Geomorphology of the fluvial–estuarine transition zone, lower Neuse River, North Carolina

Abstract

The fluvial-estuarine transition zone (FETZ) of the Neuse River, North Carolina features a river corridor that conveys flow in a complex of active, backflooded, and high-flow channels, floodplain depressions, and wetlands. Hydrological connectivity among these occurs at median discharges and stages, with some connectivity at even lower stages. Water exchange can occur in any direction, and at high stages the complex effectively stores water within the valley bottom and eventually conveys it to the estuary along both slow and more rapid paths. The geomorphology of the FETZ is unique compared to the estuary, or to the fluvial reaches upstream. It has been shaped by Holocene and contemporary sea-level rise, as shown by signatures of the leading edge of encroaching backwater effects. The FETZ can accommodate extreme flows from upstream, and extraordinary storm surges from downstream (as illustrated by Hurricane Florence). In the lower Neuse-and in fluvial-to-estuary transitions of other coastal plain rivers-options for geomorphological adaptation are limited. Landscape slopes and relief are low, channels are close to base level, sediment inputs are low, and banks have high resistance relative to hydraulic forces. Limited potential exists for changes in channel depth, width, or lateral migration. Adaptations are dominated by the formation of multiple channels, water storage in wetlands and floodplain depressions, increased frequency of overbank flow (compared to upstream), and adjustments of roughness via vegetation, woody debris, multiple channels, and flow through wetlands.

Contingent partitioning and adaptation in hydrological systems

“…These scenarios are based on field observations of the fluvial–estuarine transition zone (FETZ) of the lower Neuse River, North Carolina (Figure 3), made in the context of evaluating impacts of Hurricane Florence (Phillips, 2022a) and examining the geomorphology and hydrology of the FETZ (Phillips, 2022b).…”

“…Within the generally wet valley bottom, characterized by fine‐grained and organic hydric alluvial soils, there occur ‘islands’ of sandier fluvial terrace soils where non‐hydrophytic vegetation also occurs. Further details on the study area and on observation methods and data sources are provided by Phillips (2022a, 2022b).…”

Lowland rivers: Geomorphology, human impacts, and management

“…Because of their vast size, fine‐grained deposits, and low relief, flood basins are associated with complex inundation processes. This includes overbank, groundwater, local (small) tributaries, pluvial sources, and at the coast, sea level rise (Hudson, 2021, p. 48; Keim et al, 2022; Phillips, 2022). And flood basin inundation can be exacerbated by ground subsidence, particularly when soft organic‐rich deposits undergo oxidation due to embankment and drainage.…”

“…Towards the coast, the boundary between the fluvial and marine setting is not abrupt, but instead is a transitional zone that in itself represents a unique environment. Indeed, the fluvial‐estuarine transition zone (FETZ) is the key environment in which sea level rise initially influences the fluvial system (Phillips, 2022), and can be considered the proverbial ‘canary in the coal mine’. Southern Atlantic coastal plain rivers in particular are undergoing higher rates of eustatic sea level rise than the global average (Marra et al, 2022; Sweet et al, 2022).…”

“…The 10 special issue contributions herein reviewed concern important fundamental and applied topics within lowland river science, specifically highlighting three key themes. These include ‘natural’ river channel dynamics within settings that vary from the standard body of knowledge (Guha et al, 2022; Rogério et al, 2022; Sinha et al, 2022), geomorphic response to channel engineering (Mossa & Chen, 2022; Marteau et al, 2022; Tang et al, 2022; Wu et al, 2022), and flood basin management in response to environmental change (Keim et al, 2022; Phillips, 2022; Valentine et al, 2022). The categories are not mutually exclusive, and most contributions have linkages across multiple themes.…”