The Chehalis Gap is a gap in the Coast Range of

Washington state Washington (), officially the State of Washington, is a state in the Pacific Northwest region of the Western United States. Named for George Washington—the first U.S. president—the state was formed from the western part of the Washingto ...

between the southernmost foothills of the

Olympic Mountains The Olympic Mountains are a mountain range on the Olympic Peninsula of the Pacific Northwest of the United States. The mountains, part of the Pacific Coast Ranges, are not especially high – Mount Olympus is the highest at ; however, the east ...

called the

Satsop Hills The Satsop Hills are foothills of the Olympic Mountains in Mason County, Washington north of Matlock, Washington, between Wynoochee Lake to the west and Lake Cushman to the east. Geography The Satsop River, with a watershed, rises in the h ...

, and the

Willapa Hills The Willapa Hills is a geologic, physiographic, and geographic region in southwest Washington. When described as a physiographical province, the Willapa Hills are bounded by the Pacific Ocean to the west, the Columbia River to the south, the Olym ...

. The gap is a major geographic feature of the northwestern United States. Other geographic features in the gap include Chehalis River,

Grays Harbor Grays Harbor is an estuarine bay located north of the mouth of the Columbia River, on the southwest Pacific coast of Washington state, in the United States of America. It is a ria, which formed at the end of the last ice age, when sea levels flood ...

, and

Grays Harbor National Wildlife Refuge Grays Harbor National Wildlife Refuge is located within Grays Harbor, at the mouth of the Chehalis River, which makes up the second largest watershed in Washington. It is one of four major staging areas for migrating shorebirds in the Pacific Fly ...

in its estuary.

U.S. Route 12 U.S. Route 12 (US 12) is an east–west United States highway, running from Aberdeen, Washington, to Detroit, Michigan, for almost . The highway has mostly been superseded by Interstate 90 (I-90) and I-94, but unlike most U.S. routes tha ...

runs through the gap from Elma near

Capitol State Forest The Capitol State Forest is a state forest in Thurston and Grays Harbor counties of the U.S. state of Washington. It includes part of the unusual Mima Mounds geologic feature. The Capitol State Forest is managed by Washington State Department ...

to Aberdeen on Grays Harbor near the coast, paralleled by the former U.S. Route 410 and

Puget Sound and Pacific Railroad The Puget Sound and Pacific Railroad operates over 150 miles of track serving the U.S. State of Washington, and is headquartered in Centralia, Washington where interchanges with the BNSF Railway and Union Pacific Railroad are made. Histor ...

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.

Climate and meteorology

The gap is responsible for climate and meteorological effects in

Puget Sound region The Puget Sound region is a coastal area of the Pacific Northwest in the U.S. state of Washington, including Puget Sound, the Puget Sound lowlands, and the surrounding region roughly west of the Cascade Range and east of the Olympic Mountai ...

, including the

Puget Sound Convergence Zone The Puget Sound Convergence Zone (PSCZ) is a meteorological phenomenon that occurs over Puget Sound in the U.S. state of Washington. It is formed when the large-scale air flow splits around the Olympic Mountains and then converges over Puget Sound. ...

, marine push, diurnal wind circulation (

sea breeze A sea breeze or onshore breeze is any wind that blows from a large body of water toward or onto a landmass; it develops due to differences in air pressure created by the differing heat capacities of water and dry land. As such, sea breezes ar ...

), and the relatively high precipitation compared to areas in the Olympic Mountains'

rain shadow A rain shadow is an area of significantly reduced rainfall behind a mountainous region, on the side facing away from prevailing winds, known as its leeward side. Evaporated moisture from water bodies (such as oceans and large lakes) is carri ...

. Without the gap, the climate would be more like that of semiarid

Eastern Washington Eastern Washington is the region of the U.S. state of Washington located east of the Cascade Range. It contains the city of Spokane (the second largest city in the state), the Tri-Cities, the Columbia River and the Grand Coulee Dam, the Ha ...

. The cities closest to the gap, like Olympia, get considerably more rain than those farther away, like

Seattle Seattle ( ) is a port, seaport city on the West Coast of the United States. It is the county seat, seat of King County, Washington, King County, Washington (state), Washington. With a 2020 population of 737,015, it is the largest city in bo ...

Air quality Air pollution is the contamination of air due to the presence of substances in the atmosphere that are harmful to the health of humans and other living beings, or cause damage to the climate or to materials. There are many different types ...

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

Water

Municipal water supply A water supply network or water supply system is a system of engineered hydrologic and hydraulic components that provide water supply. A water supply system typically includes the following: # A drainage basin (see water purification – source ...

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"

. Seattle, and Everett and

Bremerton Bremerton is a city in Kitsap County, Washington. The population was 37,729 at the 2010 census and an estimated 41,405 in 2019, making it the largest city on the Kitsap Peninsula. Bremerton is home to Puget Sound Naval Shipyard and the Bremerto ...

all have municipal reservoirs in the hills surrounding each city, sometimes tens of miles (kilometers) away.

References

Notes

Sources

* * * * * * * * * * * * * * * * * * *{{citation , title=The Restless Northwest: A Geological Story , first=Hill , last=Williams , publisher=Washington State University Press , year=2002 , isbn=9780874222500 , url=https://books.google.com/books?id=z8ESAQAAIAAJ , pages=111–112 , quote=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. Landforms of Grays Harbor County, Washington Landforms of Mason County, Washington Water gaps of Washington Valleys of Washington (state)

Formation

Climate and meteorology

Water

See also

References

Notes

Sources