Drainage Network Development on a Porous Landscape

Most landscape evolution models force drainage development using stream power laws, but this concept cannot be applied where a permeable substrate precludes surface runoff. Such a situation is common in volcanic landscapes throughout the world. Key processes and timescales that reduce the permeability of lava to the point where the stream power law can be applied are poorly documented. On Quaternary basaltic lava flows of the Oregon Cascades, surface drainage network development involves several stages. Measurements of soil depth show that lava <3000 years old is mostly bare rock, with no surface drainage. By ~15,000 years, soils are 5 to >30 cm deep and surface runoff may occur for short periods during peak snowmelt. We propose that, in this initial stage, permeability is reduced by void infilling from eolian or organic material as the soil mantle accumulates. GIS analysis and rock dating show that a second stage persists for several hundred thousand years, during which time drainage is primarily via groundwater, expressed in large springs at flow toes and valley walls. Incision during this stage is primarily the result of glaciation. Up slope of the springs, a low-density, ephemeral drainage network forms, often on glacial deposits. During this period, we hypothesize that lava permeability reduction occurs by weathering, secondary mineralization in voids, and compaction from overburden. In the third stage, permeability becomes low enough that surface runoff begins to dominate over groundwater. This handoff occurs between 680,000 and 2 million years, based on rock dates and drainage density. Beyond this point, further incision and drainage network expansion are stream power driven.