In Malay language, sungai is river. As I could not visit Sungai Pahang, Sungai Kinabatangan and Sungai Rajang, I visited instead Sungai Petani Post Office (Kedah)and Sungai Bakap Post Office, Sungai Dua Post Office and Sungai Puyu Mini Post Office (all in Penang) to get First Day Cover and post office cancellations.
I included Thailand longest stamp as comparison too.
Also collected from others First Day Cover cancellations from Kuantan General Post Office and Kuching General Post Office. Enjoy river viewing.