Rivergrove is a microcity located in the Portland metropolitan area, between Lake Oswego and Tualatin, and also just north of the Tualatin River and just east of Interstate 5. It has a population of 505 (according to the city) or 545 (according to the US Census) people, and is about 1.5 miles by 1000 feet long. Despite its small size, it manages to straddle two counties, with most of it in Clackmas County, but some of it in Washington. There is no city hall, with the contact for the city being a PO Box in Lake Oswego. The city has one park. It has no retail businesses. A casual visitor would probably not know that they had crossed from Lake Oswego into another city.
The reason for the city's incorporation probably had something to do with taxes or zoning, and the city is generally affluent and quiet. However, the exact reasons and circumstances for the city's formation are not something I can find, with even the city's own webpage giving a rather sparse description of its founding. It does, however, have photos from an ice cream social. However, the existence of Rivergrove shows the diversity of communities sizes and types in the Portland area, many of which developed as the area grew at a rapid pace after World War II.