Chehalis Gap

The Chehalis Gap is a gap in the Coast Range of Washington State between the southernmost foothills of the Olympic Mountains called the Satsop Hills, and the Willapa Hills.^[3][4][5][6]

The gap is a major geographic feature of the northwestern United States.^[7] Other geographic features in the gap include Chehalis River, Grays Harbor, and Grays Harbor National Wildlife Refuge in its estuary.^[3][8] U.S. Route 12 runs through the gap from Elma near Capitol State Forest to Aberdeen on Grays Harbor near the coast, paralleled by the former U.S. Route 410 and Puget Sound and Pacific Railroad.

Formation

The broad valley in the gap is considered outsized for the river it now carries, the Chehalis River, theorized to be due to its enlargement during the Pleistocene ice age when it carried meltwater from mile-high glaciers in what is now Puget Sound.^[9][10][11]

Climate and meteorology

The gap is responsible for climate and meteorological effects in Puget Sound region, including the Puget Sound Convergence Zone,^[12][13] marine push,^[4][14] diurnal wind circulation (sea breeze),^[15][16] and the relatively high precipitation compared to areas in the Olympic Mountains' rain shadow. Without the gap, the climate would be more like that of semiarid Eastern Washington.^[17] The cities closest to the gap, like Olympia, get considerably more rain than those farther away, like Seattle.^[18] Air quality in the Puget Sound region is also affected by availability of fresh Pacific air from the Chehalis Gap to recharge the air which otherwise can stagnate and trap air pollution from industry and vehicles in the Seattle metropolitan area.^[19]

There are indications that a low level jet may be associated with the Chehalis Gap.^[20]

Water

Municipal water supply in the Puget Sound metropolitan area is dependent on the rainfall which is in part attributed to the presence of the Chehalis Gap. The unique climate and topography of the Puget Sound region mean that water from weather systems from the Pacific, particularly winter storms, falls preferentially in southwestern facing, elevated areas exposed to moist Pacific air, especially where the gap causes an "anti" rain shadow.^{[21][22][5][23]} Seattle, and Everett and Bremerton all have municipal reservoirs in the hills surrounding each city, sometimes tens of miles (kilometers) away. Seattle and Everett's primary reservoirs are Chester Morse Lake and Spada Lake Reservoir in the Cascade Mountains foothills, respectively.^[22] Bremerton's reservoir is Union River Reservoir in the Blue Hills, but filled by local rainfall rather than snowmelt like most large Puget Sound cities.

See also

• Mountain-gap wind

References

Notes

Sources

• The Great Northwest: The Story of a Land and Its People, American West Independent Publishing, 1973, p. 38, ISBN 9780910118323, Because of the Chehalis Gap and the Olympic Mountains, precipitation decreases markedly from southwest to northeast through the Puget Sound. The gap allows moist winds to enter the southern part of the sound, giving Olympia fifty inches of annual rainfall. 

