Lava Flows, Rivers, and Lakes: Complex Interactions Along the McKenzie River, Central Oregon

There are few studies of lava - surface water interactions, undoubtedly because most contemporary research on lava flows has been carried out in places with little surface water (e.g., Hawaii and Mt Etna). However, as described by a written account of the 1783 Laki eruption in Iceland, this interaction can be quite dynamic and dramatic: lava flows can disrupt water sources and rivers, simultaneously causing water shortages downstream and severe flooding upstream. In the Cascade volcanic range there are numerous examples of pre-historic Holocene lava - surface water interaction. For example, multiple lava flows have entered the McKenzie River, which occupies the western margin of High Cascades graben. Clear Lake, at the head of the McKenzie River, formed when lava flows from the Sand Mountain chain entered the ancestral McKenzie River and dammed it; dated drowned trees preserved on the lake bottom suggest that damming occurred c. 3000 years ago. While the modern forest masks the location and extent of the damming lava flow, principle components analysis of Landsat imagery helps to define flow boundaries and areal extent. This extensive flow is at least 54 meters thick and flowed west until it encountered the graben wall, at which point it flowed south, burying and damming the ancestral McKenzie River. The river currently overtops the lava dam and travels south along the graben wall. Poorly vegetated flows enter Clear Lake on its eastern margin; while early workers mapped two separate flow units around Clear Lake with younger flows distinguished by their lack of vegetation, recent workers have mapped all flows bordering Clear Lake as part of the same complex. We agree with earlier interpretations and additionally use bathymetric studies to show that the lake prematurely stopped the advance of these younger lava flows. Further downstream, flows from Belknap volcano entered the McKenzie River approximately 1500 years after the formation of Clear Lake. While these flows probably formed a temporary dam, the river now flows underneath the lava through a high permeability zone along its paleodrainage, most likely along a contact between two lava flows. There is no evidence anywhere along the McKenzie of a breakout flood. Thus, along the McKenzie River, there are three types of lava - surface water interactions: (1) older lava flows damming a river, creating a lake, (2) younger lava flows stopped by the presence of that lake, and (3) a lava flow burying a river with the river subsequently reestablishing itself underneath the lava flow.