Munroe, Jeffrey S., Quaternary Research Group, Dept. of Geology & Geophysics, University of Wisconsin-Madison, Madison, Wisconsin, 53706, USA

Clastic and organic sediments exposed in two stream cutbanks just above modern treeline in the headwaters of the Henrys Fork record multiple contrasting episodes of fluvial, lacustrine and wetland deposition.  The lower basin at 3273 m asl was apparently impounded behind a moraine dam that persisted until the later Holocene.  Likely mechanisms of water impoundment for the upper section (3485 m) include beaver activity and landsliding.  The overall sedimentary package in both exposures is dominated by organic-rich slackwater deposits that are frequently interrupted by gravels and coarse sands containing rip-up clasts of reduced silty clay.  These higher-energy sediments likely represent flood deposits, possibly produced by collapse of upstream dams, although extreme snowmelt and summer thunderstorm events may also have been responsible.
     A date of 9310 ± 70 14C years BP on a Salix fragment from the base of the lower section indicates that the upper reaches of the Henrys Fork were vegetated by riparian willows at that time.  Four other dates on wood and bulk organics ranging up to 4070 ± 70 14C years BP indicate that these sections span more than half of the Holocene.  Ongoing study of the pollen record from 12 separate organic layers will provide snapshots of the basin vegetation at discrete time slices and should allow synthesis of a paleoclimatic history for this basin.