With a variable topography, the Portland area ranges from river valley floors to terraces reaching elevations of 400 feet (120 m).[17] The Willamette Valley is marked by hills reaching heights of more than 1,000 feet (300 m),[18] and it is also physically separated from the lower Columbia River valley.[17] The Columbia River flows west from the eastern Portland region, merging with the Willamette near Portland before moving north. Tributaries for the Willamette include the Pudding, Molalla, Tualatin, Abernethy, and Clackamas Rivers; tributaries for the Columbia River include the Washougal and Sandy Rivers.[19] The Columbia River has significantly shaped the geology of the area.[20] Multnomah Creek drains from Larch Mountain, one of the volcanic cones in Boring Lava Field.[21] Local streams near the community of Boring receive seepage from the local aquifer. This unit, part of the greater Troutdale sandstone aquifer, is also made of sandstone and conglomerate and bears water well.[22] It also supplies water to domestic wells in the Mount Norway area.[23] Boring Lava is known to have formed intrusions into local sedimentary rock, and thus it may guide flow of groundwater locally.[23] Portland's climate is moderate, with long growing seasons, moderate rainfail, mild winters, and warm, dry summer seasons. The area has more than 200 frost-free days annually. Temperature can vary widely, reaching a historic maximum of 107 °F (42 °C), though the usual July maximum is below 80 °F (27 °C), and the average minimum for January is above 32 °F (0 °C).[19] Yearly, precipitation averages between 35 to 45 inches (89 to 114 cm) in most river valleys, with a mean of 42.04 inches (106.8 cm) from 1871 through 1952. It shows variability, however, with a historic low of 26.11 inches (66.3 cm) at Portland in 1929 and a maximum of 67.24 inches (170.8 cm) in 1882.[19] More than 75 percent of this precipitation occurs between October and March; July and Au
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