Delta distributary dynamics in the Skagit River Delta (Washington, USA): Extending, testing, and applying avulsion theory in a tidal system W. Gregory Hood ⁎ Skagit River System Cooperative, P.O. Box 368, LaConner, WA 98257, USA abstract article info Article history: Received 11 March 2010 Received in revised form 8 July 2010 Accepted 12 July 2010 Available online 16 July 2010 Keywords: Avulsion Distributary dynamics Skagit delta restoration Analysis of historical aerial photos shows that Skagit Delta (Washington, USA) distributary dynamics are consistent with the Slingerland and Smith model of avulsion dynamics where the ratio of the water surface slopes of the two branches of a bifurcation predicts avulsion stability. This model was extended to predict distributary inlet (upstream) width and bankfull cross-sectional area. The water surface gradient ratio for a bifurcation pair predicted distributary width well; the lowest R 2 was 0.61 for the 1937 data points, but R 2 ranged from 0.83 to 0.90 for other year-specific regression lines. Gradient ratios were not constant over the historical record; from 1937 to 1972 the mainstem river channel lengthened by 1250 m in the course of marsh progradation, while distributary lengthening was comparatively negligible. Consequently, the gradient advantage of the distributaries increased and their channels widened. After the mainstem river terminus stabilized from 1972 to the present, the distributaries continued to lengthen with marsh progradation, so that distributary gradient advantage steadily declined and the distributaries narrowed. While distributary cross sections were not available for the historical period, they were surveyed in 2007 near the distributary inlets. Gradient ratio was more closely related to distributary inlet bankfull cross- sectional area (R 2 = 0.95) than to minimum distributary width for any photo year examined. Applying this form of analysis to Skagit Delta distributaries that have been dammed in the course of agricultural development suggests that their restoration to stabilize eroding marshes at their outlets and recover salmon migration pathways would be feasible without significant risk of full river avulsion. © 2010 Elsevier B.V. All rights reserved. 1. Introduction River distributaries are the framework upon which river deltas are built. As a river delivers sediment to its delta, the delta progrades and the river progressively divides into distributaries. Thus, the processes of delta and distributary network formation are inextricably interre- lated (e.g., Edmonds and Slingerland, 2007; Stouthamer and Berend- sen, 2007). The tight coupling between distributary and delta dynamics are illustrated by the classic description of delta lobe switching in the Mississippi Delta, where periodic river avulsion has caused the location of the active delta to shift hundreds of kilometers (Coleman, 1988). Distributary network geometry is potentially the most important factor controlling delta landforms (Coleman, 1988; Syvitski et al., 2005) and related hydrological, geological, and ecological processes. In addition to distributing river water over a delta, distributaries also distribute river-borne sediments, nutrients, stream wood, fish, and other aquatic organisms to estuarine and riverine floodplain wetlands along the distributaries. Because distrib- utary network geometry in river-dominated estuaries affects the spatial distribution of estuarine salinity gradients and sedimentation patterns and these affect vegetation distribution, distributary geom- etry also affects wildlife distribution patterns through its effect on their habitat. Consequently, an understanding of distributary dynam- ics can be useful to sustainable habitat management (e.g., habitat protection and restoration) for important fish and wildlife in deltaic systems. Human engineering significantly influences river distributaries and the growth and evolution of their associated deltas (Pasternack et al., 2001; Syvitski and Saito, 2007). Direct human modifications of distributary networks can include distributary blockage with dikes or distributary excavation to redirect river flows. Indirect impacts to distributaries result from system modifications such as dam con- struction, which moderates seasonal flood pulses and results in sediment retention in the dam reservoirs, or water withdrawals for irrigation or direct human consumption that effectively reduces the hydraulic size of the river basin (Syvitski, 2008). Sustainable system management requires better understanding of geoecological con- straints on management sustainability and a better understanding of distributary network dynamics in particular. River distributaries are primarily formed by avulsion (Slingerland and Smith, 2004) or channel bifurcation during mouth bar develop- ment and delta progradation (Edmonds and Slingerland, 2007). Geomorphology 123 (2010) 154–164 ⁎ Tel.: +1 360 466 7282; fax: +1 360 466 4047. E-mail address: ghood@skagitcoop.org. 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2010.07.007 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph