Sedimentation and Climate Flathead Lake Centrally located within the COTC, Flathead Lake and the surrounding area have been the focus of a large research campaign funded primarily by the NSF and the USGS-Edmap program. The Flathead Lake basin is located at the former terminus of the Cordilleran Ice Margin and is ideally positioned to capture a sensitive record of hydrologic change associated with deglaciation. Research conducted to date focuses mainly on the recent history of sedimentation, the geomorphic evolution of the area, and the record of climate change preserved in lake sediments and their equivalents onshore. Work in the Flathead Lake region has utilized a variety of data sets and approaches. These include 1) analysis of over 270 km of 3.5 kHz seismic reflection profiles shot across Flathead Lake; 2) recovery and analysis of 19 sediment piston cores and a similar number of shorter gravity cores from the Flathead Lake basin; 3) onshore geologic mapping of >150 km² in areas surrounding the lake; and 4) reconnaissance-level investigations of older glacial Lake Missoula sediments that crop out widely in the area.     Figure 2 - Kullenberg coring vessel To accomplish the coring of Flathead Lake we employed the use of a Kullenberg coring vessel (Figure 2) available through the Climatological Research Center on the University of Minnesota campus. Core locations were selected on the basis of available sub-bottom 3.5 kHz high-resolution seismic reflection data and designed to yield the most information about the Holocene lake-level history related to climate changes and the late Pleistocene deglaciation record.     Figure 3- High-resolution seismic reflection portion of 3.5 kHz seismic line shot in the western part of Big Arm Bay. Piston core FL-00-9P (7.05m) which was collected from a central lake position preserves the longest continuous record. This core contains the Mount Mazama ash (7,630 cal. yr. B.P.) and the Glacier Peak ash (GPA; 13,755 cal. yr. B.P.), strongly suggesting that a complete uppermost Pleistocene to Holocene record is available. A substantial difference in sedimentary style exists between glacial and post-glacial sediments recovered in our core suite. Late glacial sediments consist of a series of cm-scale, clay-rich sediment packages that we interpret as annual varves, based on our ability to correlate individual varves across the lake basin (figure 4).     These varved sediments are abruptly overlain by a series of anomalously coarse silt beds that we infer to represent large discharge pulses of melt water associated with rapid retreat of the Flathead Lobe. Overlying Holocene sediments are finer-grained and locally laminated on a mm- to cm-scale by black sulfides. We estimate that the Holocene sedimentation rate for the northern half of Flathead lake ranges between 0.05cm/yr (lake center) and 0.09cm/yr (delta proximal) +/-0.02cm, based upon chronostratigraphy derived from the tephras, along with 137Cs and 210Pb dating of two gravity cores.   Figure 4 - Pleistocene-Holocene Sediment Record All text, images, and figures are property of U of M Geology. This content may not be reproduced without the explicit authorization and written consent of hereby constituents.