![]() Continued channel avulsions in the alluvial fan changed the sediment loads of the multiple river pathways created by meander belt migration, leading to delta switching downstream. Before this, westward tilting because of movements along the two faults that bound the alluvial plain and switching of the main distributary channel of the Agno alluvial fan caused the lower Agno River to migrate episodically to the southwest. These paleodeltas formed during the sea level fall from 2.4 ka to the present. Within 7 kilometers of the bayhead plain, at least 15 paleodelta lobes were alternately, successively, or simultaneously built by three river systems. The pair of wedge-shaped sets was used to map paleodelta lobes the continuous-parallel sets record transgressive events. In the more landward portions of the coastal plain, similar but older pairs of wedge-shaped beach ridge sets occur between more continuous to parallel sets that also truncate their apical ends. The shoreline retreated in the abandoned delta in contrast, paired sets of beach ridges that diverge toward the new river mouth formed a cuspate delta. The latest delta switching occurred after 1935, when the downstream portion of the Agno River, the largest river discharging into the bayhead, was artificially diverted to a more direct route. Beach ridges and relict channel patterns record delta progradation and switching that, together with meander belt migration, constructed the bayhead plain. ![]() Remotely sensed images, maps, charts, and historical accounts document the evolution of the Lingayen Gulf bayhead plain in the northwestern Philippines. ![]()
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