Refers to a physically large, broad river. Often used in literary or descriptive contexts, and in proper names like 大河ドラマ (Taiga drama).
See also: 大川
The Amazon is the world's largest river.
A large river flows through this plain.
The village spread out along the banks of the great river.
They crossed the great river by boat.
小川 is a small stream, the opposite in scale to the broad, sweeping 大河.
Compound of 大 (large) and 河 (river). The reading たいが is the on'yomi of both characters.