A Stratigraphic Approach to Characterize the Deposition and Storage of Organic Matter in Reservoir Sediments

The relationship between carbon burial and sedimentation in reservoirs is unknown, exposing gaps in our fundamental understanding of the transport, processing, and deposition of sediment and organic matter in fluvial and lacustrine systems and contributing to uncertainty in our understanding of the net impact of dams to the global carbon budget. The 2011-2014 removal of two large dams on the Elwha River, Washington State, the largest dam removal yet completed globally, created extensive cutbank exposures of reservoir sediments, allowing the first characterization of the facies architecture of sediments through direct observation in reservoirs worldwide and providing an unparalleled opportunity to 1) assess the relationship between environmental influences, such as and changes in sediment supply, and their expression in the stratigraphic record, 2) assess the relationship between sedimentation processes and detrital organic carbon deposition and storage, and the importance of coarse-grained organic matter and woody debris to the total carbon budget of a reservoir, and 3) apply the insight gained from these reservoirs to evaluate current global estimates of carbon storage in reservoirs and develop a conceptual model of carbon burial in reservoirs to guide further research, as defined by characteristic stratigraphic “types”.

Former Lake Mills, the younger, upstream reservoir, was characterized by a tripartite, subaerial Gilbert-style delta which prograded >1 km into the main reservoir from 1927 to 2011. Sediments were composed of coarse-grained topset beds, steeply dipping foreset beds, and a fine-grained, gently dipping prodelta. While individual event horizons were discernible in fine-grained sediments of former Lake Mills, their number and spacing did not correspond to known drawdown or flood events. Former Lake Aldwell, impounded from 1913 to 2011, was initially defined by the rapid progradation of a Gilbert-style, subaerial delta prior to the upstream completion of Glines Canyon Dam. However, the 1927 closure of Glines Canyon Dam upstream caused the delta to evolve to a fine-grained, mouth-bar type delta indicative of low, finer-grained sediment. This evolution, combined with a previously-unrecognized landslide deposit into the upper
delta plain, suggests that understanding the exogenic influences on reservoir sedimentation is critical to interpretation and prediction of the sedimentation within individual systems.

Former Lake Mills accumulated ~330 Gg of, with depositional-zone average accumulation rates from 229 to 9262 gCm-2 yr-1 , while Former Lake Aldwell accumulated ~ 91 Gg (263 to 2414 gCm-2 yr-1). Carbon storage in both reservoirs was dominated by heterogeneous, coarse organic matter and woody debris in the coarse-grained delta slope and relatively coarse-grained prodelta regions of the reservoirs, with little storage in the gravel-dominated, subaerial delta plains. Carbon accumulation in fine-grained lacustrine and prodelta sediments was relatively homogeneous, but turbidity flows from the Gilbertstyle delta slope in former Lake Mills delivered significantly more carbon to the prodelta than the mouth-bar style delta of former Lake Aldwell. C:N ratios support interpretation of most organic matter in both reservoirs as allochthonous. Sampling schemes based only on lacustrine and/or prodelta would underestimate of total carbon accumulation by up to 30% in former Lake Aldwell, but the overestimate by up to 47% in former Lake Mills.

Global estimates of carbon sequestration rates in reservoir sediments vary by three orders of magnitude, while individual-reservoir estimates vary by four orders of magnitude and over only 37 reservoirs and a literature review of predictive variables suggests weak or contradictory relationships. A conceptual stratigraphic framework of four unique reservoir types suggests that organic matter deposition is intrinsically tied to sedimentation processes and that patterns of carbon storage vary systematically with the stratigraphy of reservoir sediments. Deltaically-dominated reservoirs (whether Gilbert style or shoalwater) appear to store most carbon in their deltaic and prodelta regions, while thalweg-style reservoirs exhibit a bimodal distribution, with allochthonous carbon preferentially routed along the former river thalweg and autochthonous deposited on the former floodplain. Lacustrine-style reservoirs are dominated by suspended sediment
deposition and thus relatively homogeneous, but literature suggests these reservoirs are more variable than typically measured. Current methods of reservoir sampling fail to account for this systematic variation and tend to be biased toward fine sediment, suggesting that global reservoir carbon storage is underestimated.