Upon his return to San Francisco he was ordered to carry relief supplies for starving 49ers on the wagon trails to California, up river to Sacramento.
On November 1, 1850, the Invincible was sent from San Francisco on a mission to deliver 10,000 rations to the garrison of the remote post of Fort Yuma on the Colorado River.
Invincible drawing 8 feet of water was grounded at every ebb tide which was extreme in the Colorado River Delta.
On January 3, 1850, some 30 miles up river Captain Wilcox was forced to drop anchor, his way blocked by shoals too shallow to pass.
Arraignments were made to unload the boats at the ship's anchorage on the shore of Sonora and loaded onto wagons from the fort on 28 January.
This new boat, capable of carrying 50 tons of cargo, was very successful making round trips from the estuary to the fort in only four or five days.
[5] Two years after the competition of opposition steamboats was defeated and a monopoly on the river was achieved in 1867, the George A. Johnson and Company brought in more partners and created the California Steam Navigation Company, which now included a steamship line that ran from San Francisco to connect with their steamboats at their port and shipyard at the mouth of the Colorado River at Port Isabel, Sonora.
[4]: 71 That same year Wilcox purchased Rancho Santa Ysabel and eventually ran the largest sheep herd in San Diego County.