Assessment of the Lower Ping River’s riverbank erosion and accretion, Northern Thailand using geospatial technique; implication for river flow and sediment load management Nikhom Chaiwongsaen*, Montri Choowong Morphology of Earth Surface and Advanced Geohazards in Southeast Asia Research Unit (MESA RU), Department of Geology, Chulalongkorn University, Phayathai Road, Pathumwan, Bangkok, 10330, Thailand *E-mail: nchaiwon@hotmail.com Abstract The Lower Ping River downstream from the Bhumibol Dam has suffered from the excessive sedimentation. The rapid growth of sandbars occurs along the 129 km of the downstream reach within the succession of weir. However, within this succession of weir severe riverbank collapses can also occur locally as rapid growth of sandbars makes the river narrower which in turn increase its flow velocity and power. The objective of this research is to assess riverbank accretion and erosion using remote sensing and GIS techniques. Comparison of satellite images from 2007 and 2017 shows that the total emerged sandbar area increases up to 5,702,557 m 2 . The total area of riverbank erosion is 1,150,943 m 2 and the total area of accretion is over 10,561,530 m 2 . Digital Shoreline Analysis System (DSAS) software is also used to determine the rate of changes of riverbank erosion and accretion. The DSAS output locates significant changes of river accretion and erosion along the riverbanks with the average rate of erosion at 1.24 m/year and the average rate of accretion at 4.89 m/year between 2007 and 2017. The rapid growth of sandbars along the river reach is responsible for the shallowing and narrowing of river embankment leading to rapid overflow during flooding. The result from this study enables all authorities and stakeholders to recognize the specific location, which severely affected by riverbank accretion and erosion as well as to locate the areas experienced rapid growth of sandbar and huge river channel shifting. There are several possibility implications from this study involving construction of weir, riverbank collapse prevention, and management of intense in-channel sand mining along this river. Keywords: Lower Ping River, Sandbar, Riverbank erosion/accretion, Succession of weir, In-channel sand mining 1. Introduction The irrigation system for the Ping River has been established since the early 50’s. All development projects mainly involve flood mitigation, supplying water to farmlands, and hydroelectric power generation. The Bhumibol Dam completed since 1964 and the Lower Mae Ping Dam constructed in 1991 to be operated as an additional reversible hydropower plant system 5 km downstream of the Bhumibol Dam. In addition, a succession of 7 weirs has been installed within the past decade along the Ping River in Changwat Kamphaeng Phet and Nakhon Sawan. These irrigation projects have provided numerous socioeconomic benefits not only for agriculture in the irrigation areas, but also played the important role in flood control. When Bhumibol Dam was completed, it separates the Ping River into the Upper and Lower Ping Rivers. Nowadays, adverse effects on hydraulic and sediment regimes along the river due to irrigation projects have been recognized and documented (Baker et al., 2010; Francis et al., 2005; Magilligan and Nislow, 2005; Magilligan et al., 2016; Renshaw et al., 2014; Shields et