Carvajal, who was one of the survivors of the expedition, narrated the events in his work Relación del nuevo descubrimiento del famoso río Grande que descubrió por muy gran ventura el capitán Francisco de Orellana ("Account of the recent discovery of the famous Grand river which was discovered by great good fortune by Captain Francisco de Orellana").
In it, the friar recorded the dates of the expedition as well as a large number of notes of ethnological interest such as the sizes and dispositions of the indigenous peoples which occupied the banks of the river, their tactics of war, rituals, customs, utensils, and the like.
For over four centuries, scholars dismissed its reports of large cities, well developed roads, monumental construction, fortified towns, and dense populations.
He was chosen to be provincial of Peru in 1557, dedicating two years to the organization of the province and the two following ones to the visiting of the most remote territories and the founding of new convents.
There is evidence of a letter written by Carvajal to the king, informing him of the abuses that were being committed against the Indians in the mines of Peru, and asking for his intervention on their behalf.
Not knowing it, the expedition of Gaspar de Carvajal carried diseases of the Old World, particularly smallpox, malaria and yellow fever into the deep areas of the Amazon.
"One writer (Antonio Vieira, 1842) estimated that in the 37 years between 1615 and 1652, more than two million Indians living on the lower Amazon died as a result of these newly-introduced diseases.