In school (I think high school) a teacher explained that the reason so many Spanish explorers wandered around the US looking for a city of gold is because the Native American tribes would essentially tell them to "just keep walking; it's just beyond those mountains" in order to get rid of them.
Is this accurate?
Yes and no.
Spaniards did actively want to find sources of gold and silver (remember that precious metals were currency in this period). They also wanted to find people to conquer and then impose tribute and labor requirements.
We know that as they learned native languages they asked about resources and other native groups.
These encounters would have signaled to native people that Spaniards might pass them over if they could get to people and resources of greater value. We know that native groups recognized this and did tell Spaniards what they wanted to hear.
That said there were not endless expeditions for el Dorado. Once the Spanish realized there was not much of interest in North America they focused on consolidating their conquests in Mesoamerica and the Andes.
St. Augustine was founded to prevent a French colony from taking hold, and to secure the return sea route to Spain. New Mexico was founded shortly after to try and establish a foothold in the more populous Southwest.