INITIAL DEVELOPMENT AND EVOLUTION OF STREAM NETWORKS AND LONGITUDINAL PROFILES ON YOUNG RHYOLITE FLOWS, YELLOWSTONE NATIONAL PARK, AND THE IMPORTANCE OF BEDROCK STRUCTURES

Direct observation of the development of integrated stream networks and incision of the typical concave-up profile into bedrock for large (>1 km²) drainage basins is not possible because the processes involved operate over timescales of 10⁴-10⁷ years (Knighton, 2014). Therefore, much of what we know about these processes comes from computer models, small-scale natural laboratories and experiments, which require considerable extrapolation for relevance to large-scale natural watersheds. Young (< 1 Ma) surfaces containing independently formed stream networks are rare and provide important insights into the initial development of fluvial systems (Wells et al., 1985; Jefferson et al., 2010). The Rhyolite Plateau in Yellowstone National Park contains several young (79-484 ka) large (100-350 km²) rhyolite flows with many independent drainage basins. Using digital elevation data and aerial imagery supported by field observations, we evaluate a wide range of drainage basin, stream network and channel incision parameters on Rhyolite Plateau surfaces of different ages to determine patterns in initial development and evolution. The stream networks and incision patterns are strongly controlled by volcanic pressure ridges and the initial convex plateau morphology. Incision and reduction in convexity occur at nearly linear rates when accounting for differences in slope and drainage area (a proxy for stream power). Significant incision on the youngest flows is limited to small knickpoints, but on the oldest flow, stream incision has cut deeply into the steep flow margin and has propagated several kilometers headward. Stream junction angles on all flows correlate well with predictions for minimum power loss (Howard, 1971), indicating that efficient junctions are a characteristic of initial stream networks or evolve rapidly (< 79 ka). However, where streams of very different magnitude join, the junction angles are more rectangular (80ᵒ-90ᵒ) due to stream alignment with pressure ridge morphology. Bedrock structure and topography played an important role in modulating initial drainage development, and given the ubiquity of anisotropic bedrock and preexisting topography, the expression of quantifiable fluvial geomorphic processes in landscape evolution are likely strongly impacted by these boundary conditions.