at Larch Mountain; most vents reach an elevation of . Located in the Portland Basin, the field consists of monogenetic volcanic cones that appear as hills throughout the area, reaching heights of above their surroundings. The collection includes more than 80 small volcanic edifices and lava flows in the Portland–Vancouver metropolitan area, with the possibility of more volcanic deposits buried under sedimentary rock layers. The borders for the Boring Lava Field group are clear, except on the eastern side where distinguishing between Boring deposits and those from the major Cascade arc are less clear; many geologists have arbitrarily placed the eastern border at a longitude of 122 degrees west. In total, the Boring Lava Field covers an area of about , and it has a total volume of . Physical geography With a variable topography, the Portland area ranges from river valley floors to terraces reaching elevations of . The Willamette Valley is marked by hills reaching heights of more than , and it is also physically separated from the lower Columbia River valley. 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. The Columbia River has significantly shaped the geology of the area. Multnomah Creek drains from Larch Mountain, one of the volcanic cones in Boring Lava Field. 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. It also supplies water to domestic wells in the Mount Norway area. Boring Lava is known to have formed intrusions into local sedimentary rock, and thus it may guide flow of groundwater locally. 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 , though the usual July maximum is below , and the average minimum for January is above . Yearly, precipitation averages between in most river valleys, with a mean of from 1871 through 1952. It shows variability, however, with a historic low of at Portland in 1929 and a maximum of in 1882. More than 75