March 15th, 2017
With the heavy rains of winter in Oregon’s Western Cascades comes the surface and subsurface movement of water through volcanic soil. This gathering of water begins to trickle into tiny forest rivulets and then creeks, until we see the raging torrent which has become the upper Middle Fork Willamette River. Along the way, this movement of water transports the suspended solids of loose volcanic soil, which in turn, helps give the Middle Fork Willamette and Hills Creek Reservoir the beautiful shade of blue-green we see in winter. Some of the rock distributed throughout the watershed of the upper Middle Fork Willamette, is derived from volcanic ash deposited by pyroclastic flow, or as fallout from eruptions when the axis of mountain building was occurring in the Western Cascades some 17 to 35 million years ago during the late Eocene and Miocene epochs. Heavily eroded by winter rains and deeply incised by myriad streams, the Western Cascades, also referred to as the ‘Old Cascades’ occurs on average between an elevation of 1,500 feet on its western flank, and 5,800 feet on the eastern boundary; where they merge with the current axis of active mountain building, called the High Cascades. Some of the pyroclastic rock found in the watershed of the Middle Fork Willamette have been colored by trace minerals, giving them beautiful shades of blue, green, yellow and tan commonly found in local streams such as the North Fork Willamette, Hills Creek, Salmon Creek and the Middle Fork Willamette River. Pyroclastic rocks which have been physically altered or transported by wind and water are referred to as volcaniclastic, and can be represented by the rounded river cobbles of green and blue which many of us have collected and prominently displayed in our homes upon a windowsill or fireplace banister. Other types of pyroclastic rock found in the watershed of the Middle Fork Willamette include agglomerates, welded tuff and tuffs comprised of dacite and silicic.