­A LONG-LIVED LAKE HISTORY IN MOHAWK VALLEY, NORTHEASTERN CALIFORNIA

Mohawk Valley is located in northeastern California, at the northern margin of the glaciated Sierra Nevada Mountains and along the westward-flowing Middle Fork of the Feather River (MFFR). We used the combination of mapping, stratigraphy, tephrochronology and soil development to interpret the history of a long lived paleo-Mohawk Lake. The well-developed shorelines and nearly 200 meters of well-preserved, tephra-rich lake deposits record deposition as far back as 740 ka. The type of shoreline varies along the same elevation from wavecut notches, pediments, and outwash terraces, which grade up to 215-m-above modern base level. Basinward, the sediment size decreases dramatically, with the stratigraphic signatures of a delta and numerous tephra beds deposited within. On the north side of the valley, fluvial-deltaic deposits sourced from moderate-sized streams also grade to a former base levels at the same, or similar, elevations.

From these observations, we have interpreted a first order Mohawk Lake history for the past ~740 ka. Prior to 740 ka, Mohawk Valley occupied a similar elevation as today and lakes began to form intermittently in the valley until before about 600 ka, when the lake dramatically deepened. From ~600 ka to after MIS 8 or 6, lakes persisted throughout many glacial and interglacial transitions. Mohawk Lake almost certainly responded to these climatic changes; however, these climate fluctuations are not well documented by preserved shorelines. The stratigraphic and paleosol record thus far have provided some constraints on the magnitude of the lake level fluctuations, but this part of the study is ongoing. After MIS 8 or 6, the lake substantially increased in size and depth and ultimately overflowed westward through the MFFR, eroding its western sill which lead to incremental lowering until it emptied sometime before ~ 7 ka.

This incredible record has remained relatively unstudied for many years in part because of the presence of organic-rich beds, originally interpreted as deposited in a shallow marsh setting. We have re-interpreted these deposits as possibly having been deposited in anoxic conditions and in moderate to deep lake levels. Many of the organic-rich beds are closely associated with numerous tephra beds and may be genetically linked.