From Handbook of Texas Online sv "San Marcos River"-
The history of the river's name is complicated. In 1689 members of Alonso De León's expedition gave the name San Marcos to the first considerable river east of the Guadalupe, which scholars now believe to have been either the Colorado River or the Navidad River. Later Spanish explorers applied the name San Marcos to the first considerable river beyond the Guadalupe to the north and west-that is, to the present San Marcos River. Domingo Terán de los Ríos, however, is thought in 1691 to have called the San Marcos the San Agustín, and the Marqués de Aguayo in 1719 called it Los Inocentes. The name San Marcos was clearly the most common one applied to the river, however, being used by both Fray Isidro Félix de Espinosa and Fray Antonio de San Buenaventura Olivaresqv in 1709 and by Domingo Ramón in 1716
1709- Espinosa-Olivares-Aguirre Expedition
Read translation of Espinosa's diary of expedition. Found SM river but, no mention of springs or fountains
1716- Read Espinosa's diary of the Ramon Expedition. Mention only of the river, no description
1767- Solis Diary, only mentions river