No abstract

Following its emergence in Wuhan, China, in late November or early December 2019, the SARS-CoV-2 virus has rapidly spread globally. Genome sequencing of SARS-CoV-2 allows reconstruction of its transmission history, although this is contingent on sampling. We have analyzed 453 SARS-CoV-2 genomes collected between 20 February and 15 March 2020 from infected patients in Washington State, USA. We find that most SARS-CoV-2 infections sampled during this time derive from a single introduction in late January or early February 2020 which subsequently spread locally before active community surveillance was implemented.

Relationships between large woody debris (LWD) and pool area or pool spacing varied with channel slope and channel width for streams in second-growth forests in northwest Washington. Pool spacing (expressed as the number of channel widths between pools) decreased as number of woody debris increased in both moderate-slope (0.02 < slope < 0.05) and low-slope (0.001 < slope < 0.02) channels, but the relationship was stronger in moderate-slope channels. Percent pool was also more strongly correlated with woody debris volume in moderate-slope channels than in low-slope channels. Multiple-regression analyses showed that pool spacing and percent pool were correlated with an interaction term between LWD abundance and channel slope, suggesting that the influence of LWD on pool formation changes with channel slope. Analysis of pool-forming mechanisms indicated that low-slope channels are less sensitive to LWD abundance because pools are formed by mechanisms other than LWD when LWD abundance is low. Si/.e of LWD that formed pools increased with increasing channel width, but was not related to channel slope. Percent gravel (proportion of the bed in patches of gravel 16-64 mm in diameter) was best explained by channel slope and channel width, and there was no significant relationship between woody debris and percent gravel. A regression between median particle size of sediment on the stream bed and basal shear stress showed that the relationships among percent gravel, channel width, and channel slope are adequately explained by the channel's capacity to transport particles of various sizes.Large woody debris (LWD) in streams forms tain suitable habitat characteristics for at least 50 pools (e.g., review in Bisson et al. 1987; Mont-years (Grette 1985; Andrus ct al. 1988) and that gomery et al. 1995) and retains sediment and par-in-channel debris would not reach old-growth levticulate organic matter (e.g., Bilby 1981; Megahan els for about 250 years (Murphy and Koski 1989). 1982;Bilby and Ward 1989). Via these physical Where riparian forests have been removed or alfunctions, large woody debris can influence the tered, these studies taken together describe a seedistribution and abundance of juvenile salmonids nario of decreased numbers and area of pools, dein streams. For example, pools that are associated creased retention of sediment and particulatc orwith LWD are preferred habitats for various age-ganic matter, simplification of stream fish cornclasses of juvenile coho salmon Oncorhynchus ki-munities, and lower survival of juvenile salmonids. sutch, cutthroat trout O. clarki, and steelhead O.Studies comparing old-growth and clear-cut mykiss (Bisson et al. 1988). Higher volumes of streams provide a useful description of the effects LWD have been correlated with higher densities of riparian logging on woody debris and salmonid of juvenile salmonids in winter (Murphy et al. habitat. However, most salmonid streams in the 1984), and a greater diversity of pool and riffle Pacific Northwest no longer flow through oldtypes may be as...

Following its emergence in Wuhan,