Fluvial-tidal transition channel Channel geomorphology along the fluvial- tidal transition, Santee River, USA Raymond Torres† Department of Earth and Ocean Sciences, University of South Carolina, 701 Sumter Street, EWS617, Columbia, South Carolina 29208, USA ABSTRACT INTRODUCTION Pittaluga et al., 2015). Since terrestrial runoff, tidal forcing, and storm surge vary with time, There exists a rich understanding of chan- Along the river continuum, a single channel the tidal effects on channel form vary along the nel forms and processes for rivers with uni- can transition from fluvial to tidal dominance in channel (Wright et al., 1973; Dalrymple and directional flows, and for their estuarine flow and sedimentary processes, and in benthic Choi, 2007). For instance, river flow responses components with bidirectional flows. On the ecology (Dalrymple and Choi, 2007; Jablon- to tidal oscillations can vary temporally with lo- other hand, complementary insight on the ski and Dalrymple, 2016). In particular, many cal weather effects (<day), dam releases (days), transitional reach linking these flows has not coastal river systems experience the combined terrestrial flood waves (weeks), seasonal effects been well developed. This study highlights influence of terrestrial runoff processes and ma- (months), or changes in climate (decades). The analyses of high-resolution, high-accuracy rine storms and tides (e.g., Wright et al., 1973; way in which these conditions translate to tide- bathymetric surveys along a coastal plain Bokuniewicz, 1995; Ensign et al., 2013; Sassi influenced channel geomorphology remains river at 30–94 km upstream of the estuary and Hoitink, 2013), giving rise to at-a-station largely unexplored (Phillips and Slattery, 2007; mouth. The goal of this work was to identify flow conditions ranging from unidirectional Ensign et al., 2014). geomorphic indicators of the fluvial-tidal downstream to bidirectional tidal (e.g., Jay, Greater insight into fluvial-tidal channel form transition channel. Trends with sharp breaks 1991; Godin, 1999). Coastal plain rivers are es- is needed to help guide or constrain complemen- were detected in along-channel variations in pecially susceptible to the effects of marine forc- tary studies of the transition zone, and to provide depth, hydraulic radius, channel shape, bed ing due to their low elevations and low gradient, a context for coastal river system dynamics. A elevation, and sinuosity, but cross-section and they can be expected to have well-developed fruitful approach to studying the fluvial-tidal area of flow provided the greatest insight. transition channel reaches that link fluvial- and transition would be to combine work on hy- The transition channel is characterized as a tidal-dominant conditions (Dalrymple and Choi, drodynamics, sedimentology, land use, water- reach with greater than 50% decline in area 2007). In this context, the fluvial-tidal transition shed dynamics, and geomorphology, because of flow relative to the background values at is taken as the geomorphic transition associated all are intimately coupled. However, such an the upstream and downstream ends. Further with unidirectional to bidirectional dominant approach typically is not tractable within the downstream, the river is a mixed bedrock- flows (after Yankovsky et al., 2012). context of a single research project. Neverthe- alluvium system, and a 22 km reach of dis- Typically, as tidal waves enter the estuary and less, new insight on fluvial-tidal transition zone continuous bedrock outcrops has a marked advance upstream, their amplitudes increase due geomorphology can provide benefits to a range influence on local channel metrics, and cor- to upstream channel area convergence (Fried- of studies. For instance, the research theme of responding backwater effects on upstream richs and Aubrey, 1988), and this gives rise to processes and forms associated with the fluvial- metrics. Despite the confounding effects tidal oscillations in stage, and the potential for tidal transition zone has received a great deal of of bedrock on channel form, the transition flow reversals far upstream of both the estuary recent attention (see Ashworth et al., 2015). channel linking estuarine and fluvial chan- mouth (Wright et al., 1973; Sassi and Hoitink, Much of what is known about modern nel segments is apparent as a 13 km geo- 2013) and the zero isohaline (Allen, 1991). fluvial-tidal channel geomorphology comes morphic discontinuity in flow area along a However, further upstream, the tidal waves be- from a handful of papers. Ashley and Renwick channel reach of relatively uniform width. come increasingly distorted due to bottom fric- (1983) reported that the transition from fluvial Finally, it is proposed that bedrock outcrops tion and superposition of fluvial currents (e.g., to tidal currents, and the corresponding change enhance tidal energy dissipation and influ- Godin, 1999), resulting in an asymptotic decline in cross-section shape and other metrics, should