[Show abstract][Hide abstract] ABSTRACT: The Mediterranean climate of southern California is marked by droughts and extreme precipitation events. Here we use elemental variations generated by scanning X-ray fluorescence (XRF) to identify droughts and floods in recently deposited (1755–2008) sediments of Santa Barbara Basin (SBB) from box core SPR0901-04BC. The first principal component (PC1) of the scanning XRF elements has high loadings for elements associated with the lithogenic component of SBB laminae couplets, while the second (PC2) is associated with biogenic components. We interpret PC1 as a proxy for river runoff and PC2 as a proxy for marine productivity. High values of PC1 are associated with El Niño events and positive (warm) phases of the Pacific Decadal Oscillation (PDO), while low values of PC2 are associated with El Niño events and negative (cool) phases of the PDO. Droughts such as the 1934–40, 1949–56, and 1989–91 events coincide with low PC1 values. In addition to distinguishing interannual and decadal variability in the elemental composition of SBB sediments, several historic floods can be recognized including a gray flood layer associated with the 1861–62 flood, and a peak in PC1 associated with the flood following the St. Francis Dam disaster in 1928.
Quaternary International 02/2015; DOI:10.1016/j.quaint.2015.01.026 · 2.13 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Channel incision occurs in the “Anthropocene,” where natural river processes and climate variation increasingly interact with human activity. Causes of “Anthropocene” incision include landuses that change the ratio of discharge to sediment load, lower baselevels, or human activities that otherwise alter fluvial systems, such as channelization. This paper reports a field study of an alluvial channel incised into valley fill within the northern Coast Ranges of California. At this site, channel slope adjustments associated with incision, indicated by bank heights of ∼5–8 m, increased transport capacity and excess shear stress by over 20%. The incision exposed Holocene valley fill in eroding channel banks. Results of field surveys enabled development of a dimensionless metric “relative incision,” ht/de, that quantifies the ratio of terrace height (ht) relative to effective flow depth (de). The ratio is predicted to be near a threshold value of 1.0 in stable alluvial channels; in incised alluvial channels the ratio is predicted to exceed 1.0. Further application and testing will provide information to aide in managing incised systems where complex feedbacks in coupled human–landscape systems may promote or dampen incision.
[Show abstract][Hide abstract] ABSTRACT: Atmospheric rivers (ARs) have, in recent years, been recognized as the cause of the large majority of major floods in rivers all along the U.S. West Coast and as the source of 30%–50% of all precipitation in the same region. The present study surveys the frequency with which ARs have played a critical role as a common cause of the end of droughts on the West Coast. This question was based on the observation that, in most cases, droughts end abruptly as a result of the arrival of an especially wet month or, more exactly, a few very large storms. This observation is documented using both Palmer Drought Severity Index and 6-month Standardized Precipitation Index measures of drought occurrence for climate divisions across the conterminous United States from 1895 to 2010. When the individual storm sequences that contributed most to the wet months that broke historical West Coast droughts from 1950 to 2010 were evaluated, 33%–74% of droughts were broken by the arrival of landfalling AR storms. In the Pacific Northwest, 60%–74% of all persistent drought endings have been brought about by the arrival of AR storms. In California, about 33%–40% of all persistent drought endings have been brought about by landfalling AR storms, with more localized low pressure systems re- sponsible for many of the remaining drought breaks.
Journal of Hydrometeorology 11/2013; 14(6):1721-1732. DOI:10.1175/JHM-D-13-02.1 · 3.57 Impact Factor
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