A STUDY OF THE GEOCHEMICAL AND GEOMORPHOLOGIC EVIDENCE FOR PREHISTORIC FLOODS FROM PAULINA LAKE IN NEWBERRY CRATER, CENTRAL OREGON

Abandoned waterfalls and dramatic knickpoints line Paulina Creek showing evidence for catastrophic prehistoric floods of unknown origin. In addition to erosional features, there is a large flood apron (4.5km²) that formed just upstream of the confluence of Paulina Creek and the Little Deschutes River. Paulina Creek originates in Paulina Lake (620 ha) which lies within the western part of the crater of Newberry Volcano. Newberry Volcano is a Holocene shield-type volcano composed mainly of basalt flows, but with some flows ranging to rhyolitic composition. The flanks of the volcano are composed of ash flows, pyroclastic deposits and cinder cones.

Potential origins of the floods range from landslides and volcanic eruptions displacing water from Paulina Lake, to a catastrophic collapse of the outlet associated with the upstream migration of a knickpoint. The knickpoints are a consequence of differential erosion of alternating layers of resistant lava flows and weak pyroclastic deposits. Floodwaters exaggerated knickpoint relief throughout the canyon creating waterfalls up to 40m high. Chitwood and Jensen, in their paper on the prehistoric floods, suggest a flood discharge of 110-280 m³/s at peak flow. This is drastically higher than current annual peak flows (1-2 m³/s).

Current stream baseflow discharge (0.34 m³/s) does not significantly change between the outlet of Paulina Lake and the flood apron, implying that most of the streamwater originates in Paulina Lake and that there is little groundwater inflow. Paulina Lake has high concentrations of As in the water column (0.014ppm) and very high concentrations in the bottom sediment (250ppm). Paulina Creek sediments contain much less As (6 ppm) than Paulina Lake. The downstream flood apron sediment has the lowest As concentrations (1ppm).

The current sediment load in Paulina Creek is very low and there is little bedload transport due to a cemented streambed. Under current conditions it is unlikely that there is significant knickpoint migration. It is most likely that the flood was caused by a volcanic event that displaced Paulina Lake water downstream. The low concentrations of As in the downstream flood deposits makes it unlikely that the bottom of Paulina Lake was disturbed enough to contribute to the downstream sediments.