ence the position of the fluvial-tidal transi- in amplitude (e.g., Jay, 1991). Complete tidal occur in a gradual manner. However, they did tion reach, and associated geomorphic and wave dissipation occurs tens to hundreds of not address the effects of variable bedrock li- hydrodynamic features. kilometers upstream of the coast, at the “tidal thology and slope associated with their chan- limit” (or “head of tide,” “limit of tidal effects,” nel reach crossing the “fall line,” the crystalline “tidal length”), which is taken as the upstream bedrock to coastal plain boundary (Ashley et limit of measureable tidal oscillations in stage al., 1988). Hence, their channel properties re- (e.g., Wright et al., 1973; Dalrymple et al., 1992; ported as being responsive to fluvial-tidal in- †[email protected] van den Berg et al., 2007; Ensign et al., 2013; teractions remain speculative. Gurnell (1997) GSA Bulletin; November/December 2017; v. 129; no. 11/12; p. 1681–1691; doi: 10.1130/B31649.1; 4 figures; Data Repository item 2017186; published online 30 June 2017. Geological Society of America Bulletin, v. 129, no. 11/12 1681 © 2017 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/129/11-12/1681/3949807/1681.pdf by guest on 25 September 2021 R. Torres found systematic channel variations in 122 estuary of a coastal plain river (sensu Savenije flux over 20 yr time intervals to estimate the cross sections along an 18 km channel reach. et al., 2008). The detailed observations and pre-European, predam, and postdam conditions. However, the study site only experienced the ef- analyses presented here will help improve our The corresponding values are 2.24, 5.80, and fects of the higher tides and no current reversals understanding of transition zone morphody- 0.81 Mt yr–1, respectively. At this time, it is not due to the placement of a downstream weir, and namics, and they will help to support the util- known if the channel has attained morphody- as a consequence, those findings provide lim- ity of generalized facies models that are taken namic equilibrium in response to dam effects. ited insight. Further, Gardner and Bohn (1980) to represent average conditions over much lon- However, a comparison of aerial images from and later Ensign et al. (2013) provided a con- ger time scales (e.g., Bokuniewicz, 1995; Blum 1939 and 2011 indicates that the channel pattern ceptual view on how terrestrial and tidal chan- and Tornqvist, 2000; Cattaneo and Steel, 2003; and width have remained relatively stable over nel cross sections should differ by highlighting Phillips and Slattery, 2007; van den Berg et al., 72 yr, except for ~12 m of widening near the the characteristic change in channel properties 2007; Dalrymple and Choi, 2007; Jablonski rediversion canal where flows are returned to the due to the onset of tidal effects, with particular and Dalrymple, 2016). Also, given the dynamic system (Fig. 1). emphasis on width. Inokuchi (1989) and later nature of the transition channel, it is likely that This study is focused on an ~64 km reach Nittrouer et al. (2011b) examined particle size, both process and form respond at time scales between 30 km and 94 km from the mouth. channel cross sections, and long profiles of commensurate with land-use change, climate For clarity, hereafter “mouth” is taken to mean hundreds of kilometers of the lower Mississippi change, and sea-level rise (Florsheim et al., “estuary mouth,” where the estuary meets the River. Overall, inconsistencies in sediment cali- 2008). Moreover, with the relative uniformity of ocean (sensu Savenije, 2012). The site was cho- ber and bed slope were interpreted as resulting the modern southeastern U.S. coastal plain land- sen because a U.S. Geological Survey (USGS) from coastal backwater effects (e.g., Fernandes scape (Hayes, 1994), it is likely that the channel gauging station at 59 km upstream of the mouth et al., 2016). Tidal distortion and tidal wave- features reported here may apply to many rivers shows that the reach has intermittent tidal- and length effects on channel properties were re- that discharge to the southeastern U.S. Atlantic non-tidal-dominant flow conditions. The down- ported by Wright et al. (1973) and expressed as Bight (Fig. 1), and perhaps to rivers that tra- stream limit of the study reach was set by the equilibrium between the tidal prism and equal verse coastal plains in general, landscapes that position where distributary channels begin, by work per unit channel bed area for the channel occupy 5.7 × 106 km2 worldwide (Colquhoun, changes in land cover, and by the presence of of a funnel-shaped estuary. Phillips and Slattery 1968). Overall, this work attempts to fill a gap dikes along the channel. The upstream limit was (2008) analyzed long- and cross-channel river in knowledge of the geomorphic structure of the set by the position where <0.04 m tidal oscilla- profiles to assess the role of topography and fluvial-tidal transition channel, and it is driven tions were detected during low-flow conditions.