Ecological Monographs, 0(0), 2019, e01363 © 2019 by the Ecological Society of America Emerging reservoir delta-backwaters: biophysical dynamics and riparian biodiversity 1,4 1 2 3 MALIA A. VOLKE, W. C ARTER JOHNSON, MARK D. DIXON, AND MICHAEL L. SCOTT 1Department of Natural Resource Management, South Dakota State University, SNP 138 Box 2140B, Brookings, South Dakota 57007 USA 2Department of Biology, University of South Dakota, 414 E. Clark Street, Vermillion, South Dakota 57069 USA 3Watershed Sciences Department, Utah State University, 5210 Old Main Hill, NR 210, Logan, Utah 84322 USA Citation: Volke, M. A., W. C. Johnson, M. D. Dixon, and M. L. Scott. 2019. Emerging reser- voir delta-backwaters: biophysical dynamics and riparian biodiversity. Ecological Monographs 00(00):e01363. 10.1002/ecm.1363 Abstract. Deltas and backwater-affected bottomlands are forming along tributary and mainstem confluences in reservoirs worldwide. Emergence of prograding deltas, along with related upstream hydrogeomorphic changes to river bottomlands in the backwater fluctua- tion zones of reservoirs, signals the development of new and dynamic riparian and wetland habitats. This study was conducted along the regulated Missouri River, USA, to examine delta-backwater formation and describe vegetation response to its development and dynam- ics. Our research focused specifically on the delta-backwater forming at the confluence of the White River tributary and Lake Francis Case reservoir. Objectives of the research were to: (1) describe and analyze the process of delta-backwater formation over space and time; (2) determine by field sampling and GIS mapping how vegetation has responded to devel- opment of the delta-backwater; and (3) compare the woody plant communities of the delta-backwater to those along free-flowing and regulated remnant river reaches. In response to base level changes caused by reservoir filling, the thalweg of the lower 31 km of the original White River channel and adjacent floodplain aggraded by up to 12 m between 1954 and 2011. The overall channel slope flattened from 0.70 to 0.29 m/km. Riparian Populus–Salix forests increased in area by nearly 50% during the post-dam period by colonizing new deltaic and floodplain deposits. Many of the native woody species found along natural and regulated river reaches were also found on the delta-backwater. Woody species sorted along a fluvial to delta gradient; wetland affiliated species (Salix spp., Typha spp.) dominated the delta-backwater near the reservoir while riparian species (Populus, Fraxinus) dominated in upstream portions of the delta-backwater. This habitat complex supports young stands of native riparian vegetation now in decline in remnant reaches protected from flooding. Key words: dams; ecosystem services; flow regulation; Populus; reservoir backwater; restoration; riparian; Salix. dams currently proposed in developing economies (Zarfl INTRODUCTION et al. 2015, Winemiller et al. 2016). The world’s rivers are regulated by over 59,000 large Sediment that historically moved downstream and dams (>15 m high; ICLD 2018). Flow regulation by sustained coastal deltas is now accumulating in and dams has major ecological consequences for riverine upstream of reservoirs. Riparian and wetland ecosystems ecosystems and biota. Dams transform rivers by frag- are forming on deltaic and related fluvial deposits that menting river networks (Jansson et al. 2000, Nilsson were non-existent along the free-flowing rivers of the et al. 2005), replacing bottomlands with artificial reser- past (Volke et al. 2015). The physical processes involved voirs, disrupting natural patterns of sediment transport, in delta formation are well understood from coastal and altering seasonal variation in stream flow (Poff et al. delta research (Giosan and Bhattacharya 2005) with 1997). The global pace in new hydropower dam con- delta form and vegetation assemblages dictated by struction is rapidly increasing, with thousands of new stream flow, sediment particle size, sediment deposi- tional patterns, and the timing, frequency, and duration of inundation on newly deposited substrates (Johnson Manuscript received 3 March 2018; revised 15 December 2018; accepted 7 January 2019. Corresponding Editor: Christer et al. 1985, White 1993). Reservoir deltas form in Nilsson. response to the same physical processes that produce 4 E-mail: [email protected] river mouth deltas, but on different time scales. All river Article e01363; page 1 Article e01363; page 2 MALIA A. VOLKE ET AL. Ecological Monographs Vol. 0, No. 0 mouths are dynamic and subject to changes in base level, Similarly, Johnson (2002) and Volke et al. (2015) have or the elevation at which a river enters still water. Tidal inquired to what extent reservoir