LAVA FLOWS ESTABLISHING LAKES, RIVER PATHS, AND WATERFALLS: A CASE STUDY IN THE UPPER MCKENZIE RIVER VALLEY, OREGON

Lava flows are geologic agents capable of resurfacing and reorganizing landscapes. As most contemporary research on lava flows is carried out in places with little surface water (e.g., Hawaii and Mt Etna), little work has been done on how lava flows influence the location and path of surface water. Here we use field observations, LiDAR imagery, bathymetric surveys, and geochemical analysis of lava samples to reconstruct the interplay between lava flows and surface water in the upper McKenzie River valley, Oregon. The head of the McKenzie River, Clear Lake, formed when lava flows from the Sand Mountain volcanic chain entered and dammed the ancestral McKenzie River from the east; the modern McKenzie River flows out of the southwestern end of Clear Lake. Lava flows at the lake’s southern margin are covered by old growth forest, while relatively barren lava flows form the lake’s eastern margin. The southern portion of the lake covers a submerged platform with drowned trees. These observations suggest that lava flows affected the Clear Lake area on at least two separate occasions: first, a lava flow dammed the river and created the lake, then later, a lava flow entered and raised lake levels. Upon exiting Clear Lake, the McKenzie River traverses Sand Mountain lava flows for 2.5 km. Initially, the river channel marks the boundary between a lava flow margin (east) and a Pleistocene graben-bounding fault (west). 1 km downstream of the outlet the channel crosses over the lava flow for 300 meters, then for the remainder of the traverse it marks the boundary between lava flows of two different eruptive events. These observations raise questions as to why the river crosses the lava flow after flowing along its margin, and where the channel was prior to and between the emplacements of its two bounding lava flows. Two waterfalls, both over 25 meters tall, mark lava flow fronts. These waterfalls have amphitheater shapes, presumably formed from plucking along joints and removal of material. Thus, in the upper McKenzie River valley, lava flows from the Sand Mountain volcanic belt (1) dammed the river, creating a lake, (2) dictated the location of the river channel, and (3) formed two large waterfalls. In the future, lava flows are likely to enter the valley again, further reorganizing the hydrology; lava flows are indeed powerful movers of surface water.