SINUOSITY OF COASTAL RIVERS IN NORTHERN SUMATRA OVER THE LAST 150 YEARS: RESPONSE TO ENVIRONMENTAL CHANGES AND TECTONIC FORCING

Aerial imagery and Dutch colonial maps that extend to 1838AD provide a detailed history of rapid change in the sinuosity of coastal rivers in northern Sumatra. Such morphological changes suggest that major tectonic events, among other environmental parameters, may have played a role in the meandering patterns. The west coast of Sumatra runs parallel to the Sunda Trench, and the northern end of the island was heavily impacted by the magnitude 9.1 Sumatra-Andaman earthquake and subsequent tsunami in December of 2004. Previous studies have investigated the effect of tectonic forces on coastline evolution in the area, a result of sediment transport and regional co-seismic subsidence and inter-seismic uplift. These vertical adjustments of the coast may also affect the morphology of coastal rivers, and effects may be particularly apparent in rivers that run perpendicular to the coast/trench as they are subject to gradient changes along the length of their valley. Colonial maps from the Dutch National Archives, recent topographic maps, and aerial images were traced and analyzed using GIS. The sinuosity (ratio of stream length to valley length) was calculated for three rivers – the Woyla, Bubon, and the Meulaboh. An overall increase in sinuosity over the last 150 years can be observed, including rapid changes around 1900AD and 1940AD. This pattern is compared to known major earthquakes and tsunamis and the deforestation history of northern Sumatra. An evaluation of these environmental factors will lead to an understanding of the factors influencing river morphology, as well as estimate the relative magnitude of their influence. Such a detailed history of coastal rivers in an active tectonic setting will provide insight into complicated interactions of various environmental parameters on fluvial morphology.