text: I don't have a complete answer, particularly on the naming for the full length of the waterway, but I did want to point out that there are a lot of situations where "St." shouldn't be expanded to Saint, because "St." is a part of the proper name. For instance St. Louis, MO is not expanded to Saint Louis, because the city charter gives the name as "City of St. Louis" very explicitly. So in this case I think we probably should not expand to Saint unless there is some sort of evidence that it was originally named "Saint Vrain".
  
  As for creek vs. river, that is a very local thing - I'd say that in dry CO, we have a lot of "rivers" that wouldn't pass muster as a river in other regions. Heck, you can practically jump across the mighty Colorado River in some locations!