The Namoi flows past Manilla and joins the Manilla River near the town. After passing into Keepit Dam it joins the Peel River below before passing by Carroll, then joining the Mooki River near Gunnedah on the Liverpool Plains. From here it flows north-westwards past Boggabri, Narrabri, Wee Waa and Burren Junction and eventually joins the Barwon River, near the town of Walgett.
Keepit Dam near Gunnedah is an important dam for flood control and irrigation. The floodplains of the lower Namoi around Wee Waa are an important area for growing cotton which is irrigated using water from the dam.
The Namoi River Road Bridge, which was built at Manilla in 1886, has been placed on the Register of the National Estate. This bridge has eleven metal lattice truss spans and is one of the longest six or seven metal truss bridges completed in Australia by that time.
Murray cod in the Namoi grow to a very large size, with fish of forty kilograms or larger being not uncommon. European carp and extensive irrigation practices have had adverse impact on the native fish population over the last forty years, and restocking programs have been undertaken, mainly by volunteers, to ensure the survival of the native fish population.
The Namoi River Snapping Turtle or Bell's turtle (Elseya belli) is a species of turtle found only in the upper reaches of the Namoi River, Gwydir and Macdonald Rivers on the North West Slopes of NSW Australia.