Geology

Leavenworth sits on the southeast side of the North Cascades collage, which is a group of terranes that accreted to North America all about the same time. Marine fossils indicate that the terranes were probably a group of islands originating in the South Pacific hundreds of million years ago. The terranes arrived at North America about 90 million years ago in the middle of the Cretaceous period. When they smashed into their new home, they were a puzzle of north–south slices. As accretion continued, they were cut into horizontal (east-west) slices.

During the Eocene epoch, about 50 million years ago, the area was once again cut into vertical slices, creating among others, the Leavenworth fault and the Entiat fault. In between these two faults the Chiwaukum graben was created. This graben is about 12 miles wide and trends northwest from Wenatchee for about 50 miles. As the graben dropped, it began to fill with clastic sediment from the surrounding hills, creating the Chumstick formation. About 30 million years ago in the Oligocene epoch, the Chiwaukum graben underwent compressional deformation creating several folds in the region that are visible today. Leavenworth is on the western edge of the graben; in fact, the Leavenworth fault runs through the western edge of town. The area to the west and southwest of Leavenworth was created in the middle Cretaceous period with the uplift of the Mount Stuart batholith, forming the granite rock seen today in Icicle Ridge and Tumwater Mountain.

During the Pleistocene and into the Holocene epochs, an alpine glacier originating from the southwest in the Mount Stuart range made its way to where the town is today. Leavenworth sits on the terminal moraine of that glacier. The residential parts of town display many glacial erratics that originated 20 miles up the Icicle Valley near Mount Stuart.

About 19,000 years ago, a large rock slide dammed the Columbia River near Rock Island, just south of Wenatchee. The temporary dam, in conjunction with one of the Lake Missoula floods, caused the water to flow back up the Wenatchee Valley, where it was stopped by the glacier at Leavenworth. As the leading edge of the glacier interacted with the flood, ice rafts formed carrying granite erratics from the Stuart batholith, which ended up in the town of Dryden about 15 miles down the valley from Leavenworth. As the glacier retreated, the south side of Leavenworth was a lake dammed up by the moraine. The bridge on the east side of town is a good vantage point to see where the Wenatchee River cuts through the moraine today.

Climate

Leavenworth has a continental Mediterranean climate (Köppen Dsb) with summers characterized by hot, sunny days and chilly nights, and cold, snowy winters.

During the summer, the weather is typically anticyclonic due to the presence of the North Pacific anticyclone, with resultant clear skies and large diurnal temperature ranges. Rainfall is limited by the Cascade rain shadow as well as by the anticyclone, and all months from May to October have recorded zero precipitation on occasions, including 114 consecutive days without precipitation from June to October 2003. When a continental flow enters the Columbia Basin, the temperature can be very hot during the day, with the hottest temperature being 110 °F or 43.3 °C during the heatwave of July 1941. However, on cooler, stiller days, summer nights can still be cold and frosts have been recorded as early as August 29 in 1980.

The fall months have steady cooling and a gradual increase in the frequency of frontal storms producing rainfall, while winter weather is typically cold and snowy, with an annual mean snowfall of 90.1 inches or 2.29 metres and a maximum monthly total of 92.3 inches (2.34 m) recorded in December 1996. The snowiest season has been from July 1968 to June 1969 with 217.2 inches (5.52 m) and the least snowy from July 1962 to June 1963 when just 19.4 inches (0.49 m) of snow fell. The lowest temperature recorded in Leavenworth was −36 °F (−37.8 °C) on December 30, 1968, but typically five mornings per year will fall at or below 0 °F or −17.8 °C.

The wettest "rain year" was recorded from July 1955 to June 1956 with a total of 41.13 inches (1,044.7 mm) and the driest was from July 1929 to June 1930 with 11.77 inches (299.0 mm). The spring months see gradual warming and drying, though frosts remain frequent well into the start of April. During an average year, over 168 mornings see temperatures fall to freezing or below. May 16 is the average date of last freeze; 2016 was the first year to record a last frost in March.

Transportation

Leavenworth is located on U.S. Route 2, which continues west across Stevens Pass to Snohomish County and east towards Wenatchee.^[25] The Chumstick Highway, formerly known as State Route 209, travels north from Leavenworth to Plain and Lake Wenatchee State Park.

The city is served by Amtrak's Empire Builder train, which stops daily at Icicle Station near downtown.^[25] Link Transit provides local and inter-city public transit services that connect Leavenworth to Wenatchee, as well as local paratransit and a downtown shuttle.^[26] The city also has several private taxi companies and inter-city bus operators, including stops for Northwestern Trailways and Amtrak Thruway.^[27]

Further reading

• Generalized geologic map of the Chelan 1:100,000 quadrangle
• Swauk Formation (Caroline J. Cloudas, 1998)
• Field Trip Guide: Geology and Art
• Chiwaukum Schist in the Nason Terrane (Jessy Ryan)
• A geological trip along Snoqualmie, Swauk, and Stevens Pass Highways. Washington state geology department, 1963
• Northwest Exposures. David Alt and Donald W. Hyndman
• Roadside Geology of Washington. David Alt and Donald W. Hyndman
• Genealogy of Chelan County. Wayne and Linda McGahuey