Abstract: The flat valleys of braided channels in the Yangtze River source region are about 1 - 3 km wide and feature with well developed side bars attaching to two banks, and their morphologic variations are strongly affected by the runoff. This paper uses Landsat imagery and hydrological data to analyze and compare these variations, and demonstrate three types of stable, sub-stable, and active side bars. We define a percentage change in the morphology of active side bars to describe their quantitative variations and reveal its two peak values ? 80% due to erosion and up to 300% due to deposition. Our analysis shows that the main factors affecting side bar morphology are river discharge and high water duration. In large discharge periods, overbank flow erosion cuts a side bar through and causes a sudden decrease in its elevation, thereby dividing the bar into new central bars and new side bars; in low discharge, as water level drops and deposition occurs, central bars and side bars are merging together, leading to an instant increase in side bar area and a drastic percentage change of active side bars.

Key words: Yangtze River source, braided channel, side bar, river branches, runoff, remote sensing image