Forest Park is a public municipal park in the Tualatin Mountains west of downtown Portland, Oregon, United States. Stretching for more than on hillsides overlooking the Willamette River, it is one of the country's largest urban forest reserves. The park, a major component of a regional system of parks and trails, covers more than of mostly second-growth forest with a few patches of old growth. About of recreational trails, including the Wildwood Trail segment of the city's 40-Mile Loop system, crisscross the park. As early as the 1860s, civic leaders sought to create a natural preserve in the woods near Portland. Their efforts led to the creation of a municipal park commission that in 1903 hired the Olmsted Brothers landscape architectural firm to develop a plan for Portland's parks. Acquiring land through donations, transfers from Multnomah County, and delinquent tax foreclosures, the city eventually acted on a proposal by the City Club of Portland and combined parcels totaling about to create the reserve. Formally dedicated in 1948, it ranks 19th in size among parks within U.S. cities, according to The Trust for Public Land. More than 112 bird species and 62 mammal species frequent the park and its wide variety of trees and shade-loving plants. About of rain falls on the forest each year. Many small tributaries of the Willamette River flow northeast through the woods to pipes or culverts under U.S. Route 30 at the edge of the park. One of them, Balch Creek, has a resident trout population, and another, Miller Creek, supports sea-run species, including salmon. Threats to the park include overuse, urban traffic, encroaching development, invasive flora, and lack of maintenance money. Occasional serious crimes and more frequent minor crimes occur in the park. Geology and geography Solidified lava from Grande Ronde members of the Columbia River Basalt Group underlie Forest Park. About 16 million years ago during the Middle Miocene, the Columbia River ran through a lowland south of its modern channel. Eruptions from linear vents in eastern Oregon and Washington flowed down this channel through what later became the Willamette Valley. These flows, some of which reached the Pacific Ocean, recurred at intervals between 16.5 and 15.6 million years ago and covered almost . About eight separate Grande Ronde Basalt flows have been mapped in the Tualatin Mountains (West Hills), where they underlie the steepest slopes of Forest Park and form the columned rocks