The Jordan River: big river, lots of people-- very crowded, lots of activity happening all around, miracles happening, many important people are just there, chaotic. This is a place that is included in many different stories throughout the Bible.
This detail is only included in the baptism accounts of Matthew and Mark, but not in Luke and John. The first two find it to be a very important detail, while the others just say it was water--not even specifically a river.