Stratton and Grant Autopsy of a reservoir: Facies architecture in a multidam system, Elwha River, Washington, USA Laurel E. Stratton1,† and Gordon E. Grant2 1College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin Building, 101 SW 26th Street, Corvallis, Oregon 97331, USA 2U.S. Department of Agriculture Forest Service, Pacific Northwest Research Station, 3200 SW Jefferson Way, Corvallis, Oregon 97331, USA ABSTRACT the 1927 closure of Glines Canyon Dam up- critically, the sediment dynamics of multiple stream caused the delta to evolve to a fine- reservoirs arranged longitudinally in a single The 2011–2014 removal of two large dams grained, mouth-bar–type delta indicative of river or watershed remain largely unexplored. on the Elwha River, Washington State, the low, finer-grained sediment. This evolution, As a result, our understanding of the character largest dam removal yet completed globally, combined with a previously unrecognized of sedimentation in individual reservoirs and created extensive cutbank exposures of reser- landslide deposit into the upper delta plain, the processes controlling them, as well as the voir sediments, allowing the first character- suggests that understanding the exogenic in- response to changing sediment regimes in mul- ization of the facies architecture of sediments fluences on reservoir sedimentation is critical tidam systems, is limited. through direct observation in reservoirs to interpretation and prediction of the sedi- At the scale of individual dams, most study worldwide and providing an unparalleled op- mentation within individual systems. has focused on developing approaches to aid portunity to assess the relationship between managers in determining the local sediment environmental influences, such as changes in INTRODUCTION yield or volumetric accumulation rate. In its sediment supply, and their expression in the comprehensive design manual Small Dams stratigraphic record. Using a combination As of 2018, there were an estimated 59,000 (a legacy title retained through multiple edi- of facies description from observation of 49 large (>15-m-high) dams worldwide, intercept- tions despite its expanded applicability to large measured sections and >100 exposures and ing 40% of total river flow volume annually, dam design), the U.S. Bureau of Reclamation analysis of digital elevation models and his- impounding an area as large as 723,000 km2, (USBR) developed a series of morphology- toric aerial photographs, we delineated the and increasing the terrestrial water surface area and operations-based type curves to plot the characteristic depositional zones of each res- by >7% (Nilsson et al., 2005; Downing et al., relationship between reservoir depth and depo- ervoir and mapped the evolution of the sub- 2006; Lehner et al., 2011; International Com- sition. These curves recognize that sedimenta- aerial delta over the life span of the reservoir. mission on Dams, 2018). These dams represent tion is typically weighted toward the upstream, Former Lake Mills, the younger, upstream humans’ greatest impact on global land-ocean inflow-adjacent regions of reservoirs, with the reservoir, was characterized by a tripartite, sediment transport, with as much as 25% of assumed delta volume equal to the volume of subaerial Gilbert-style delta that prograded annual global sediment discharge impounded sand-sized or greater sediment input to the >1 km into the main reservoir from 1927 to (Vörösmarty et al., 2003). In fact, despite reservoir (Strand and Pemberton, 1987). The 2011. Sediments were composed of coarse- an estimated 2300 Tg yr–1 increase in global USBR considers a “typical” delta profile to be grained topset beds, steeply dipping fore- sediment transport during the Anthropocene, defined by distinct topset and foreset slopes set beds, and a fine-grained, gently dipping ~1400 Tg yr–1 less sediment reaches the world’s separated by a pivot point located at the median prodelta. While individual event horizons oceans (Syvitski et al., 2005). reservoir operating elevation, but notes that “the were discernible in fine-grained sediments of The resulting disconnectivity in river sys- prediction of delta formation is still an empirical former Lake Mills, their number and spac- tems and the global transfer of sediment via procedure based upon observed delta deposits in ing did not correspond to known drawdown the “sediment cascade” has been the subject of existing reservoirs” (p. 549) and requires exten- or flood events. Former Lake Aldwell, im- much study. However, most work has focused sive data collection. pounded from 1913 to 2011, was initially de- on estimating the net sediment volume impact The “typical” profile, as defined by the fined by the rapid progradation of a Gilbert- at