The lateral stability of unlined sandbed irrigation canals is a problem in the operation and maintenance of gravity irrigation systems. This problem has been studied from the aspect of meandering thalwegs that develop in otherwise straight channels. Hydrodynamic stability analysis of two-dimensional flow in sandbed channels has been used to predice the conditions under which meandering thalwegs will develop and lead to lateral instability. The analytical study shows that, with realistic functions for hydraulic resistance and bedload transport used herein, the two parameters which determine if meandering thalwegs will develop are the discharge intensity, q and channel width W. Results of analysis are confirmed by extensive data, especially collected for this purpose from straight irrigation canals in Pakistan. This study leads to criteria for stable channel width.