Linking channel hydrology with riparian wetland accretion in tidal rivers

Abstract

[1] The hydrologic processes by which tide affects river channel and riparian morphology within the tidal freshwater zone are poorly understood yet are fundamental to predicting the fate of coastal rivers and wetlands as sea level rises. We investigated patterns of sediment accretion in riparian wetlands along the nontidal through oligohaline portion of two coastal plain rivers in Maryland, U.S., and how flow velocity, water level, and suspended sediment concentration (SSC) in the channel may have contributed to those patterns. Sediment accretion was measured over a 1 year period using artificial marker horizons, channel hydrology was measured over a 1 month period using acoustic Doppler current profilers, and SSC was predicted from acoustic backscatter. Riparian sediment accretion was lowest at the nontidal sites (mean and standard deviation = 8 ± 8 mm yr À1 ), highest at the upstream tidal freshwater forested wetlands (TFFW) (33 ± 28 mm yr À1 ), low at the midstream TFFW (12 ± 9 mm yr À1 ), and high at the oligohaline (fresh-to-brackish) marshes (19 ± 8 mm yr À1 ). Channel maximum flood and ebb velocity was twofold faster at the oligohaline than tidal freshwater zone on both tidal rivers, corresponding with the differences in in-channel SSC: The oligohaline zone's SSC was more than double the tidal freshwater zone's and was greater than historical SSC at the nontidal gages. The tidal wave characteristics differed between rivers, leading to significantly greater in-channel SSC during floodplain inundation in the weakly convergent than the strongly convergent tidal river. High sediment accretion at the upstream TFFW was likely due to high river discharge following a hurricane.

Contemporary Deposition and Long-Term Accumulation of Sediment and Nutrients by Tidal Freshwater Forested Wetlands Impacted by Sea Level Rise

“…In summary, temporal variability in vertical accretion in our study was dependent on spatial scale, from plot to river to region, without obvious explanation. In addition to this scale-dependent variation in spatial and temporal patterns of sedimentation under typical conditions, tropical cyclones can result in very large amounts of episodic deposition in intertidal forested wetlands (Castañeda-Moya et al 2010;Ensign et al 2014b). The large discrepancy in sediment mass trapping rate over time is likely reflective of changes in sediment availability over time.…”

“…No difference in sediment deposition was observed between tidal freshwater marsh and salt marsh, however (Temmerman et al 2003), suggesting greatest contemporary deposition rates in estuaries likely occur in oligohaline marsh. Within the TFFW zone, sediment accretion has been shown to decrease from the upper reaches of tidal freshwater rivers (near the limit of tide) to the downriver limit of TFFW (near oligohaline wetlands) (Baldwin 2007;Ensign et al 2014b). These longitudinal gradients in contemporary deposition suggest greater sedimentation in the upper reaches of TFFW due to greater availability of watershed-derived fluvial sediment, minimization of sedimentation in the lower reaches of TFFW after exhaustion of watershed sediment load by sediment removal in the upper reaches, and an increase in sedimentation in oligohaline marshes due to the ETM increasing the availability of estuarine or marine sediment.…”

“…This enhanced availability of sediment to tidal freshwater and oligohaline marshes along lower tidal rivers is often assumed to be associated with the ETM (Darke and Megonigal 2003;Loomis and Craft 2010). Flood-tide dominant transport also can move upstream fine sediment from the coast, estuarine sediment mobilized by dredging, and entrained channel bed sediment in tidal rivers (Meade 1969), leading to enhanced deposition in oligohaline marshes (Ensign et al 2014b). Similarly, oceanic and coastal sediment can be transported to and deposited in estuaries (Newell et al 2004).…”

“…Deposition rates in the upriver reaches of TFFW are generally greater than those of downriver reaches of TFFW as watershed sediment delivered to the tidal zone is trapped by upriver TFFW (Kroes et al 2007;Ensign et al 2014b;Hupp et al 2015). Increased sediment deposition farther downriver of TFFW in oligohaline marsh coincides with increases in herbaceous biomass that may lead to a positive feedback mechanism between plant biomass and sediment trapping (Ensign et al 2014a).…”

“…Increased sediment deposition farther downriver of TFFW in oligohaline marsh coincides with increases in herbaceous biomass that may lead to a positive feedback mechanism between plant biomass and sediment trapping (Ensign et al 2014a). Channel hydraulics also increase sediment concentrations in lower, oligohaline tidal rivers (Ensign et al 2014b). Proximity to an estuarine turbidity maximum (ETM) in oligohaline reaches of estuaries also leads to enhanced wetland sediment deposition and accumulation rates (Darke and Megonigal 2003;Loomis and Craft 2010).…”