The St. Louis Corridor shows a number of Northern Cities Shift (NCS) features originating from the Inland North dialect area despite being geographically situated in the Midland region. Past studies have shown evidence of the Corridor's special status as an Inland North enclave (Labov 2007, Bigham 2010) but have not demonstrated how this dialectal relationship has grown and changed over time, showing mixed results when explaining or defining the Inland North influence (Kenny and Stanford 2013). In this dissertation, I demonstrate how the NCS rise and retreat in the Corridor is a result of population movements tied to both the development of Route 66 and historical periods of population movement. By analyzing new interviews collected for this project as well as existing interviews collected as early as 1972, this dissertation shows both dialect influence and retreat over the course of a generation. This discovery has implications for the communication patterns involved in dialect spread and retreat, including the role of child speakers, adult speakers, and location in diffusion as well as the overlap of speech communities. Analysis of the data also shows dialect interaction and interference stemming from the contact between the two competing Corridor dialects (NCS and Midland). Instead of demonstrating dialect leveling or movement of existing dialect boundaries, the NCS and Midland dialect features interact both at the phonological and broad dialectal level. During the period of Inland North influence, the strength of the NCS meant that even Midland sound changes unrelated to NCS dialect features (fronting of /aw/, /ow/, and non-coronal /uw/) showed a dip during the NCS influence period across the entire Corridor in both rural and urban areas. However, many individual vowel changes present in these dialect regions interfere directly (specifically Midland /ʌ/ fronting vs. NCS /ʌ/ backing, Midland conditioned low-back merger vs. NCS /o/ fronting, and the US nasal /æ/ system vs. NCS /æ/ raising). The strengthening of the Midland dialect ultimately led to the retreat of the NCS in the region. The exploration of these dialectal interactions contributes important insight to the study of competition between linguistic systems.