DRAINAGE NETWORK EVOLUTION IN VOLCANIC LANDSCAPES: HOW MUCH TIME DOES IT TAKE TO GET THE RIVER FLOWING?

Young volcanic terranes are typically characterized by extensive lava fields having some topographic relief but little or no fluvial dissection. In these landscapes, development of drainage networks is fundamentally limited by the very high permeability of the rocks, resulting in virtually all precipitation infiltrating the surface and contributing to deep groundwater. In the absence of surface runoff, drainage networks develop extremely slowly and by different suites of processes than in landscapes where surface and shallow sub-surface flow prevail. These processes may include sapping and headward migration of springs. Over geologic timescales, development of fluvial drainage networks in volcanic landscapes is constrained by the time required to weather parent material into soil or fines and fill intersitial voids, thereby decreasing permeability and promoting near-surface flow. Mechanisms and rates associated with these weathering processes are poorly understood, and likely to vary by climatic and geomorphic setting.

We explore the timescales required to develop drainage networks in young volcanic terranes, drawing on examples from the both the wet and dry sides of the Oregon Cascades and Hawaii. We use a simple algorithm for calculating drainage densities from digital elevation models, and examine the degree of drainage network development as a function of rock age and annual precipitation, the latter representing a measure of climatic intensity. Drainage densities developed on the east (dry) side of the Cascades average 0.25 km/km² for Quaternary basalts as opposed to 0.75 km/km² for Tertiary lavas. Preliminary analysis of Hawaiian DEMs suggests that 500 Ka years or more may be required to initiate drainage development in tropical environments. Shorter time periods may be required in temperate zones, where glacial advances and retreats may jumpstart initiation of drainage networks through at least three mechanisms: 1) etching and scouring topographic lows that can later be occupied by rivers; 2) compacting lava flows, hence reducing porosity and permeability; and 3) increasing rates of weathering and production of fine-grained material that can fill interstitial spaces. This geoclimatic framework underscores the importance of incorporating geologic history into models of landscape evolution.