河 he 江 jiang
(1) The smaller one is called 河，the bigger one is called 江；
The river that flows in to the lake or deep water is called 河(hé)，
The river that flows in to the ocean is called 江(jiāng)
(2) 江(jiāng)，it is frequently used in south of China,Such as Chang Jiang(长江)，Zhujiang(珠江)，HuangpuJiang(黄浦江),Ganjiang(赣江)，and so on.
河(hé),it is often used in north of China,Such as Huang he(黄河)，Haihe(海河)，Tarim he(塔里木河)，and so on.
However, there are some special occasions, for instance, there are few rivers in northeast China, such as Songhua Jiang(松花江),Heilongjiang(黑龙江).