Rio Grande participated in the Passage of Humaitá on 19 February 1868 and provided fire support for the army for the rest of the war.

The Pará-class monitors were designed to meet the need of the Brazilian Navy for small, shallow-draft armored ships capable of withstanding heavy fire.

The monitor configuration was chosen as a turreted design did not have the same problems engaging enemy ships and fortifications as did the casemate ironclads already in Brazilian service.

The engines produced a total of 180 indicated horsepower (130 kW) which gave the monitors a maximum speed of 8 knots (15 km/h; 9.2 mph) in calm waters.

[3] Rio Grande carried a single 70-pounder Whitworth rifled muzzle loader (RML) in her gun turret.

[5] Most unusually the gun's Brazilian-designed iron carriage was designed to pivot vertically at the muzzle; this was done to minimize the size of the gunport through which splinters and shells could enter.

She arrived at Montevideo on 6 January 1868 and steamed up the Paraná River, although her passage further north was barred by the Paraguayan fortifications at Humaitá.

Rio Grande and her two sister ships, Alagoas and Pará, were lashed to the larger ironclads in case any engines were disabled by the Paraguayan guns.

[8] After the war Rio Grande was assigned to the newly formed Alto Uruguay Flotilla, based at Itaqui.