A conceptual model of depositional, rather than erosional, tidal channel development in the rapidly prograding Skagit River Delta (Washington, USA)

Abstract

The origin and growth of blind tidal channels is generally considered to be an erosional process. This paper describes a contrasting depositional model for blind tidal channel origin and development in the Skagit River delta, Washington, USA. Chronological sequences of historical maps and photos spanning the last century show that as sediments accumulated at the river mouth, vegetation colonization created marsh islands that splintered the river into distributaries. The marsh islands coalesced when intervening distributary channels gradually narrowed and finally closed at the upstream end to form a blind tidal channel, or at mid-length to form two blind tidal channels. Channel closure was probably often mediated through gradient reduction associated with marsh progradation and channel lengthening, coupled with large woody debris blockages. Blind tidal channel evolution from distributaries was common in the Skagit marshes from 1889 to the present, and it can account for the origin of very small modern blind tidal channels. The smallest observed distributary-derived modern blind tidal channels have mean widths of 0·3 m, at the resolution limit of the modern orthophotographs. While channel initiation and persistence are similar processes in erosional systems, they are different processes in this depositional model. Once a channel is obstructed and isolated from distributary flow, only tidal flow remains and channel persistence becomes a function of tidal prism and tidal or wind/wave erosion. In rapidly prograding systems like the Skagit, blind tidal channel networks are probably inherited from the antecedent distributary network. Examination of large-scale channel network geometry of such systems should therefore consider distributaries and blind tidal channels part of a common channel network and not entirely distinct elements of the system. Finally, managers of tidal habitat restoration projects generally assume an erosional model of tidal channel development. However, under circumstances conducive to progradation, depositional channel development may prevail instead.

Signatures of sea level changes on tidal geomorphology: Experiments on network incision and retreat

“…Some studies suggest tidalchannel development to be the result of depositional rather then erosional processes [e.g., Redfield, 1965;Hood, 2006], whereas others consider erosion as the dominant process [e.g., Fagherazzi and Sun, 2004;Perillo et al, 2005;D'Alpaos et al, 2005D'Alpaos et al, , 2007bVlaswinkel and Cantelli, 2011]. In the erosional scenario, differential erosion drives the evolution: Network configuration is driven by local excess in the bed shear stress compared to the critical threshold for sediment motion, as modelled in this experimental context.…”

“…Recently, several mathematical models have been developed to describe the morphogenesis and development of tidal networks [e.g., Fagherazzi and Sun, 2004;D'Alpaos et al, 2005;Marciano et al, 2005;Kirwan and Murray, 2007;Temmerman et al, 2007]. These models deepen our understanding of tidal network growth, otherwise analyzed solely on the basis of field observations and related conceptual models [e.g., Redfield, 1965;Allen, 1997;Perillo et al, 2005;Hood, 2006;D'Alpaos et al, 2007b;Hughes et al, 2009]. In two cases tidal network initiation and development have been addressed with physical models [Stefanon et al, 2010;Vlaswinkel and Cantelli, 2011].…”

Contrasted sediment processes and morphological adjustments in three successive cutoff meanders of the Danube delta

“…The TM and ETM+ data had 28, 15, and 20 m pixels. Further details of the map analyses, including estimation of rectification and digization error, have been previously described (Elliott and Gyetvai, 1999;Hood, 2004Hood, , 2006Hughes et al, 2006). The error sources (RMSE) include different spatial resolution of TM and ETM+ data, inaccuracies from manual delineation of banklines, water level differences, effects of vegetation, and other systematic errors.…”

Currents in a small channel on a sandy tidal flat

“…Despite their potential importance as pathways for sediment transport , Christiansen et al 2000, Hood 2006, Green and Coco 2007, Ralston and Stacey 2007 many others) and as roughness elements affecting the large-scale flow (Whitehouse et al 2000), there are few observations of currents in small dendritic channels. In a small channel in the salt marshes and mudflats of San Francisco Bay, observations of currents were made along 3 cross-channel transects for two few-day (Ralston and Stacey 2005b) and two 2-week periods (Ralston and Stacey, 2006).…”