.g., Coppus and Imeson, 2002;Rustomji and Wilkinson, 2008;Duvert et al, 2011Duvert et al, , 2012Conaway et al, 2013;Gray et al, 2015), which show that infrequent events produce the majority of water and sediment discharge from these watersheds.…”

“…These headwater studies suggest that there are fundamental differences in the frequency and magnitude of sediment discharge in the smaller, low-order drainage basins compared to the larger, high-order drainage basins (Walling, 1974;Graft, 1988). For example, sediment discharge from small watersheds is commonly ephemeral, and the majority of the long-term sediment discharge occurs during and immediately following infrequent heavy precipitation when suspended-sediment concentrations can rise to grams or hundreds of grams per liter (Tropeano, 1991;Coppus and Imeson, 2002;Milliman and Kao, 2005;Galewsky et al, 2006;Mano et al, 2009;Grodek et al, 2012;Conaway et al, 2013). Additionally, several factors can exacerbate erosion within and sediment yields from these small watersheds, including: ground shaking from seismic activity (Dadson et al, 2004;Hovius et al, 2011); vegetation clearing and sediment release after wildfire (Shakesby and Doerr, 2006;Malmon et al, 2007;Lamb et al, 2011;, glacial processes (Hinderer et al, 2013); shifts in climate (Galewsky et al, 2006); geomorphic change of the watershed landscape (Nearing et al, 2007;Nadal-Romero and Regüés, 2010); human-derived disturbances from land use and channel alterations (Trimble, 1981(Trimble, , 1997Owens et al, 2010;de Vente et al, 2011); and combinations of these effects (Madej and Ozaki, 1996;Pinter and Vestal, 2005;Warrick and Rubin, 2007;García-Ruiz et al, 2013).…”

A regime shift in sediment export from a coastal watershed during a record wet winter, California: Implications for landscape response to hydroclimatic extremes

“…The 1973-1982 SSC measurements indicated annual sediment yield ranging from 1.93 t/km 2 in 1977 to 5580 t/km 2 in 1982. To better constrain the episodic nature of sediment and organic-pollutant flux, Conaway et al (2013) measured SSC near station 11161000 over water-years 2009 (WY09, a dry year with rainfall 73% of average and no floods), and 2010 (WY10, a year with normal rainfall, 100% of average, and with a 5-year flood). The higher value, 931 t/km 2 / year, was used subsequently in regional marine sediment budgets (Best and Griggs, 1991;Eittreim et al, 2002).…”

“…Best and Griggs (1991) used those measurements to infer an average annual sediment yield from the San Lorenzo watershed of 931 t/km 2 , although their measurements at station 11160500 in 1986 implied a yield of 553 t/km 2 . Conaway et al (2013) calculated annual sediment loads of 7000 t and 41 000 t for WY09 and WY10, respectively, equivalent to yields of 24 t/km 2 and 138 t/km 2 . To better constrain the episodic nature of sediment and organic-pollutant flux, Conaway et al (2013) measured SSC near station 11161000 over water-years 2009 (WY09, a dry year with rainfall 73% of average and no floods), and 2010 (WY10, a year with normal rainfall, 100% of average, and with a 5-year flood).…”

Post-fire Response of Little Creek Watershed: Evaluation of Change in Sediment Production and Suspended Sediment Transport

“…River watershed neighboring Scotts Creek to the east was not affected by the Lockheed fire and displayed reverse hysteresis for the same storm event, which is likely due to slower release of sediment from large erosion features such as landslides and eroded banks (Conaway et al, 2012;Williams, 1989;Bogen, 1980). The SF monitoring station also observed an early spike in SSC, but UNF SSC was closely associated with flow and does not appear to display a hysteresis effect.…”