Why were African slaves imported to work on encomiendas in Latin America?

Las presiones ejercidas por sectores de la Iglesia y la disminución de la población indígena, determinaron que la corona permitiese el ingreso de negros africanos para que sustituyeran a la mano de obra aborigen.  

Explanation:

Por lo tanto, la encomienda no implicaba la propiedad sobre los nativos; era una concesión no heredable

an effect of the steamboat's popularity was that people began building canals in the united states and europe. this was done to find another way to transport goods across the country and in that time, transportation using the river was very slow and dangerous. it depended entirely on manpower and the current of the river. that all changed in the late 1700's when steamboats were introduced. they soon revolutionized travel and trade through the river and dominated the waterways. eventually, other means of transportation took over from the steamboats, but there was a time that they the united states to expand to the west.