delta-backwaters may fluctuations affect coastal deltas and upstream river be able to replace some of the geomorphic processes, reaches on daily time scales, whereas sea level changes shallow aquatic environments, and early successional influence these systems over millennia. Reservoirs are vegetation dynamics that have been lost because of river superimposed on rivers, and base level changes affecting regulation. In regulated river systems, where the restora- newly forming deltas and upstream river reaches fluctu- tion of natural processes is not likely, delta-backwaters ate seasonally in response to reservoir management as may recover some important ecological functions on a well as over multi-year wet and dry cycles. large spatial scale within a river regulation infrastructure Considerable research effort has focused on under- designed primarily to support human economic, munici- standing the physical and biotic processes that structure pal, and recreational interests (Acreman et al. 2014). and maintain riverine and riparian ecosystems (Wolman The Missouri River of the north-central United States and Leopold 1957, Junk et al. 1989, Gregory et al. 1991, is an example of a highly regulated large river where Hupp and Osterkamp 1996, Poff et al. 1997) as well as ecosystems have been altered by decades of flow regula- the downstream effects of dams on riparian ecosystems tion, but little mitigation has occurred. Between 1937 (Johnson et al. 1976, Petts 1984, Friedman et al. 1998, and 1963, six large U.S. Army Corps of Engineers Nilsson and Berggren 2000). Aside from work on devel- (USACE) dams and associated reservoirs were con- opment and succession of shoreline plant assemblages in structed along the mainstem Missouri River in Mon- the zone of fluctuation around reservoir margins (Nils- tana, North Dakota, South Dakota, and Nebraska. son and Keddy 1988, Nilsson et al. 1997, Merritt et al. Numerous studies have been conducted to gauge the 2010), comparatively little is known about the upstream ecological effects of damming on the remnant sections effects of dams, especially in those dynamic zones where of channel and floodplain below or between dams (John- a river or stream enters a reservoir (Xu and Shi 1997, son et al. 1976, 2012, NRC 2002, 2011, Dixon et al. Liro 2019). 2012, 2015, Scott et al. 2013). A major finding of these Deltas and associated backwater zones form in two studies is that the cessation of flooding and channel locations in reservoirs: where tributaries enter reservoirs movement has curtailed cottonwood (Populus deltoides) laterally and where the mainstem river enters the forest reproduction, while pre-dam forests are aging and upstream end of the reservoir. We define a delta-back- senescing. Under current management, these forests are water as a continuum of landforms where sediment expected to lose much of their biodiversity-rich cotton- accumulates due to the presence of a reservoir. To iden- wood component during this century (Dixon et al. 2012, tify different hydrologic influences in delta-backwater Scott et al. 2013). environments, we distinguish shorter-term flooding Prominent reservoir delta-backwaters have been form- related to fluvial processes from longer-term inundation ing at nine locations along the Missouri River, including related to reservoir fluctuations (Flick et al. 2012, Liro where the free-flowing White River enters Lake Francis 2019). Both mainstem and tributary delta-backwaters Case. Surprisingly, despite the worldwide abundance of comprise a complex that can be subdivided into two dammed rivers, there is virtually no published research parts: a delta that progrades into the reservoir and a on delta-backwater ecosystems (Volke et al. 2015, Liro backwater that aggrades upstream into the former river 2019). The dearth of research on these systems, yet their bottomland. Reservoir levels set a new base level that potential to contribute to the recovery of some of the represents the downstream end of the drainage network ecological values of riparian and wetland ecosystems in where fluvial processes end and delta formation begins regulated riverscapes, stimulated us to study the White (Leopold and Bull 1979). Fluctuations in reservoir River delta-backwater. Our study had three main objec- backwaters shift base level and thus the locations of tives as follows: (1) describe and analyze the process of delta-backwater processes, which include sediment delta-backwater formation over space and time; (2) aggradation in upstream reaches (Holste 2013), alter- determine by field sampling and GIS mapping how veg- ation of channel form and process (Liro 2017), inunda- etation has responded to development of the delta-back- tion of existing bottomland vegetation, and exposure of water;