Stratigraphy of Carbon Preservation in Reservoir Sediments, Elwha River, Washington

Abstract:

Dam removal revealed the facies architecture and distribution of detrital carbon in the sediments of two former large reservoirs, providing an unparalleled opportunity to develop a stratigraphic framework for understanding processing and storage of organic carbon in reservoir sediments. Incision following the 2011-2014 removal of Elwha (34 m high; Lake Aldwell reservoir) and Glines Canyon (64 m; Lake Mills reservoir) Dams on the Elwha River in Washington State exposed 85+ years accumulation of reservoir sediment in cross section, creating the first known opportunity to characterize post-impoundment sediments of a large reservoir in situ. In Lake Mills, the upstream reservoir, the Elwha River deposited an estimated 1.56x10⁶ m³ sediment primarily in a steep-fronted, Gilbert-style delta, while deposition in downstream Lake Aldwell comprised only ~3.0x10⁶ m³ sediment, deposited broadly in an elongate, low-angle delta. Allochthonous carbon is primarily preserved in delta foresets and complexly-bedded prodelta sands, with secondary lenses of coarse-grained organics deposited as channel-lag in migrating topset channels. Organic units tend to be coarse-grained, open-framework lenses and beds consisting of well-preserved branches, cones, and needles in the topset and foreset beds, while prodelta sands consist of well-sorted, well-preserved needles and leaves associated with fine sands and silts. Compared to Lake Mills, Lake Aldwell’s delta deposits are lower angle and have finer, more organic-rich topset beds. Lacustrine beds in the main body of the reservoir are dominated by mineral sediments. Taking the average total organic carbon (TOC) content and geometry of individual facies provides a first-order estimate of the total organic carbon load in the reservoir sediments, and provides insight into the spatial variability of carbon deposition, a key challenge in understanding the carbon footprint of large water reservoirs.