the local, basin, or global scale (cf. Wall- USBR, represents a Gilbert-style delta, first style, subaerial delta prior to the upstream ing and Fang, 2003; Vörösmarty et al., 2003; described in the deposits of Pleistocene Lake completion of Glines Canyon Dam. However, Syvitski et al., 2005; Kummu et al., 2010; Yang Bonneville (Gilbert, 1885). The Gilbert delta and Lu, 2014) and resulting downstream effects, is a process-based paradigm, in which the de- both geomorphic and ecologic, of damming crease in slope and rapid expansion of flow †U.S. Geological Survey Oregon Water Science river systems (e.g., Nilsson and Berggren, 2000; abruptly decrease the competence of inflowing Center, 2130 SW 5th Avenue, Portland, Oregon Bunn and Arthington, 2002; Graf, 2005). The discharge, causing rapid, inertia-based sedimen- 97201, USA; [email protected]. dynamics of in-reservoir sedimentation and, tation (Nemec, 1990a, 1990b). Accordingly, GSA Bulletin; November/December 2019; v. 131; no. 11/12; p. 1794–1822; https://doi.org/10.1130/B31959.1; 16 figures; 2 tables; Data Repository item 2018204; published online 2 April 2019. 1794 GeologicalFor permission Society toof copy, America contact Bulletin,[email protected] v. 131, no. 11/12 © 2019 Geological Society of America Downloaded from https://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/131/11-12/1794/4855511/1794.pdf by National Forest Service Lib Forest Products Laboratory user on 21 February 2020 Facies architecture in a multidam system, Elwha River, Washington incoming sediment is deposited as a prograding of sedimentation studies addressing cascaded styles of deltaic progradation over the course foreset wedge, defined by a gravelly, subaerial reservoirs can be found. of the reservoirs’ lifetimes. We then used these topset bed, heterolithic foreset slope prograding observations to broadly interpret how reservoir at about the angle of repose, and downstream- Approach and Scope cascades with multiple impoundments can con- fining bottomset wedge. The Gilbert paradigm trol depositional styles and rates and to assess has informed a vast literature of delta dynam- The poorly understood dynamics of sedi- the expression of known environmental events ics and remains the dominant model in lacus- ment deposition in reservoirs may be attributed (e.g., an 18 m reservoir drawdown in 1989 or a trine delta interpretation (e.g., Colella and Prior, in part to an absence of rigorous description 50 yr flood in 2007) in the stratigraphic record. 1990; Talbot and Allen, 1996; Reading and Col- and observation across a range of reservoir sys- linson, 1996; Wetzel, 2001; Snyder et al., 2006). tems. Unlike natural lakes, which are well rep- STUDY AREA Examples of Gilbert-style reservoir deltas resented within the geologic record, the archi- have been documented in Trinity and Eng- tecture of reservoir sediments has thus far been Elwha River Hydrology and lebright Lakes, two reservoirs in northern Cali- relatively inaccessible to detailed study except Geomorphology fornia (Spicer and Wolfe, 1987; Snyder et al., through coring and remote-sensing techniques. 2004, 2006). However, results from other stud- The accelerating pace of dam removals in the The Elwha River watershed (833 km2) is lo- ies of reservoir sedimentation suggest that the late twentieth century and early twenty-first cated on the northern Olympic Peninsula, Wash- Gilbert paradigm is oversimplified or not appli- century, however, has provided an unparalleled ington (Fig. 1). Watershed elevation ranges from cable to many reservoirs. For example, studies opportunity to examine reservoir sedimentation 2432 m in the glaciated core of the Olympic in Lake Mead, located on the Colorado River within a watershed context (O’Connor et al., Mountains to sea level, where the Elwha River and the largest reservoir in the United States, 2015; Major et al., 2017; Foley et al., 2017). discharges to the Strait of Juan de Fuca. Most show that turbidity currents appear to be the With the advent of intentional dam removal, we precipitation occurs from October to March, primary mechanism of sediment transport to the now have a brief window within which to ex- with snow dominating above ~1200 m. Pre- stagnant-basin portions of the reservoir, modify- amine the way in which sediment accumulates cipitation is strongly controlled by elevation and ing the shape and distribution of the delta front in reservoirs before the deposit is eroded by a ranges from more than 6000 mm yr–1 on Mount and transporting relatively coarse-grained sedi- free-flowing river. From this, many questions Olympus to only 1000 mm yr–1 at the mouth ments as far as the dam, >100 km downstream become more approachable, including how of the Elwha River; climate records