Before the 1920s, the southern border of Dearborn was the portion of Monroe Street heading southwest from the Downtown West Dearborn area. In 1925, the border extended south to Carlysle Street. This was extended further south again to Dartmouth Street in 1929 as part of the whole consolidation of Dearborn and Fordson. Presumably the floodplain of Ecorse Creek is why the border wasn’t extended even further south..
Dearborn’s southern border continues east along Dartmouth until it reaches a line 110 ft from the southern limit of what is now Outer Drive. It continues southeast along a line south of Outer Drive until it reaches the old Dearborn / Ecorse Township line. From there, the city line continues northeast along the old Dearborn Township line to the River Rouge.
In 1956, Dearborn attempted to annex the portion of Ecorse Township where the Fairlane Green (The Hill) shopping center is now. The area known at the time as the “Ford’s Woods” was eventually annexed by Allen Park because that community submitted paperwork 12 hours before Dearborn
An interesting area of the Dearborn / Allen Park border is the Detroit Lions training facility which opened in the early 2000s. While much of the parking lot and most neighboring buildings are in Allen Park, over 80% of the training structure is in Dearborn. Thanks to an agreement between Allen Park and Dearborn, both cities split tax money from the Lions and Allen Park is considered their home.
From the old Dearborn Township southern border, the Dearborn border continues eastward along the River Rouge. When modern Dearborn was established in 1929, the border followed the middle of the River Rouge channel at the time. When the River Rouge was straightened in the 1970s, the border with Allen Park was adjusted in 1979 to match the new channel. However the Dearborn border with Melvindale was not adjusted. As a result, pockets of Dearborn lie south of the Rouge connected to Melvindale and bits of Melvindale are north of the river.
The most unusual portion of Dearborn’s border along the River Rouge is the segment next to Fordson Island. That island was created in 1917 when the Rouge was channelized and dredged to better allow ships on the river. Even after the project, the border of Springwells Township and later Dearborn remained in the middle of the original Rouge channel. For decades Fordson island was inhabited with residents but is now vacant and closed to the public.