if there are any Peruvian colonial era history experts out there, I'm looking for a citable source of some kind for the following scenario.
In my research I've noticed an interesting discrepancy in Quechua dialects in the upper Amazon. One of these dialects, called 'Southern Pastaza Kichwa' is very linguistically distinct from its sister dialects upriver in Ecuador. I discussed this with a colleague and their best guess that this was because of contact from other Quechua speakers down in San Martin/Lamas area of Loreto.
To verify this hypothesis, I've been looking through a lot of literature and colonial area documents and while I have found some interesting things, nothing yet on missionary contact activity, or any activity at all between San Martin and the area around the Pastaza river.