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In 1827 Lister-Maw, in 1834 and 1835 Smyth, in 1844 the French lieutenant in command of the "Boulonnaise," the Brazilian Valdez in 1840, the French "Paul Marcoy" from 1848 to 1860, the whimsical painter Biard in 1859, Professor Agassiz in 1865 and 1866, in 1967 the Brazilian engineer Franz Keller-Linzenger, and lastly, in 1879 Doctor Crevaux, have explored the course of the river, ascended many of its tributaries, and ascertained the navigability of its principal affluents.