Tidal_PAD_V1_Sec4.pdf - Snohomish County PUD

Snohomish County PUD –Pre-Application Document
4 – Description of Existing Environment
shorelines and channels, while mud and mixed fine sediments derived from biogenic processes
and the discharge of numerous rivers blanket the seafloors of the deeper benthos and
the sheltered near-shore environments (DON 2006; EPRI 2007; Gustafson 2000; Palsson
et al. 2003).
As the drainage endpoint for a watershed of more than 33,000 km 2 , and incorporating an average
of 3.4 trillion liters of fresh water into its volume each day, Puget Sound is a marine body that is
further characterized by estuarine properties (Gelfenbaum et al. 2006; Gustafson et al. 2000).
Salinities within the basin range from 5 to 42 ppt, while water exported from the Sound through
the Strait of Juan de Fuca average 31 ppt (DON 2000, 2006). Stratification of the water column
is typical of an estuary, with denser, saltier, inflowing waters of oceanic origin overlain by
fresher waters derived from the basin flowing seaward (DON 2006; Gustafson et al. 2000;
NOAA and WSDE 1997; Taylor 2000). Stratification can be weak due to intense tidal mixing
but generally tends to increase from west to east and varies seasonally, with upwelling occurring
at sill structures, specifically those adjacent to the Tacoma Narrows (DON 2006; Gustafson et al.
2000). Sea surface temperatures average 10-12°C (50-54°F) but can occasionally be as low as
3°C (37°F) during the winter and as high as 23°C (73°F) in enclosed areas during the summer
(DON 2000, 2006). The thermocline is typically found between 2 and 20 m of water depth
(DON 2006; Dunne et al., 2002).
In addition, Puget Sound is subject to a complex current regime, driven primarily by tidal
oscillation and to a lesser extent by freshwater flux and wind (DON 2006; Gustafson et al.
2000). The large tidal exchanges that occur basin-wide generate significant flow velocities that
range from typical values of 25-80 cm/s to more than 400 cm/sec in restricted channels. These
exchanges reduce the flushing rate of nutrients and result in consistently elevated levels of
nitrates, phosphates and other bio-essential compounds (DON 2006; Gustafson et al. 2000).
4-4 January 31, 2008