A SPACE-FOR-TIME ASSESSMENT OF STREAM NETWORK DEVELOPMENT AND INCISION PATTERNS ON THE PLATEAU RHYOLITE FLOWS IN YELLOWSTONE NATIONAL PARK

The development of stream networks and concave-up profiles are classic problems of geomorphology. Natural, observable examples of stream network development from an initial state are rare, and often limited to areas <1 km² or soft sediments. We examine differences in stream characteristics, incision and longitudinal profiles on several large (>100 km²) flows of the 70 – 480 ka Plateau Rhyolites in Yellowstone.

The flows have broad surfaces and steep margins, giving streams an initially convex long profile. They accumulate a mantle of aeolian and glacial sediments between lava flow pressure ridges. The mantle is thicker and more continuous on older surfaces and on gentler slopes. Where the fractured bedrock is exposed or thinly buried, there is little evidence for runoff. Most first-order streams develop (1) below small gaps in pressure ridges that concentrate runoff from sediment-mantled areas above or (2) in other sediment-mantled areas with steep or convergent slopes. Where streams have incised into bedrock, pressure ridges form knickpoints that increase gradient, sediment size and infiltration. Drainage density and stream frequency generally increase with surface age, indicating that networks on younger flows are still in the classic extension and elaboration phases. Incision increases with surface age, drainage area, and stream segment slope. Deep incision near flow margins reduces stream profile convexity, and the most rapid incision occurs in downstream reaches within 70 kyr. Incision rates generally decrease with surface age except in headwaters, where first-order channels are mostly extending headward on younger surfaces, but incising on older surfaces.

Younger flows have large (>1 km²), sometimes dry closed basins, few perennial streams and major spring discharge along flow margins, whereas older flows typically have fewer and smaller closed basins, and more perennial streams with springs closer to their headwaters. These differences may indicate a shift over time from deep percolation and fracture flow to surface runoff and shallow subsurface flow facilitated by (1) accumulation of mantle sediments and (2) incision into the zone of fracture flow. Overall, stream characteristics change systematically with flow age, under strong controls by initial bedrock form and surficial sedimentation.