Other Spaniards and, later, Mexicans, also were familiar with the Green; the old Spanish Trail from New Mexico to California crossed the Green just above the present-day town of Green River, Utah. These later explorers, who were probably traders, prospectors, and slavers, left no written records mentioning the river.
Although trapping parties from the Hudson's Bay Company were in the upper basin of the Green as early as 1819, it wasn't until 1825 that American trappers explored the river in Utah. In April of that year, William Ashley and a party of trappers floated down the river from north of the Uinta Mountains to the mouth of the White River. This marked the first recorded time that anyone had actually floated on the river. In the next decade, Browns Park and the bottoms around the mouth of the White became favorite wintering grounds and places of rendezvous for the trappers, as they also had been for the Indians before them. Several trading posts were established in the basin of the Green, at the mouth of the White, near Whiterocks, Utah, and in Browns Park by Antoine Robidoux, Denis Julien, and others. Julien was also one of the few trappers to actually float the river after Ashley's pioneering voyage. Most refused that means of transportation as John C. Frémont, who explored the area around the upper Green in 1843, noted: "Though offering many temptations, and often discussed, no trappers have been found bold enough to undertake a voyage with so certain a prospect of a fatal termination."