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Incised valleys are canyon‐like features that initially form near the highstand shoreline and evolve over geological time as rivers erode into coastal plains and continental shelves to maintain equilibrium‐gradient profiles in response to sea‐level fall. Most of these valleys flood during sea‐level rise to form estuaries. Incised‐valley morphology strongly controls the rate of creation of sediment accommodation, valley‐fill facies architecture and the preservation potential of coastal lithosomes on continental shelves, and affects coastal physical processes. Nonetheless, little is known about what dictates incised‐valley size and shape and whether these metrics can be used to explain principal formation processes. The main control on alluvial channel morphology over human time scales is discharge; this is based on numerous empirical studies and is well‐constrained because all variables are easily measured at this short time scale. Knowledge of long‐term river evolution over a complete glacio‐eustatic cycle, on the contrary, remains largely conceptual, experimental and based on individual systems because variables that are thought to drive morphological change are not easily quantified. In spite of this difficulty, existing models of incised‐valley formation at the coast suggest that valley evolution is driven largely by downstream forcing mechanisms, highlighting sea‐level and shelf gradient/morphology as the dominant controls on valley incision. Although valleys are cut by rivers, whose channels are a direct reflection of discharge, little empirical data exist in coastal areas to address the degree to which valley evolution is governed by upstream controls. The late Quaternary is the best time period to examine because it provides the most complete sedimentary record and many variables, including sea‐level, tectonics, substrate lithology and drainage network characteristics, are accurately constrained. Here, 38 late Quaternary valleys along the coast of two different passive continental margins are compared, which suggests that valley shape and size are governed primarily by upstream, intrinsic controls such as discharge. Valley width, depth and cross‐sectional area are found to be predictable at the highstand shoreline and are scaled with the size of their drainage basin, which has important implications for estimating sediment discharge to continental shelves and deep water environments during periods of low sea‐level.

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[1] Low-relief fluvial systems that originate in the lower coastal plain and discharge into estuaries are common along passive margins. These watersheds are thought to be disconnected from their termini by floodplains, which buffer the sediment-routing system by sequestration. Here, we present a detailed study of the Newport River, a typical lower coastal-plain system, which reveals high connectivity between watershed and delta. Connectivity is measured as the time lag between initiation of a silviculture operation, which increased landscape erosion, and when the sediment appeared at the bay-head delta. The time lag, measured from aerial photographs and sedimentation rates calculated from 210 Cs-activities in cores from the watershed and delta, is <3 years. Most lower coastal-plain rivers are steeper and have less floodplain accommodation available for storage than their larger counterparts that originate landward of the fall line, which promotes higher connectivity between upstream and downstream.

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Mattheus, C.R. and Fowler, J.K., 2015. Paleotempestite distribution across an isolated carbonate platform, San Salvador Island, Bahamas. Journal of Coastal Research, 31(4), 842-858. Coconut Creek (Florida), ISSN 0749-0208. Sediment cores were collected from coastal ponds across San Salvador Island, an isolated Bahamian carbonate platform, and integrated with aerial photographs and field-survey data to provide insight into how spatially variable storm records are at the island scale and what drives this variance. A better understanding of local tempestite generation and preservation potential is crucial for correlating events across more regional scales. Sandy storm layers, products of beach overwash and/or high-energy aeolian transport from the shore into coastal ponds, are easily distinguished from flocculated carbonate muds, microbial mats, and other organics that ordinarily sequester in the stagnant, low-energy coastal water bodies. Interpreted paleotempestites do not correlate well between studied ponds using a strictly stratigraphic approach, although all sites investigated are situated within 150 m of a sandy beach, are similar in size and morphology, and coastal terrains across the small island are comparable. While wrack lines show that individual storms produce surges that vary in height along the island's 60 km-long coastline, differences in the spatial distribution of storm deposits cannot be explained solely as a function of variable surge heights, storm trajectories, and shore aspect; they also reflect intrinsic, biogeomorphic variables such as pond-fringing vegetation characteristics and coastal geomorphic configuration. A complete storm chronology for the island is not generated in any single pond location, and diagenetic processes compound this variance, producing a high degree of spatial heterogeneity of storm records. Nonetheless, the Caribbean-wide Hurricane Hyperactivity Period is distinguished in core from the modern period of relative inactivity by an up-core transition from sandy, stratified pond deposits containing storm layers to heavily bioturbated sediments indicative of low-E deposition. This facies transition, recognized across the island, provides a regional correlation tool for evaluating the distribution of paleotempestites within context of major climate regimes. ADDITIONAL INDEX WORDS:Coastal pond, washover fan, carbonate, Hurricane Frances, storm deposit.