The second Spanish coastal voyage to search for the Sieur de La Salle's Texas colony, the Rivas-Iriarte expedition, was also the most important. It found the wreckage of La Salle's ships at Matagorda Bay and made a complete circuit of the Gulf of Mexico. The circumnavigation provided the basis for a detailed diary by the chief pilot, Juan Enríquez Barroto, and his coastal map, which has not yet come to light. The voyage was made in two piraguas (small, open vessels equipped with oars and a single sail) built at Veracruz especially for the purpose. The two craft, named Nuestra Señora del Rosario and Nuestra Señora de la Esperanza, had keels of fifty-four and sixty feet and were of shallow draft. Rosario, commanded by Martín de Rivas, was the larger. Esperanza was captained by Pedro de Iriarte. The ships were designed on the recommendations of the two chief pilots, Enríquez Barroto and Antonio Romero, who had conducted the first search voyage early in 1686, from Havana to the Mississippi River. Each had twenty oars per side, carried provisions for 3 ½ months, and towed canoes for exploring in shallow water. Manned with sixty-five soldiers and sailors, each vessel was mounted with six bronze swivel guns.
After sailing from Veracruz at nine o'clock on Christmas morning, 1686, the tiny ships soon encountered northers that set them back repeatedly and heavy seas that soaked the men and spoiled provisions. More than a month was required to reach Tampico, where the voyage was delayed another month to reprovision while awaiting more favorable weather. It sailed again on March 8. Progressing along the western and northern Gulf shore, the voyagers encountered Coahuilteca, Karankawa, and Atákapa Indians. At the mouth of the Rio Grande, near three native villages, the mariners sounded the bar while naked Indians watched from the beach with bows strung. As they sailed north along Padre Island, a continuous parade of natives moved in and out of the dunes. At Cedar Bayou (called Río de Flores), the explorers found in an abandoned Indian canoe fittings from a large ship, their first sign of the French interlopers. Exploring around Matagorda Bay, which they named San Bernardo, they found the wreckage of La Salle's bark Belle on Matagorda Peninsula and, in the entrance channel, the rudder post of his storeship Aimable, which had run aground while trying to enter the bay. They nevertheless concluded, because of the shallowness of the bay, that any attempt to establish a settlement here could only have met disaster and therefore no longer posed a threat. The explorers accorded scant attention to Galveston Bay because of its shoals. On reaching the Calcasieu River, they heard news-possibly of La Salle's men living among the Indians-that caused them to turn back to make the first known exploration of both Galveston Bay and Sabine Lake. At the Calcasieu also, they made the first known European contact with the Louisiana Atákapas. With the aid of two castaways from a lost Spanish galley-one an Apalachino Indian from Florida, the other a "Mexican" boy-whom they found living among these natives, the voyagers obtained significant ethnological data.
Thanks to their shallow-draft ships, members of the Rivas-Iriarte expedition may have been the first Europeans to glimpse the shoal-studded western Louisiana coast. Because of the wide, shallow shelf, larger vessels were unable to see the shore from a safe depth, and even the wilderness-built craft of the Pánfilo de Narváez and Hernando de Soto expeditions had given it a wide berth. The piraguas were able to explore many of the bays and inlets for the first time, including Atchafalaya Bay and the Mississippi passes. After reexploring Mobile Bay, the voyagers passed up Pensacola Bay and proceeded to San Marcos de Apalache (St. Marks, Florida) to reprovision, then returned, via Havana and the southern Gulf Coast, to Veracruz on July 3–4, 1687. Although a landmark of exploration, the voyage had failed to identify the elusive "Espíritu Santo Bay or Michipipi," where the French were believed to have settled. The captains and pilots could offer only an opinion, based on the signs at San Bernardo (Matagorda) Bay, that the French settlement had failed. Until additional evidence emerged, the matter could not be put to rest.