The Ōpaoa (Opawa) River is a spring fed river on the Wairau Plain which passes through Blenheim. The river flows into the Wairau Lagoons. Tributaries include the Taylor, Omaka, and Fairhall Rivers. The river forms a single channel with a silty bed. Median flow near the mouth of the river is 2.5 cumecs.
The river has become a highly modified and degraded environment with significant issues with pest weed species such as Lagarosiphon (oxygen weed). In spite of this, the river provides a habitat for a number of species including shortfin eel, brown trout, kōura and Inanga. A number of waterfowl also can be found in and around the river. Prior to European settlement, the river provided an important food gathering environment for Māori.
Following an earthquake in 1855 that lowered the land, the Ōpaoa became navigable by vessels with a shallow draft as far as its confluence with the Taylor River in Blenheim leading to the establishment of a port which operated until 1968 providing shipping across Cook Strait. More recently, the River Queen has offered cruises on the river since 1999.
A historic bridge over the Ōpaoa at the northern entrance to Blenheim is one of the earliest examples of concrete arch construction in NZ.