• Bikos, Dan; Braun, Jeff (2007), "Puget Sound Convergence Zone", Satellite Interpretation of Orographic Clouds (talking points), NOAA Regional and Mesoscale Meteorology Branch—Virtual Institute for Satellite Integration Training (VISIT) / Colorado State University, p. Slide 53, Along the coast, west of Seattle, westerly winds off the Pacific Ocean have a relatively difficult time crossing the Olympic Mountains. One of the easiest paths for the air to follow inland is through the Strait of Juan de Fuca, into the Georgian Basin and then south through the channels of Puget Sound. Air flow further south, however, will often traverse the lower coastal range, and move through the Chehalis Gap to the fjords, finally pushing north into Puget Sound. 
• Brewer, Matthew C.; Mass, Clifford F. (2014), "Simulation of Summer Diurnal Circulations over the Northwest United States" (PDF), Wea. Forecasting (Department of Atmospheric Sciences, University of Washington) (29): 1208–1228 
• Casad, C. Cliff; Cunningham, John W. (May 2, 1957), "Construction of the Union River Dam at Bremerton, Wash.", Journal (American Water Works Association) (American Water Works Association), JSTOR 41254984 
• Puget Sound Georgia Basin air quality, U.S. Environmental Protection Agency, 2011, The atmospheric characteristics and meteorological conditions that dictate the transport, dispersion and deposition of airborne chemicals are strongly influenced by the topography of the Basin. The Georgia Basin is dominated by the west-to-east axis of the Lower Fraser Valley, the northwest-to-southeast Strait of Georgia and the west-to-east Strait of Juan de Fuca. This is in contrast to the well-defined north-south alignment of the Puget Sound airshed. The southern portion of the Puget Sound airshed is affected by air flowing through the Chehalis Gap from the Pacific Ocean. Both airsheds are influenced by the flow of air up and down “tributary” valleys and mountain slopes. 
• Foster, Tom (2008), "The Pleistocene Epoch", The Ice Age Floods (hugefloods.com) (Pasco, Washington), retrieved 2014-11-17 
• Kitsap County Initial Basin Assessment (PDF), Kitsap Public Utility District, October 1997, pp. 2–3, Winter storms generally approach the County from the southwest. The southwestern portion of the County receives relatively high winter rainfall from storms which enter the area through a topographic gap between the Olympic Mountains and the Black Hills. 
• Mass, Cliff (February 23, 2012), "The Spada Lake Anomaly", Cliff Mass Weather Blog 
• Mass, Clifford F.; Brewer, Matthew C. (2013), On the Existence and Impacts of Summertime Northwest U.S. Weather Phenomena (PDF), p. 9, Joint Fire Science Program Project ID Number: 13-3-01-4 
• McShane, Dan (January 18, 2011), "Chehalis River, a Bit Different Than Other Western Washington Rivers", Reading the Washington Landscape, The Chehalis follows the former valley of a much larger river. During the maximum ice extent during the last glacial period melt water from the Puget lobe ice sheet drained to the ocean via what is now the Chehalis River. The river that carved that valley was a much bigger river than the Chehalis. 
• Overland, James E.; Walter, Bernard A. (Jr.) (January 1983), Marine Weather of the Inland Waters of Western Washington, Pacific Marine Environmental Laboratory—National Oceanic and Atmospheric Administration, p. 57, NOAA Technical Memorandum ERL PMEL-44, The figure-eight-shaped holograhp at Olympia is interesting because Olympia is situated at the boundary between two regional sea breeze circulations; it is influenced by the Chehalis Gap to the south and Puget Sound to the north. 
• Pool, Steve; Sistek, Scott (2005), Somewhere, I was right: why Northwest weather is so predictably unpredictable, Classic Day/Peanut Butter Press, p. 73, ISBN 9781598490008, At night, we'll get a push of marine air from the ocean, through the Strait of Juan de Fuca, the gaps in the Oregon Coastal Range and the Chehalis Gap, which is the break in the hills between the Olympic Mountains and the Coastal Range. 
• Renner, Jeff (2005), Mountain Weather (First ed.), The Mountaineers Books, p. 202, ISBN 9781594851629, The Puget Sound Convergence Zone ... develops after the passage of a cold front, as high pressure building along the coast produces coastal winds ... When the onshore flow of air runs into the Olympic Mountains, it splits, some flowing through the Strait of Juan de Fuca to tne north, some through the Chehalis Gap to the south. The Cascades present an almost insurmountable barrier to the east, so some of the air moving through the strait is forced south into Puget Sound, while some of the air moving through the Chehalis Gap is forced north. These two opposing currents collide, forcing some of the air to rise, and then it is pushed into the Cascades by the winds passing over the Olympics. 
• Saunders, Edwin J. (1913), "Physiography of the Cowlitz, Nisqually, Puyallup, White, Green and Cedar Drainage Basins", in Henshaw, Fred F.; Parker, Glenn L., Congressional Serial Set: Water-Supply Paper 313, U.S. Government Printing Office, p. 20, [T]he Olympic Mountains ... extend southward along the coast from the Strait of Juan de Fuca, gradually becoming lower to the south and west until they give place to the broad, low gap occupied by the Chehalis River and Grays Harbor. South of the gap the mountains are of rather low relief... 
• "Climate of the Puget Sound Basin", Smalley's Magazine, 1889: 13–14, There are two main ocean winds: The northwest or dry wind, blowing chiefly in summer, and blowing up the Straits of Fuca, the Chehalis and Columbia River: this makes our cool summers; then there is the southwest or west wind, warm and moist, which blows diagonally across the gaps, made in the series of mountains called the Coast Range, by the Straits of Fuca, and the Chehalis and Columbia Rivers. Nearly all the rain falling in Washington Territory is brought from the ocean in storm clouds by the southwest wind. .. Were there no gaps in the Coast Range from the Straits south to California, but a solid mountain wall over four thousand feet high, the climate of Puget Sound would be like that of Eastern Washington. ... [W]et winter winds reach the Sound through the Chehalis gap. 
• Trandum, William I. (May 1, 2013), "The Chehalis Gap, The Coriolis effect, and the Aleutian Low", Key Peninsula News, Our fall, winter, and springtime weathers here on the Key Peninsula pretty much come from the same place. The Chehalis Gap. And no, you can’t buy jeans there. This gap is the space between the southernmost of the Olympic Mountains and the Willapa Hills to the south. The Gap is an opening that allows winds and weather systems to come ashore from the Pacific Ocean where they are born. 
• Terrain of the Pacific Northwest, UW Atmospheric Sciences, 2000, There is a gap in the Coastal Range at 47N called the Chehalis Gap where the Chehalis River runs into the Pacific. 
• Weaver, Charles E. (1916), "The Tertiary formations of western Washington", Washington Geological Survey (Olympia: State of Washington) 
• Williams, Hill (2002), The Restless Northwest: A Geological Story, Washington State University Press, pp. 111–112, ISBN 9780874222500, That is why the Chehalis gap of today seems oversized for the river occupying it. It once carried huge volumes of meltwater. The outwash accumulating ahead of the advancing ice sheet filled and leveled the Puget lowland. ...Meltwater again formed huge, temporary lakes that drained to the sea through the Chehalis gap.